Chapter 6 — Changes Around Us

What is a change? A change is when an object or a situation becomes different from what it was before. Changes can affect appearance (colour, shape, size), state (solid, liquid, gas), position, or properties (taste, temperature, hardness).

Types of changes

• Physical change: Only the physical form or state of a substance changes but its chemical identity remains the same (e.g., ice melting to water, tearing paper). Physical changes are often reversible.
• Chemical change (or irreversible change): New substances with different properties are formed (e.g., burning wood, cooking an egg, rusting). These are usually not easily reversed.
• Reversible vs irreversible: Reversible changes allow the original form to be recovered (freezing, melting, dissolving then evaporating). Irreversible changes do not return to the original form by simple means (burning, baking).
• Fast vs slow changes: Some changes happen quickly (a match flame), others take a long time (rusting, growth of a plant).

How to identify a change

• Observe: look for change in colour, shape, size, state or temperature.
• Test reversibility: can you get the original back by simple methods (cooling, evaporation)?
• Look for new substances: gas release (bubbles), permanent colour change, formation of solid (precipitate) often indicate chemical change.

Why this chapter matters

Understanding changes helps us explain everyday events (melting ice, cooking food, rusting bicycles, growth of plants) and decide whether a change can be reversed or prevented.

What is a change? A change is any process in which an object or substance becomes different in appearance, shape, state or composition. Changes are common in daily life — ice melting, an apple being cut, wood burning, clothes drying, etc.

Main types of changes

• Reversible changes: Changes in which the original substance can be recovered by reversing the process. These are usually physical changes that do not form new substances. Examples: melting, freezing, evaporation, condensation, and dissolving (when no chemical reaction occurs).
• Irreversible changes: Changes in which the original substance cannot be recovered by simple reversing. These usually involve chemical changes that form new substances or permanent physical changes. Examples: burning, rusting, cooking, formation of curd from milk, and ripening of fruits (chemical processes).

Physical change vs Chemical change

• Physical change: Only the physical properties (shape, size, state) change; the composition remains the same. Often reversible. E.g., ice to water, tearing paper (composition still paper).
• Chemical change: New substances with different properties are formed. Often irreversible. Signs include change in colour, evolution of gas, formation of a precipitate, change in temperature, or emission of light/sound. E.g., iron rusting, wood burning, milk turning sour.

How to tell them apart (simple tests):

• Try to recover the original substance by reversing the process (cooling, evaporating the solvent, etc.). If recovery is possible without chemical treatment → likely reversible/physical.
• Look for signs of chemical change: new colour, gas bubbles (not from boiling), change in temperature without heating, formation of a solid from two liquids (precipitate), or light/smoke. These indicate an irreversible/chemical change.

Important notes for Class 6 CBSE

• Not all physical changes are reversible in practice (cutting a sheet of paper is a physical but practically irreversible change).
• Some changes involve both physical and chemical aspects; use the criterion of whether a new substance is formed to classify.

Definition: A physical change is a change in the form, shape, size or state of a substance without altering its chemical identity. No new substance is formed.

Key characteristics:

• The chemical composition remains the same before and after the change.
• Mass is conserved (total mass of closed system remains the same).
• Often reversible (e.g., melting/freezing), but some physical changes may be practically irreversible (e.g., cutting paper).
• Energy may be absorbed or released (heat, mechanical work) but no chemical bonds are newly formed or broken in a chemical sense.

Particle-level explanation: In a physical change the arrangement, spacing or motion of particles changes. For example, when ice melts to water, particles gain freedom to move and the rigid structure of the solid breaks down into a fluid arrangement, but the H2O molecules remain the same.

Types and examples of physical changes: Changes of state (melting, freezing, evaporation, condensation, sublimation), mechanical changes (cutting, crushing, bending), mixing and dissolving (salt in water), and changes in shape or size.

How to tell a physical change from a chemical change:

• No new substance formed — test: original substance can often be recovered by simple physical methods (evaporation, filtration, magnetic separation).
• No permanent colour change, no gas evolution with new odour, no formation of a precipitate caused by new chemical reaction (although some physical changes can show temporary bubbles e.g., boiling).

Everyday importance: Physical changes are used in daily life and technology — freezing food, melting metals for casting, filtering mixtures, crystallising salt from seawater, drying clothes by evaporation.

