Overview
Introduction: This chapter introduces water as a vital natural resource that supports all life and many activities on Earth. It describes where water is found (rivers, lakes, ponds, wells, rain and underground), basic physical properties of water (colourless, odourless, tasteless; exists as solid, liquid and gas) and simple everyday observations and experiments children can do to understand water's behaviour. Importance: The chapter explains why water is essential — for drinking, cooking, cleaning, agriculture, industry and habitat for plants and animals. It emphasises the limited availability of clean drinking water, the need to keep water sources clean, and simple ways to save water at home and school. Key themes: Major ideas covered are sources of water, states of water and changes between them (melting, freezing, evaporation, condensation, precipitation), the water cycle, groundwater and wells, how water gets polluted, methods to make water safe for drinking (settling, filtration, boiling), and practical steps to conserve water. What the student will learn: By the end of the chapter students will be able to identify different sources of water, describe how water changes its…
Learning Objectives
- Define key terms related to water (evaporation, condensation, precipitation, transpiration, groundwater, water table, aquifer) and use them correctly in answers.
- Describe the three states of water (solid, liquid, gas) with everyday examples and explain the physical changes between these states (melting, freezing, evaporation, condensation).
- Explain the water cycle by naming and sequencing its main processes and by labelling a diagram showing evaporation, condensation, precipitation and runoff.
- Identify major sources of water (rain, rivers, lakes, groundwater, glaciers) and classify them as surface or underground sources.
- Demonstrate simple classroom experiments to show that water is a good solvent and record observations to distinguish between soluble and insoluble substances.
- Measure and compare the relative solubility of common materials (salt, sugar, sand) in water through simple experimentation and interpret the results.
- Explain common methods of water purification (settling, filtration, chlorination) and outline simple household ways to obtain safe drinking water.
- Calculate, using given data, the proportion or percentage of Earth's water that is fresh and usable for humans and interpret the implications for water availability.
Topics in this chapter
15 topics · tap a topic title to jump straight to it.
Importance and uses of water
What is water and why it is important
Water is a colourless, tasteless liquid essential for all known forms of life. It makes up a large part of living things and covers about 71% of the Earth's surface. Water is involved in almost every biological, physical and chemical process on Earth.
Key reasons why water is important
- Supports life: All plants, animals and humans need water to live. Cells use water to carry nutrients and remove wastes.
- Solvent for chemical reactions: Many reactions inside living organisms and outside (e.g., dissolving minerals in soil) occur in water because it dissolves many substances.
- Regulates temperature: Water heats up and cools down slowly, so it helps keep climates and bodies at stable temperatures.
- Agriculture and food production: Water is essential for growing crops and raising animals.
- Industry and energy: Many industries use water for manufacturing, cooling, cleaning and generating power (e.g., hydroelectricity).
- Habitat and biodiversity: Rivers, lakes, wetlands and oceans are homes for many plants and animals.
- Transport and recreation: Water bodies are used for transport (boats, ships) and for activities like swimming and fishing.
Uses of water in daily life
People use water for drinking, cooking, bathing, washing clothes and dishes, cleaning homes, gardening and flushing toilets. Water is also used for making medicines, producing food products, and in hospitals for sanitary purposes.
Environmental role and conservation
Water cycles through evaporation, condensation and precipitation, connecting oceans, atmosphere, land and living things. Freshwater is limited: only a small fraction of Earth's water is easily available for use. Therefore, conserving water — by fixing leaks, using water-saving taps, reusing water where safe, and practising efficient irrigation — is important to ensure supply for future generations.
- Drinking water keeps the human body hydrated and helps digestion.
- Irrigation waters crops so farmers can grow food (e.g., watering rice fields).
- Water cools machines in factories and in power plants (cooling towers, hydroelectric dams).
- Boating and fishing use rivers and lakes for transport and food supply.
- Washing clothes, dishes and cleaning the house use domestic water daily.
- Wetlands and rivers provide homes for fish, birds and many other animals.
- Chemical formula of water: H2O (each molecule has 2 hydrogen atoms and 1 oxygen atom).
- Water cycle (simple): Evaporation → Condensation → Precipitation → Collection.
- Mass–volume relation: mass = density × volume (useful to calculate water mass; density of water ≈ 1 g/cm³ or 1000 kg/m³ at 4°C).
- Heat to change temperature (basic physics): Q = m × c × ΔT (shows why water moderates temperature; specific heat c of water ≈ 4.18 J/g°C).
- Earth water facts (useful percentages): ~71% of Earth's surface is covered by water; about 2.5% of Earth's water is freshwater, and less than 1% is easily accessible freshwater.
Sources of water
What are sources of water?
Sources of water are places and processes from which we get water for drinking, farming, industry and nature. Water on Earth exists in several forms and comes from several sources: rain (precipitation), surface water (rivers, lakes, ponds, reservoirs), groundwater (wells, springs, aquifers), glaciers and snow, the oceans (saline water) and the atmosphere (dew, fog, humidity). People also create sources such as tanks, canals and rainwater harvesting systems.
Main sources — short descriptions:
- Rain (precipitation): Water that falls from clouds as rain, snow or hail. It is a direct and primary source that recharges rivers, lakes and groundwater.
- Surface water: Water held in rivers, lakes, ponds and reservoirs. It is easily visible and commonly used for irrigation, industry and household needs.
- Groundwater: Water stored beneath the ground in soil pore spaces and in rock cracks (aquifers). We access it using wells, tube wells and springs. Groundwater supplies many villages and cities, especially during dry seasons.
- Glaciers and snow: Large amounts of fresh water are stored in ice at high altitudes and release water gradually as they melt, feeding rivers (for example, Himalayan glaciers feeding rivers like the Ganga).
- Oceans and seas: Contain most of Earth’s water but it is saline (salty). It supports marine life and transport; desalination can make it usable for drinking in some areas.
- Atmosphere and near-surface sources: Water vapour, dew and fog provide small but important amounts of water in some climates and help in natural moisture cycles.
- Man-made sources: Reservoirs, canals, tanks, and harvested rainwater; these help store and distribute water where needed.
How these sources are connected — the water cycle:
Evaporation (and transpiration from plants) sends water into the air as vapour. This vapour cools and condenses into clouds and falls as precipitation. Precipitation either runs off into rivers/lakes, soaks into the ground (infiltration) to become groundwater, or is stored as snow/ice. This continuous movement links all sources.
Importance and challenges:
Only a small fraction of Earth’s water is usable fresh water. Rivers and groundwater are essential for life, agriculture and cities. Threats include pollution, over-extraction (groundwater depletion), loss of wetlands, and climate change affecting rainfall and glacier melt. Conserving and managing sources (for example, rainwater harvesting and protecting recharge areas) is vital.
- Collecting rainwater from a home roof into a barrel for gardening — simple rainwater harvesting.
- A village using groundwater from a dug well or tube well for drinking and irrigation.
- Rivers like the Ganga being fed by melting Himalayan glaciers and rainfall; the river water used for irrigation and transport.
- Reservoirs (dams) storing river water in the monsoon to use for irrigation in dry months.
- Seawater used for fishing and, in some coastal cities, desalinated to produce drinking water.
- Ponds and tanks in villages that recharge groundwater and supply cattle and household needs during summer.
- Volume of rainwater collected (litres) = Rainfall (mm) × Collection area (m²) (because 1 mm rainfall = 1 L/m²)
- Volume (m³) = (Rainfall in mm / 1000) × Area (m²)
- Mass of water (kg) = Volume (m³) × Density (1000 kg/m³) (so 1 m³ water = 1000 kg)
- Simple water balance: Precipitation (P) = Evaporation/Transpiration (E) + Runoff (R) ± Change in storage (ΔS)
Types of water
In Class 6 Science, "Types of water" means how water is classified by where it is found and by its quality. The common ways to classify water are by salinity (amount of dissolved salts), by source or location, and by suitability for use.
1. By salinity
- Saline (sea) water: Water of the oceans and seas contains a high amount of dissolved salts (mainly sodium chloride). Typical ocean salinity ≈ 3.5% (35 g of salt per litre or 35,000 ppm). It is not suitable for drinking or most agriculture without desalination.
- Brackish water: Water with salinity between freshwater and seawater (found in estuaries, mangroves). It has more salt than freshwater but less than seawater.
- Fresh water: Water with very low dissolved salts (rivers, lakes, ponds, most groundwater used for drinking). Freshwater has salinity much less than seawater and is the main source for drinking and irrigation.
2. By source/location
- Surface water: Water that collects on the Earth's surface such as rivers, lakes, ponds and reservoirs. Easily visible and often used directly after treatment.
- Groundwater: Water stored under the ground in soil pore spaces and in rock cracks (wells, tube wells, springs). Groundwater is an important source for drinking and irrigation.
- Rainwater: Water that falls from clouds. It is relatively pure when it forms but can pick up impurities while falling or after collecting.
3. By quality or use
- Potable (drinking) water: Water treated so it is safe for humans to drink.
- Hard water: Water that contains dissolved calcium and magnesium salts. Hard water reduces soap lather and forms scale in kettles and pipes.
- Soft water: Water low in dissolved calcium and magnesium. It lathers well with soap and does not form scale.
- Distilled/mineral/bottled water: Special forms: distilled water is purified by boiling and condensation; mineral water naturally contains dissolved minerals in safe amounts.
Why these distinctions matter
- Saline water cannot be drunk or used for irrigation without desalination.
- Groundwater and surface water require different methods of protection and recharge (e.g., stopping pollution, rainwater harvesting to recharge groundwater).
- Hard water affects household chores and industrial equipment; softening may be needed in some uses.
Simple classroom observations and experiments: Collect samples from a river (surface water), a hand pump (groundwater) and a glass of tap water. Test for taste/saltiness (sea water vs freshwater), for lathering with soap (hard vs soft), and for visible impurities. Discuss which is safe to drink and why.
- Sea water — saline; not fit for drinking without desalination (example: Arabian Sea, Bay of Bengal).
- River water (Ganga, Yamuna) — surface freshwater used for irrigation and domestic uses after treatment.
- Groundwater from a well or tube well — often used for drinking and irrigation; may be hard.
- Rainwater — collected on roofs and stored (rainwater harvesting) for domestic use and groundwater recharge.
- Estuary water — brackish water where rivers meet the sea (mix of fresh and saline).
- Bottled mineral water — commercially treated/filtered and contains safe levels of minerals.
- Percent concentration (mass percent) of salt = (mass of salt / mass of solution) × 100. Example: seawater ≈ 3.5%.
- ppm (parts per million) = mg of solute per litre of solution. 1 mg/L = 1 ppm. Example: seawater ≈ 35,000 ppm.
- Convert percent to ppm: percent × 10,000 = ppm. Example: 3.5% × 10,000 = 35,000 ppm.
- Hardness units: concentrations in mg/L are commonly reported as mg/L as CaCO3. (Practical note: 1 mg/L = 1 ppm.)
- Volume from percentage: volume of a type = (percentage / 100) × total volume. (Useful to calculate how much water of a given type is available.)
States of water
What are the states of water? Water exists in three states: solid (ice), liquid (water), and gas (water vapour). The state depends on temperature and pressure.
Particle explanation: In solid form, water molecules are closely packed in a regular structure and only vibrate about fixed positions. In the liquid state, molecules are close but can move past each other, allowing flow. In the gaseous state (water vapour), molecules are far apart and move freely in all directions.
Changes of state (phase changes) and what happens to the molecules:
- Melting (ice → water): On heating, molecules gain energy, vibrate more and break the rigid structure. Melting point of pure water at 1 atm = 0 °C.
- Freezing (water → ice): On cooling, molecules lose energy and arrange into a solid structure at 0 °C.
- Evaporation (liquid → vapour): Occurs at any temperature from the surface of a liquid as faster-moving molecules escape. Evaporation causes cooling (e.g., sweating).
- Boiling (rapid vapour formation throughout the liquid): Occurs when vapour pressure equals atmospheric pressure. Boiling point of water at 1 atm ≈ 100 °C.
- Condensation (vapour → liquid): Vapour loses energy and becomes liquid (e.g., water droplets on a cold glass).
- Sublimation (solid → gas) and deposition (gas → solid): Under certain conditions ice can change directly to vapour (sublimation) or vapour to ice (deposition, forming frost).
Energy during phase changes: During a change of state temperature remains constant while energy is absorbed or released. Energy used to change state (without temperature change) is called latent heat. Examples: latent heat of fusion (melting) and latent heat of vaporisation (boiling).
Effect of pressure: Boiling point depends on external pressure. At lower pressure (high altitude) water boils at a lower temperature. A pressure cooker raises pressure so water boils at a higher temperature and food cooks faster.
Everyday relevance: Understanding states of water explains rain and clouds (condensation), drying of wet clothes (evaporation), ice formation (freezing), and why sweating cools you (evaporative cooling).
- Ice cube left at room temperature melts into liquid water (melting).
- Puddles disappear on a sunny day (evaporation from the surface).
- Steam from a boiling kettle condenses into drops on a cooler surface (condensation).
- Frost forming on a cold morning when water vapour in air becomes ice on surfaces (deposition).
- Drying of clothes: water evaporates from fabric even when the water temperature is below boiling.
- Using a pressure cooker: water boils at a higher temperature under pressure, so food cooks faster.
- Q = m × c × ΔT (heat required to change temperature of a substance). Units: Q in joules (J), m in kilograms (kg), c in J/(kg·°C), ΔT in °C.
- Q = m × L (heat required for a change of state at constant temperature). L is latent heat. For fusion (melting) use L_f; for vaporisation use L_v.
- Typical values for water: c ≈ 4184 J/(kg·°C), L_f (fusion) ≈ 334000 J/kg, L_v (vaporisation) ≈ 2260000 J/kg.
Water cycle
What is the water cycle? The water cycle (also called the hydrological cycle) is the continuous movement of water on, above and below the surface of the Earth. Energy from the Sun drives the cycle and causes water to change its state — from liquid to vapor and back to liquid or ice — so water keeps circulating through different reservoirs (oceans, atmosphere, land, groundwater).
Main stages of the water cycle
- Evaporation: Liquid water from oceans, lakes, rivers and puddles warms up and turns into water vapour. The Sun provides the energy for evaporation.
- Transpiration: Plants release water vapour from their leaves into the air. Together with evaporation this is often called evapotranspiration.
- Condensation: Water vapour in the air cools and changes into tiny water droplets, forming clouds or fog.
- Precipitation: When cloud droplets combine and grow heavy, they fall as rain, snow, sleet or hail back to the ground.
- Runoff and Infiltration/Collection: Water that falls on land either flows over the surface into rivers, lakes and oceans (runoff) or soaks into the ground (infiltration). Some water collects in groundwater stores and later returns to surface water or is used by plants.
Why the water cycle is important
- It renews fresh water supplies needed by living things.
- It controls weather and climate patterns.
- It supports plant growth and replenishes rivers and groundwater.
Simple classroom observations and experiments
- Place a shallow dish of water in the sun and watch the water level fall (evaporation). Collect condensed water on a cold lid above boiling water to show condensation.
- Leave a wet cloth or puddle in sunlight vs shade to compare evaporation rates.
Human effects
Activities like deforestation, urbanisation and pollution can change evaporation, runoff and groundwater recharge, affecting the local water cycle and water availability.
Summary
The water cycle is a closed loop driven by the Sun: evaporation and transpiration → condensation → precipitation → collection (runoff, infiltration). This continuous movement keeps Earth's water available for ecosystems and humans.
- A puddle from rain disappears on a sunny day — water evaporates into the air.
- Dew on grass in the morning is water vapour that condensed as temperatures dropped overnight.
- Steam rising from a kettle demonstrates evaporation; droplets on a cool lid show condensation.
- Clouds forming over mountains as moist air rises, cools and condenses, then causes rainfall on the windward side.
- Plants releasing water vapour through leaves (transpiration); this helps add moisture to the air in forests.
- Rainwater collecting into streams and rivers that flow back into the sea (runoff and collection).
- Simple water budget (mass balance): P = E + T + R + ΔS where P = precipitation, E = evaporation, T = transpiration, R = runoff, ΔS = change in storage (soil moisture + groundwater).
- Conservation of mass principle (qualitative): Water in = Water out + Change in storage. This means total amount of water is conserved (ignoring very slow geologic exchanges).
- Percentage calculation (useful for water distribution): percentage = (part / whole) × 100. Example: percentage of freshwater = (freshwater volume / total water volume) × 100.
- Qualitative relation (class-level): Evaporation rate increases with temperature and wind. We can write a simple proportionality (not a precise formula): Evaporation ∝ Temperature × Wind speed.
Properties of water
Water is a simple molecule (H2O) but has many special physical properties that make it vital for life and everyday use. Below are the main properties with short explanations:
- Colourless, odourless and tasteless (pure water): Pure water appears transparent and has no smell or taste. Impurities can change these.
- Exists in three states: Solid (ice), liquid (water), and gas (water vapour). It changes state with temperature: melting, freezing, evaporation and condensation.
- No fixed shape, fixed volume (as a liquid): Liquid water takes the shape of its container but keeps its volume. As a gas it expands to fill the container; as a solid it has a definite shape.
- Universal solvent: Many substances (like salt and sugar) dissolve in water because its polar molecules attract ions and polar molecules. This makes water important for transport of nutrients and waste in living organisms.
- Cohesion and adhesion: Cohesion: water molecules stick to each other by hydrogen bonds. Adhesion: water molecules stick to other surfaces. These cause phenomena like water droplets and wetting.
- Surface tension: The top layer of water behaves like a stretched skin because of cohesion. Small insects (water striders) can walk on water and small metal needles can float if placed carefully.
- Capillary action (capillarity): Combination of cohesion and adhesion makes water rise in thin tubes or spaces (capillaries). Plants use this to move water from roots to leaves.
- Density behaviour and ice floats: Water has maximum density at about 4 °C. When water freezes it expands and ice is less dense than liquid water, so ice floats. This insulates lakes and helps aquatic life survive cold seasons.
- High specific heat: Water requires more heat to change temperature than many substances. This stabilises climate and helps organisms maintain temperature.
- High latent heat: Evaporation and melting of water require large amounts of energy (latent heat), which provides cooling effects (e.g., sweating cools the body).
- Wetting behaviour: On some surfaces (glass) water spreads (wets) because of strong adhesion; on waxy surfaces it forms beads (does not wet).
These properties together explain many everyday observations (rain, rivers, boiling, ice on ponds) and are essential for biological, chemical and physical processes.
- Ice floating on a lake: ice is less dense than liquid water so it stays on the surface and insulates the water below.
- Salt or sugar dissolving in water: shows water's property as a universal solvent (e.g., making a saline solution).
- Water climbing up a thin glass tube or inside plant xylem: capillary action helps water rise in plants.
- Water droplets on a waxed car bonnet form beads: poor wetting on hydrophobic surfaces.
- A water strider insect standing on a pond: surface tension supports its weight.
- Sweating cools the body: evaporation needs energy (latent heat) and removes heat from the skin.
- Density: ρ = mass / volume (ρ in kg/m³). Example: ρ = m / V.
- Heat to raise temperature: Q = m · c · ΔT, where Q = heat (J), m = mass (kg), c = specific heat (J/kg·°C), ΔT = temperature change (°C).
- Latent heat: Q = m · L, where L is latent heat of fusion (Lf) or vaporisation (Lv) (J/kg).
- Hydrostatic pressure: P = ρ · g · h, where P = pressure (Pa), ρ = density (kg/m³), g = acceleration due to gravity (~9.8 m/s²), h = depth (m).
- Capillary rise (in a thin tube): h = (2·γ·cosθ) / (ρ·g·r), where γ = surface tension (N/m), θ = contact angle, r = tube radius (m). (This is a more advanced relation but explains capillarity.)
Solubility and mixtures involving water
What is solubility?
Solubility tells how much of a substance (the solute) can dissolve in a given amount of a liquid (the solvent). In our chapter, water is the most common solvent. A substance that dissolves is called soluble (e.g., sugar in water); one that does not dissolve is insoluble (e.g., sand in water).
Parts of a solution
- Solvent: the liquid that dissolves the solute (water).
- Solute: the substance dissolved in the solvent (salt, sugar).
Types of mixtures involving water
- Solution: a uniform mixture where the solute particles are at molecular or ionic size and do not settle (e.g., salt or sugar dissolved in water).
- Suspension: a mixture where large particles are temporarily dispersed in water but settle on standing (e.g., muddy water, sand in water).
- Colloid: particles intermediate in size between solutions and suspensions; they do not settle and often scatter light (e.g., milk, fog).
Saturated, unsaturated and supersaturated solutions
- Unsaturated: more solute can still dissolve at that temperature.
- Saturated: the maximum amount of solute has dissolved at that temperature; any extra remains undissolved.
- Supersaturated: contains more solute than normally possible at that temperature (unstable; extra solute may crystallize out).
How dissolution happens
When a solute is added to water, water molecules surround solute particles and pull them apart (for ionic or polar solutes). Stirring (agitation), increasing surface area (crushing the solute), and raising temperature (for most solids) speeds up dissolution.
Factors affecting solubility
- Temperature: For most solids in water solubility increases with temperature (e.g., sugar dissolves more in hot tea). For gases (like oxygen, carbon dioxide) solubility in water decreases as temperature increases (warm water holds less dissolved oxygen).
- Pressure: Important for gases — higher pressure increases gas solubility in water (this is why soft drinks are fizzy when bottled under pressure).
- Nature of solute and solvent: "Like dissolves like" — polar solutes dissolve well in polar solvents (salt in water); nonpolar solutes (oil) do not dissolve in water.
Everyday examples and observations
- Making sweet tea (sugar dissolves faster in hot water and with stirring).
- Salt on icy roads: salt lowers the freezing point because it dissolves in the thin layer of water on the ice surface.
- Oil spill in water: oil floats and does not mix (immiscible).
- Soda bottle: opening reduces pressure and carbon dioxide escapes because gas solubility falls at lower pressure.
How solubility is expressed
In school we often express solubility as the grams of solute that dissolve in 100 g of water at a given temperature (for example: 36 g of NaCl per 100 g water at 20°C). Concentration can also be given as mass percent.
- Sugar in hot tea: sugar (solute) dissolves faster in hot water and more sugar can dissolve compared with cold water.
- Salt in seawater: salt dissolves in water to form a solution; if water evaporates, salt crystals are left behind.
- Sand in water: sand particles do not dissolve and settle to the bottom — a suspension.
- Oil and water: oil is immiscible with water and forms a separate layer due to being nonpolar.
- Carbonated drink: CO2 gas is dissolved under pressure; when the bottle is opened, pressure drops and gas escapes as bubbles.
- Solubility (common school form) = (mass of solute that dissolves / mass of solvent) × 100 g. Example: if 36 g salt dissolves in 100 g water at 20°C, solubility = 36 g per 100 g water.
- Mass percent concentration = (mass of solute / mass of solution) × 100%
- For gases (Henry's law, simple form) C = k × P, where C is concentration of dissolved gas, P is partial pressure of the gas above the liquid and k is a constant (shows gas solubility increases with pressure).
- Optional/advanced: Molarity = moles of solute / litres of solution (M).
Methods to obtain clean water
Clean water is water free from harmful microbes, visible dirt and dangerous chemicals so that it is safe to drink and use. There are several simple and industrial methods to obtain clean water. Each method targets different types of impurities (suspended solids, microbes, dissolved salts or chemicals).
- Sedimentation: Allowing water to stand undisturbed so heavy suspended particles settle at the bottom. Used as a first step in many treatment processes. After settling, clearer water on top is decanted or passed to the next step.
- Filtration: Passing water through layers that trap suspended particles. Simple household filters use cloth or sand and gravel; improved filters add charcoal (activated carbon) to remove bad taste and some chemicals. Ceramic and membrane filters remove bacteria and protozoa.
- Boiling: Heating water until it reaches a rolling boil kills disease-causing microbes (bacteria, viruses, protozoa). After boiling, allow water to cool and store it in a clean covered container. (WHO guideline: one minute at sea level; longer at high altitudes).
- Chlorination / Chemical disinfection: Adding chlorine (e.g., bleaching powder or sodium hypochlorite) kills microbes. Used by municipalities and for household emergency disinfection. After dosing, wait the recommended contact time (usually 30 minutes) before use. Follow product instructions for safe dosage.
- Distillation: Boiling water and condensing the steam collects pure water, leaving most impurities and salts behind. Useful when removing salts or many dissolved chemicals; requires heat and collection equipment.
- Reverse Osmosis (RO) and Membrane processes: Water is forced through a semipermeable membrane that blocks salts and many dissolved substances. RO is widely used for desalination and producing very pure water but wastes some water and needs electricity and maintenance.
- Solar Disinfection (SODIS): Filling clear PET bottles with water and exposing them to strong sunlight for several hours. UV rays and heat reduce microbes. Low-cost method suitable for small volumes in sunny climates.
- Ultraviolet (UV) treatment: Passing water under UV lamps inactivates microorganisms. It does not remove particles or chemicals, so water should be clear before UV treatment.
- Coagulation and Flocculation (used in waterworks): Chemicals (coagulants like alum) are added to water to bind fine particles into larger clumps (flocs) that settle out more easily. This is followed by sedimentation, filtration and disinfection in a municipal plant.
- Rainwater harvesting and simple protection: Collecting and storing rainwater in clean containers reduces reliance on contaminated surface water. Proper first-flush diversion and covered storage reduce contamination.
Choosing a method depends on the contamination type, the volume of water needed, available resources, cost and safety. Often several methods are combined (e.g., coagulation → sedimentation → filtration → chlorination) to produce safe drinking water at scale.
- Household: Boiling water for 1 minute and storing it in a clean container to make well or river water safe for drinking.
- Rural/household filter: A DIY filter made with layers of gravel, sand and charcoal to remove visible dirt before further treatment.
- Municipal waterworks: Water from a river is treated by screening → coagulation/flocculation → sedimentation → sand filtration → chlorination before distribution.
- Disaster relief: Using chlorination tablets or chlorine solution to disinfect water in emergencies.
- Desalination: Reverse osmosis plants to convert seawater into fresh water for coastal cities.
- SODIS: Placing clear PET bottles filled with contaminated water in bright sunlight for 6+ hours to reduce microbes for small household use.
- Concentration (mass/volume): C = m / V (e.g., grams per litre). Use to describe amount of a dissolved substance in water.
- Dilution (useful for preparing disinfectant solutions): C1 × V1 = C2 × V2. Example: diluting a stock bleach solution to a safe disinfecting concentration.
- Parts per million (ppm): ppm = (mass of solute / mass of solution) × 10^6. Used for low-concentration contaminants such as residual chlorine or pollutants.
- Percent removal (efficiency): % removal = ((C_initial − C_final) / C_initial) × 100. Use to compare how well a method removes a contaminant (e.g., turbidity or microbes).
Water pollution
What is water pollution? Water pollution is the way by which harmful substances — chemicals, microbes, solid waste, oil, heat or noise — get into water bodies (rivers, lakes, groundwater and seas) and make the water unsafe for use by plants, animals and people.
Common types of water pollution
- Physical pollution: floating plastic, sediments, oil films, or heat (thermal pollution).
- Chemical pollution: acids, alkalis, heavy metals (lead, mercury), pesticides, detergents and industrial chemicals.
- Biological pollution: disease-causing organisms such as bacteria, viruses and parasites from sewage and animal waste.
Main sources (how pollution reaches water)
- Domestic sewage and wastewater from homes (untreated sewage, soaps, detergents).
- Industrial effluents released without treatment (toxic chemicals, dyes, heavy metals).
- Agricultural runoff that carries fertilisers and pesticides into streams and ponds.
- Plastic and solid waste dumped into rivers or on land that washes into water bodies.
- Oil spills from ships, ports and pipelines.
Effects of water pollution — Polluted water harms fish and aquatic plants, reduces oxygen levels (so aquatic life can die), causes diseases in humans (cholera, typhoid, diarrhoea), makes water unsafe for drinking, farming and recreation, and damages ecosystems and livelihoods (fishing, tourism).
How to prevent and reduce water pollution
- Treat sewage before releasing it into water bodies and build proper toilets to stop open defecation.
- Install and enforce rules for industries to treat effluents to safe limits before discharge.
- Use fewer chemical fertilisers and pesticides; adopt organic farming and buffer strips near rivers.
- Reduce, reuse and recycle plastics; do not throw solid waste into drains or rivers.
- Pick up oil spills quickly, and reduce use of harmful household chemicals; use biodegradable detergents.
- Raise community awareness and follow local laws to protect water sources.
Simple observation activity (class-friendly): Collect two small water samples — one from a clean tap or bottle and one from a nearby pond or drain. Note colour, smell, presence of solids, and test pH with strips. Let samples stand in clear glasses to observe sedimentation. Discuss differences and likely causes.
Key message: Clean water is essential for life. Small actions (proper waste disposal, avoiding harmful chemicals, conserving water) by individuals and communities help keep water bodies clean and safe.
- Household sewage discharged into a local pond without treatment leads to foul smell, algae growth and fish deaths.
- Farm runoff after heavy rain carries fertiliser into a river, causing excess algae (eutrophication) that reduces oxygen for fish.
- Plastic bags and bottles thrown near roads are carried by stormwater into drains and then into the sea, harming marine life.
- An oil spill from a boat creates a thin film on a river surface that prevents oxygen exchange and coats birds' feathers.
- An industry releasing coloured dye into a river turns the water, killing plants and making it unsafe for nearby villagers to use.
- Concentration (mg/L) = mass of pollutant (mg) ÷ volume of water (L)
- Dilution law (useful to estimate after mixing): C1 × V1 = C2 × V2 (where C = concentration, V = volume)
- Percent concentration (%) = (mass of solute ÷ mass of solution) × 100
- pH definition (basic concept) = -log10[H+] (shows acidity/alkalinity of water; pH 7 is neutral)
- BOD (simple idea for pollution) ≈ DO_initial − DO_after_5_days (mg/L) — higher BOD means more organic pollution
Water conservation
What is water conservation?
Water conservation means using water carefully and avoiding waste, so there is enough clean water for people, animals, plants and future generations. Although the Earth is covered with water, only a tiny part is fresh and easily available for use. Conserving water helps protect this limited resource.
Why is it important?
- Protects freshwater sources (rivers, lakes, groundwater).
- Ensures water for drinking, farming and industry.
- Prevents water scarcity and reduces energy used to treat and pump water.
- Helps maintain ecosystems and biodiversity.
Simple methods of water conservation
- At home: Turn off taps while brushing teeth, fix leaks, use a bucket for washing vehicles instead of a running hose, collect and reuse rinse water for plants.
- In gardening and farming: Use drip irrigation, water plants early morning or late evening, mulch soil to reduce evaporation.
- Rainwater harvesting: Collect roof runoff in tanks or recharge wells to store water for later use and to replenish groundwater.
- Reduce, reuse, recycle: Reuse greywater (from washing) for gardening, and choose water-efficient appliances.
How kids can help
- Turn taps off tightly and report leaks.
- Use a bucket for play with water and avoid leaving taps running.
- Plant native and drought-tolerant plants.
- Encourage rainwater harvesting at school or home.
Effects of good water conservation
Lower water bills, healthier rivers and lakes, stable groundwater levels, and a safer future supply of freshwater for all.
Short example calculation (explained)
If a shower uses 9 litres per minute and you reduce your shower by 3 minutes, you save 9 × 3 = 27 litres each time. Save small amounts often and they add up.
- Fixing a leaking tap that drips once per second can waste up to 5–10 litres per day. Repairing it saves that water every day.
- Using a bucket and mug to bathe instead of a running shower: a bucket bath may use 30–50 L, while a 10-minute shower at 9 L/min uses 90 L.
- Collecting rainwater from a 20 m² roof during a 10 mm rain gives roughly 20 m² × 0.01 m = 0.2 m³ = 200 litres (before losses). This water can be used for gardening.
- Drip irrigation uses water directly at plant roots and can save up to 30–60% water compared to flood irrigation.
- Reusing greywater (from washing vegetables) to water plants — one wash bowl of 10 L reused daily gives 300 L/month for plants.
- Volume of water in a rectangular container: Volume = area × depth (V = A × h). Example: a tank 2 m × 1 m with water 0.5 m deep holds V = 2 × 1 × 0.5 = 1 m³ = 1000 L.
- Flow rate (simple): Flow rate = Volume ÷ Time (Q = V / t). Example: a tap delivering 9 L in 1 minute has Q = 9 L/min.
- Water saved (absolute): Saved = Initial use − New use. Example: 90 L (initial) − 63 L (new) = 27 L saved.
- Percentage saved: % Saved = (Saved ÷ Initial use) × 100. Example: (27 ÷ 90) × 100 = 30%.
- Rainwater collected (approx.): Collected volume = roof area × rainfall depth × runoff coefficient. Example: V = A × R × C (C ~ 0.8 for tiled roofs).
Groundwater and water table
Groundwater and Water Table
Groundwater is the water that soaks into the soil and fills the spaces and cracks in rocks below the Earth's surface. When rain falls, some water runs off, some is taken up by plants, some evaporates, and the rest infiltrates the soil. This infiltrated water moves downward by gravity until it reaches a zone where all pore spaces are completely filled with water. This lower zone is called the zone of saturation and the upper surface of this zone is called the water table.
Above the zone of saturation is the zone of aeration (or unsaturated zone) where pores contain both air and some water. Just above the zone of saturation there is a thin region called the capillary fringe where water is drawn up slightly by capillary action.
An aquifer is a body of permeable rock or sediment that stores and transmits groundwater easily (for example, sand and gravel). An aquiclude or aquitard is a layer that does not allow water to pass through easily (for example, clay or solid rock).
Wells and tube wells tap groundwater by reaching below the water table. If the water table falls below the bottom of a well, the well will stop supplying water. The water table is not fixed; it rises after heavy rains and falls during dry seasons or when too much water is pumped out.
Groundwater is important because it supplies drinking water, supports irrigation, and maintains river flow during dry periods. Human activities affect groundwater levels and quality: excessive pumping can lower the water table (groundwater depletion) and pollutants from sewage, chemicals, or fertilizers can contaminate groundwater.
- Key terms: infiltration, percolation, zone of aeration, zone of saturation, water table, aquifer, recharge, discharge.
- Recharge is the process by which groundwater is replenished (for example, by rain or by water from ponds). Discharge is the flow of groundwater to the surface (springs, rivers) or through wells.
- A village dug well that fills with water after rains because the water table rises.
- Tube wells used for irrigation that lower the water table when many tubes are pumped at once.
- Natural springs where groundwater reaches the Earth's surface and flows out.
- An oasis in a desert where groundwater near the surface allows plants to grow.
- A pond recharging the nearby groundwater when water soaks down from the pond bed.
- Contamination example: seepage from a leaking septic tank polluting nearby groundwater used for drinking.
- Porosity (%) = (Volume of voids / Total volume of soil or rock) × 100
- Specific yield = (Volume of water that drains by gravity / Total volume) — used to estimate usable groundwater
- Infiltration rate = Volume of water infiltrated / (Area × Time) — units like cm/hour
- Darcy's law (basic form) Q = k × A × (dh/dl) where Q is discharge (volume/time), k is hydraulic conductivity, A is cross-sectional area, and dh/dl is hydraulic gradient (useful for estimating groundwater flow)
Transpiration and plant role
What is transpiration?
Transpiration is the loss of water vapour from the aerial parts of plants (mainly leaves) into the air. Water absorbed by roots travels up through the plant and evaporates from tiny openings on the leaf surface called stomata, and also from the cuticle and lenticels.
How transpiration happens (simple steps):
- Roots absorb water from the soil.
- Water rises through xylem vessels by capillary action and by a pull created when water evaporates from leaves.
- Water evaporates from cell surfaces into air spaces inside the leaf and exits through stomata as water vapour.
Types of transpiration (brief):
- Stomatal transpiration – through stomata (major part).
- Cuticular transpiration – through the waxy cuticle on the leaf surface.
- Lenticular transpiration – through lenticels in stems.
Role and importance of transpiration:
- Transports water and dissolved minerals from roots to all parts of the plant.
- Helps in cooling plants (evaporative cooling) on hot days.
- Maintains turgor pressure (firmness) in cells which helps support the plant.
- Creates a continuous upward pull (transpiration pull) that helps water movement through xylem.
- Plays a part in the water cycle by returning water vapour to the atmosphere, affecting local climate and humidity.
Factors affecting transpiration:
- Light: more light usually opens stomata → more transpiration.
- Temperature: higher temperature increases evaporation → more transpiration.
- Humidity: low humidity increases transpiration; high humidity decreases it.
- Wind: wind removes humid air around leaves → increases transpiration.
- Soil water availability: less soil water reduces transpiration and may cause wilting.
Simple classroom observations and importance:
Covering leaves with a polythene bag will show water droplets forming inside the bag—this is direct evidence of transpiration. Plants with small or fewer stomata (xerophytes like cacti) lose less water, while water plants (hydrophytes) have different transpiration patterns. Transpiration is vital for nutrient transport and cooling, but excessive transpiration without enough water uptake causes wilting.
- Placing a clear plastic bag over a leafy branch: after some hours you see water droplets inside the bag — this shows transpiration.
- On a hot, dry, and windy day houseplants wilt because transpiration exceeds water uptake from the soil.
- Trees in a city cool the surroundings: large numbers of trees release water vapour by transpiration, lowering local temperature.
- Cactus and other desert plants have reduced transpiration (thick cuticle, fewer stomata) to conserve water.
- Using a potometer (classroom setup) shows faster water uptake on bright warm days than on cold cloudy days, demonstrating the effect of light and temperature on transpiration.
- Transpiration rate R = Volume of water lost (V) / Time (t). Example units: mL/hour.
- Percentage of water transpired (%) = (Water transpired / Water absorbed) × 100.
- Net water balance (simple) = Water absorbed by roots − Water lost by transpiration (and other losses).
Distribution of Earth's water
Overview
Most of the water on Earth is found in the oceans and is salty. Only a very small part is fresh water that we can use for drinking, irrigation and most other needs.
Simple numerical picture
If we consider all the water on Earth as 100 parts: about 97 parts are salt water (in oceans and seas) and only about 3 parts are fresh water.
Breakdown of the 3 parts of fresh water
Of that small 3 parts of fresh water:
- About 69% (≈2.07 parts of the 100) is frozen in glaciers and ice caps.
- About 30% (≈0.90 parts of the 100) is stored as groundwater (water in soil and rocks).
- Less than 1% (≈0.03 parts of the 100) is available as surface water in rivers, lakes and ponds or as water vapour in the atmosphere.
What this means
Even though there is a lot of water on Earth, only a tiny fraction is easily available fresh water. For example, the water flowing in rivers—which we commonly use for drinking, drinking-water treatment, irrigation and industry—is only a tiny fraction of the total water on Earth.
Key terms
- Saline water: water with high salt content (oceans and seas).
- Fresh water: water with little or no salt (rivers, lakes, groundwater, glaciers).
- Groundwater: water stored under the ground in soil and rock pores (aquifers).
- Glaciers and ice caps: frozen fresh water stored at the poles and on high mountains.
Why this is important
Because usable fresh water is limited, we must conserve it, prevent pollution of rivers and groundwater, use water carefully in agriculture, and recharge groundwater where possible. Melting of glaciers (due to climate change) can change the availability of water for many regions that depend on glacier-fed rivers.
- Drinking water: Although Earth is 70% covered by water, most is salty. Cities rely on groundwater, rivers or treated surface water because ocean water is not directly drinkable.
- Irrigation for farming: Farmers pump groundwater from wells or use river water for irrigation. Overuse can lower the groundwater level and wells can dry up.
- Glacier-fed rivers: Many rivers in mountainous regions (e.g., Himalaya-fed rivers) get water from melting glaciers. Reduced glacier mass can reduce river flow over time.
- Desalination: Coastal cities sometimes use desalination plants to convert seawater into fresh water — an expensive but useful option where fresh water is scarce.
- part = (percentage / 100) × total (useful to calculate how much of total water is in each store). Example: freshwater = (3 / 100) × 100 = 3 units if total = 100 units.
- percentage = (part / total) × 100. Example: if rivers contain 0.03 units out of 100, percentage = (0.03 / 100) × 100 = 0.03% of total Earth water.
- To convert a share of freshwater into share of total water: (share_of_freshwater × freshwater_percentage) / 100. Example: rivers ≈ 1% of fresh × 3% total = 0.03% of total.
Domestic water supply and sanitation
What is domestic water supply? Domestic water supply means providing safe water to homes for drinking, cooking, bathing, washing and other household needs. Water comes from sources such as rivers, lakes, wells, groundwater (borewells), springs and rainwater. The water supply system collects water from a source, treats it if necessary, stores it and distributes it to houses through pipes, tanks or by tankers.
Key parts of a domestic water supply system
- Source: rivers, lakes, groundwater, rainwater harvesting.
- Intake and conveyance: pumps and pipes that move water from the source to treatment or storage.
- Treatment: simple steps include screening (removing large particles), sedimentation (letting heavy particles settle), filtration (sand/charcoal filters) and disinfection (chlorination or boiling) to make water safe.
- Storage: overhead tanks and ground tanks store water to ensure supply during shortages and provide pressure to the taps.
- Distribution: pipes, taps and standposts that deliver water to households.
What is sanitation? Sanitation means safe handling and disposal of human waste (faeces and urine), wastewater and solid waste so they do not spread disease. Good sanitation includes toilets, sewage systems, septic tanks and sewage treatment plants. It also includes personal hygiene (handwashing) and safe disposal of household waste.
How sanitation and clean water protect health
- Proper treatment and safe storage of drinking water prevent water-borne diseases such as diarrhoea, cholera and typhoid.
- Safe toilets and sewage disposal prevent contamination of water sources and reduce spread of pathogens and parasites.
- Handwashing with soap after using toilet and before eating interrupts disease transmission.
Household methods to make water safe
- Boiling: heating water until it bubbles kills microbes.
- Filtration: using cloth, ceramic or sand filters to remove particles and some microbes.
- Chlorination: adding small amounts of chlorine (or bleach) kills bacteria and viruses.
- Solar disinfection (SODIS): leaving clear bottles of water in sunlight for several hours.
- Sedimentation: letting water stand so heavy dirt settles to the bottom, then pouring off clear water.
Sanitation systems at community and household level
- Septic tanks for individual houses: store and partially treat sewage; effluent seeps into a soak pit.
- Sewage network connected to a sewage treatment plant (STP) for towns and cities.
- Community toilets and safe desludging services where sewerage is not available.
Water conservation and good practices
- Fix leaking taps and pipes.
- Collect and store rainwater (rainwater harvesting) for non-drinking uses or after treatment for drinking.
- Reuse greywater (from baths/sinks) for gardening after simple filtration.
- Use bucket bathing instead of long showers; switch off taps while brushing teeth.
Why this matters in real life — Safe water and sanitation are essential for health, dignity and well-being. They prevent disease, save time (especially for women and children who otherwise fetch water), and protect the environment by preventing pollution of rivers and groundwater.
- An overhead water tank supplies water to a house. The pump fills the tank from a borewell; water flows down by gravity to household taps.
- In a village, a handpump provides groundwater for many families. Periodic chlorination or boiling is used if contamination is suspected.
- A family boils water for 5–10 minutes to kill germs before drinking when no treated water is available.
- A household collects rainwater from the roof into a barrel and uses it for gardening and washing clothes.
- A home toilet is connected to a septic tank where solids settle and liquid effluent seeps into a soak pit; the sludge is removed by desludging services.
- A community installs a slow sand filter and chlorination point so locals receive safer piped water.
- Flow rate: Q = V / t. Example: if 100 litres are used in 10 minutes, Q = 100 L / 10 min = 10 L/min.
- Pressure due to a column of water: P = ρ g h. For water ρ ≈ 1000 kg/m^3 and g ≈ 9.8 m/s^2, so P (Pa) = 9800 × h (m). This explains why taller overhead tanks give higher tap pressure.
- Volume of a cylindrical storage tank: V = π r^2 h. Convert cubic metres to litres: 1 m^3 = 1000 L. Example: a tank radius 0.5 m and height 1 m has V = π × 0.5^2 × 1 ≈ 0.785 m^3 ≈ 785 L.
- Water used (cost) calculation: Cost = Rate_per_unit × Volume_used. If rate is per kilolitre (kL), convert: 1 kL = 1000 L.
Simple experiments and activities
Introduction
Simple experiments and activities for Class 6 chapter 'Water' help students observe properties and behaviour of water: its states (solid, liquid, gas), evaporation and condensation, solubility, filtration, capillary action and role in everyday life. These activities are safe, use common materials and develop observational and reasoning skills.
Common experiments (aim, materials, steps, observations & explanation)
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Melting and freezing (state change)
Aim: Observe melting of ice and freezing of water.
Materials: Ice cubes, bowl, freezer or warm room.
Procedure: Place ice cubes in a bowl at room temperature and time how long they take to melt. To freeze, put water in an ice tray and into a freezer.
Observation: Ice becomes water on heating; water becomes ice on cooling.
Explanation: Heat causes molecules to move faster and break rigid structure (melting). Cooling reduces motion and forms solid (freezing).
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Evaporation and condensation (mini water cycle)
Aim: See evaporation and condensation.
Materials: Shallow bowl, warm water, clear plastic wrap, small cup (optional), sunlight or lamp.
Procedure: Put some water in a bowl, cover tightly with plastic wrap, place a small cup in the centre (optional) so condensed drops run into it. Put in sunlight or near a lamp.
Observation: Water level in the bowl decreases (evaporation); drops form on the wrap and then fall (condensation).
Explanation: Water molecules escape as vapour from the liquid (evaporation). Vapour cools at the wrap and becomes liquid again (condensation) — same as rain formation.
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Filtration and settling (cleaning dirty water)
Aim: Remove suspended solids from muddy water.
Materials: Muddy water, filter paper/coffee filter/cloth, funnel, container.
Procedure: Pour muddy water through the filter into a clean container. Allow some sample to stand for settling.
Observation: Larger particles are left on the filter; water becomes clearer. After standing, heavy particles settle to bottom.
Explanation: Filtration removes suspended solids; settling relies on gravity. This does not remove dissolved substances or kill microbes.
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Dissolving and making a saturated solution
Aim: Learn how solutes dissolve and what a saturated solution is.
Materials: Water, table salt/sugar, spoon, beaker.
Procedure: Add a spoonful of salt to a measured amount of water and stir. Keep adding until no more dissolves (solid remains). Warm the water and notice more dissolves.
Observation: Salt dissolves up to a limit; solubility increases with temperature for many solids.
Explanation: Solute particles mix with water molecules. At saturation, the maximum amount dissolvable at that temperature is reached.
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Capillary action (coloured celery experiment)
Aim: Show how plants draw water up through capillary action.
Materials: White celery stalks or white carnations, coloured water (food colouring), tall glass.
Procedure: Put the celery stalk in coloured water and leave for several hours to a day.
Observation: Coloured water moves up the celery; leaves/edges show colour.
Explanation: Thin tubes (capillaries) in plants pull water upward due to adhesion, cohesion and capillary action.
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Floating and sinking (density demonstration)
Aim: See effect of density using an egg.
Materials: Fresh egg, glass of water, salt.
Procedure: Put the egg in plain water — it sinks. Add salt gradually and stir — at some concentration the egg floats.
Observation: Floating occurs when water becomes denser than the egg.
Explanation: Density = mass/volume. Adding salt increases water density; if liquid density > object density, object floats.
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Diffusion of ink or dye in water
Aim: Observe how substances spread in water without stirring.
Materials: Clear glass of water, a drop of ink or food colour.
Procedure: Gently drop an ink drop into still water and watch.
Observation: Colour spreads gradually through water.
Explanation: Random motion of molecules causes diffusion until concentration is uniform.
Safety tips: Use adult supervision for hot water, glassware and food colouring. Clean spills immediately.
- Evaporation in daily life: Wet clothes dry because water evaporates into the air; faster in sunlight and wind.
- Condensation in daily life: Water droplets on a cold glass or morning dew form by condensation of water vapour.
- Water purification: Settling and filtration remove suspended solids; boiling kills microbes (not shown by filtration alone).
- Plants drawing water: Capillary action helps roots and stems transport water to leaves, visible in the coloured-celery experiment.
- Salt and buoyancy: Seawater is denser than freshwater, which is why people float more easily in the ocean.
- Density: ρ = mass (m) / volume (V). Example units: kg/m³ or g/cm³.
- Concentration (mass/volume percent): % (w/v) = (mass of solute in g / volume of solution in mL) × 100.
- Evaporation (average) rate: rate = (mass_initial - mass_final) / time. (Used to compare conditions.)
- Simple mass balance when dissolving: mass_solution = mass_solvent + mass_solute (mass conserved).
- Diffusion (basic rate relation): rate ∝ concentration difference / distance (qualitative for class 6).
Key Concepts
- Water cycle
- Continuous movement of water on, above and below the surface of Earth through evaporation, condensation and precipitation.
- Evaporation
- Process by which water changes from a liquid to a gas (water vapour) due to heat.
- Condensation
- Conversion of water vapour into tiny liquid droplets when it cools, forming clouds or dew.
- Precipitation
- Any form of water (rain, snow, sleet, hail) that falls from clouds to the Earth's surface.
- Transpiration
- Release of water vapour from plant leaves into the atmosphere.
- Surface run-off
- Water that flows over land into rivers, lakes or drains after rain when soil cannot absorb it all.
- Percolation
- Downward movement of water through soil and porous rock layers that helps recharge groundwater.
- Groundwater
- Water stored beneath the Earth's surface in soil pores and rock spaces.
- Water table
- Upper level of an underground surface in which the soil or rocks are permanently saturated with water.
- Aquifer
- Underground layer of permeable rock, sand or gravel that holds and transmits groundwater.
- Well (open well)
- A dug or bored hole in the ground that reaches the water table and is open at the top for drawing water.
- Tube well
- A deep, narrow well with a pipe and pump used to reach groundwater at greater depths.
- River
- A natural flowing body of freshwater that usually moves toward a sea, lake or ocean.
- Lake
- A large inland body of standing water surrounded by land.
- Pond
- A small, shallow body of still water, usually smaller than a lake.
- Glacier
- A large, slow-moving mass of ice formed from compacted snow that feeds rivers when it melts.
- Reservoir
- An artificial lake created by building a dam to store water for drinking, irrigation or power generation.
- Rainwater harvesting
- Collecting and storing rainwater from roofs or surfaces for later use.
- Filtration
- Process of removing impurities from water by passing it through materials like sand, gravel or charcoal.
- Potable water
- Water that is safe and clean for human consumption and cooking.
End-of-Chapter Trial Paper & Test Questions
Topic-wise questions to test your understanding of every concept in this chapter.
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Which process involves water changing from liquid to vapour at any temperature from the surface of a liquid? / किस प्रक्रिया में जल किसी भी तापमान पर तरल की सतह से वाष्प में बदलता है? (a) Condensation / संघनन (b) Evaporation / वाष्पीकरण (c) Precipitation / वर्षण (d) Freezing / हिमीकरण
Show answer
(b) Evaporation / वाष्पीकरण — Evaporation is the change of water from liquid to vapour and can occur at any temperature. It is driven by faster-moving surface molecules escaping into the air. / वाष्पीकरण जल का तरल से वाष्प में परिवर्तन है और किसी भी तापमान पर हो सकता है।
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Approximately what percentage of Earth's total water is fresh water? / पृथ्वी के कुल जल का लगभग कितने प्रतिशत भाग मीठा पानी है? (a) 50% (b) 97.5% (c) 2.5% (d) 10%
Show answer
(c) 2.5% — About 97.5% of Earth's water is saline (oceans), and only about 2.5% is fresh water, most of which is locked in glaciers and ice caps. / पृथ्वी का लगभग 97.5% पानी खारा (समुद्र) है और केवल लगभग 2.5% मीठा पानी है, जिसका अधिकांश भाग हिमनदों और बर्फ में बंद है।
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Which method of water purification kills disease-causing microbes by heating water to a rolling boil? / जल शुद्धिकरण की कौन सी विधि जल को उबालकर रोग फैलाने वाले सूक्ष्मजीवों को नष्ट करती है? (a) Filtration / छानना (b) Sedimentation / अवसादन (c) Boiling / उबालना (d) Distillation / आसवन
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(c) Boiling / उबालना — Boiling water at a rolling boil kills bacteria, viruses and protozoa. It is a simple and effective household method to obtain safe drinking water. / उबलते जल में उबालने से बैक्टीरिया, वायरस और प्रोटोजोआ नष्ट हो जाते हैं।
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Fill in the blank: The continuous movement of water on, above and below the surface of the Earth is called the ______. / रिक्त स्थान भरें: पृथ्वी की सतह पर, ऊपर और नीचे जल की निरंतर गति को ______ कहते हैं।
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Water cycle / जल चक्र (Hydrological cycle / जलवैज्ञानिक चक्र) — The water cycle is driven by solar energy and involves evaporation, condensation, precipitation, and collection/runoff, continuously recycling water through different reservoirs. / जल चक्र सौर ऊर्जा द्वारा संचालित होता है और इसमें वाष्पीकरण, संघनन, वर्षण तथा संग्रह/अपवाह शामिल हैं।
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Fill in the blank: Water stored beneath the ground in soil pores and rock cracks is called ______. / रिक्त स्थान भरें: मिट्टी के छिद्रों और चट्टान की दरारों में भूमि के नीचे संग्रहीत जल को ______ कहते हैं।
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Groundwater / भूजल — Groundwater fills aquifers below the water table. It is accessed by wells and tube wells and is a vital source of drinking water and irrigation. / भूजल जल स्तर के नीचे जलभृतों को भरता है। इसे कुओं और नलकूपों द्वारा प्राप्त किया जाता है।
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True or False: Water has its maximum density at 100°C, which is why ice forms at the surface of ponds. / सत्य या असत्य: जल का घनत्व 100°C पर अधिकतम होता है, इसीलिए तालाब की सतह पर बर्फ बनती है।
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False / असत्य — Water has its maximum density at about 4°C. When water cools further and freezes, it expands and becomes less dense than liquid water, so ice floats on the surface. This insulates the water below and helps aquatic life survive winter. / जल का घनत्व लगभग 4°C पर अधिकतम होता है। जब जल और ठंडा होकर जमता है, यह फैलता है और तरल जल से कम घना हो जाता है, इसलिए बर्फ सतह पर तैरती है।
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What is transpiration? How does it help plants and the water cycle? / वाष्पोत्सर्जन क्या है? यह पौधों और जल चक्र में कैसे सहायता करता है?
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Transpiration is the loss of water vapour from plant leaves mainly through stomata. It helps plants by drawing water and minerals upward from roots through xylem (transpiration pull) and by cooling the plant. In the water cycle, transpiration returns water vapour to the atmosphere, contributing to cloud formation and rainfall. / वाष्पोत्सर्जन मुख्यतः रंध्रों के माध्यम से पत्तियों से जलवाष्प का उत्सर्जन है। यह जड़ों से जल और खनिज ऊपर खींचने और पौधे को ठंडा रखने में सहायता करता है।
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List two causes of water pollution and two ways to prevent it. / जल प्रदूषण के दो कारण और इसे रोकने के दो उपाय बताइए।
Show answer
Causes: (1) Untreated domestic sewage discharged into rivers. (2) Agricultural runoff carrying fertilisers and pesticides. Prevention: (1) Treat sewage before releasing it into water bodies. (2) Use fewer chemical fertilisers; adopt organic farming near rivers. / कारण: (1) अनुपचारित घरेलू सीवेज का नदियों में प्रवाह। (2) कृषि अपवाह जो उर्वरक और कीटनाशक लेकर आता है। रोकथाम: (1) सीवेज को जल निकायों में छोड़ने से पहले उपचारित करें। (2) रासायनिक उर्वरकों का कम उपयोग करें।
Related Laws & Principles
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