Recovery methods (examples): Evaporation or crystallisation to recover dissolved solids, filtration for undissolved solids, decantation, magnetic separation, sieving.

Note: Some changes that look irreversible (like cutting paper) are still physical because the material's chemical identity is unchanged.

Definition: A chemical (irreversible) change is a change in which one or more new substances with different properties are formed. These changes cannot be reversed easily by simple physical means.

Key features: New substances are formed; there is usually a temperature change (energy released or absorbed), or gas, light or precipitate may appear; original substances lose their original properties.

Common signs of a chemical change: color change, evolution of gas (bubbles/odour), formation of a solid (precipitate), change in temperature (exothermic or endothermic), light or sound produced.

Examples in everyday life: burning wood, rusting of iron, cooking an egg, digestion of food, souring of milk, burning of paper, formation of curd from milk (with bacterial action).

Note on reversibility: Many chemical changes are called irreversible at the practical level because reversing them requires further chemical reactions rather than simple physical processes (for example, burning paper cannot be restored to paper). The law of conservation of mass still holds: atoms are rearranged to form new substances.

Changes are processes in which the appearance, shape, size, state or properties of an object or organism become different. In Class 6 Science, changes are understood under two broad groups: changes in living things and changes in non-living things.

Changes in living things

• Living organisms show characteristic changes like growth, repair, reproduction, movement and aging. These changes are usually continuous and often slow (for example, human growth) but sometimes rapid (for example, sprouting of seeds).
• Some living things undergo definite and predictable life-stage changes, e.g., metamorphosis in insects (egg → larva → pupa → adult) or seed → seedling → plant.
• Certain changes in living things are irreversible (aging, death), while others like wound healing are reversible (restorative).

Changes in non-living things

• Non-living things also undergo changes but these are classified as physical or chemical changes.
• Physical changes affect only the form or state of a substance and are often reversible (e.g., melting of ice, freezing of water, tearing paper is a shape change but may be irreversible for practical purposes).
• Chemical changes (chemical reactions) produce new substances and are usually irreversible under ordinary conditions (e.g., burning wood, rusting of iron, cooking food).

How to recognize types of change

• Signs of a chemical change: change in colour, formation of gas (bubbles), change in temperature without external heating, formation of a precipitate, or an irreversible change (e.g., food cooked cannot be turned back into raw ingredients).
• Physical change indicators: change in state or shape, no new substance formed, often reversible (e.g., water ↔ ice).

Important points

• Living things grow by increasing in size and/or number of cells; growth is a characteristic feature of life.
• Not all changes mean living: a rock can break into pieces (change) but it is non-living.
• Some processes involve both physical and chemical changes (e.g., ripening of fruit involves chemical changes in sugars and physical softening).

Changes around us occur when the properties or appearance of materials or living things are altered. Causes of change are the immediate actions or conditions that bring change (for example, heating, cooling, mixing, force, biological activity). Factors affecting changes are conditions that control how fast or how much a change happens (for example, temperature, time, surface area, moisture, light, concentration).

Main causes of change

• Heat (addition or removal of thermal energy): causes melting, evaporation, freezing, burning and some chemical changes. Example: heating ice causes it to melt to water.
• Mixing and dissolution: when two or more substances mix, a physical or chemical change can occur. Example: sugar dissolving in water (physical), and metal + acid producing hydrogen (chemical).
• Force and pressure: physical shape or state may change under force. Example: stretching a rubber band, crushing a can.
• Chemical reaction: substances react to form new substances (irreversible in many cases). Example: burning paper becomes ash and gases.
• Biological activity and growth: living things grow and change because of nutrition, cells dividing and biochemical processes. Example: a seed growing into a plant.
• Light and radiation: can cause fading of colours or trigger reactions (e.g., photosynthesis, photo-degradation).

Key factors that affect how changes occur

• Temperature: generally speeds up physical and chemical changes (higher temperature → faster melting, faster evaporation, faster reactions).
• Time: some changes need longer time (rusting, growth) while others are immediate (breaking a glass).
• Concentration or amount of reactants: more reactant usually increases the extent or speed of a change (e.g., faster dissolving when more solvent is present).
• Surface area: larger surface area speeds up changes like dissolving and rusting (powder dissolves faster than a lump).
• Presence of moisture or air: essential for processes such as rusting (iron + moist air → rust) and decay of food.
• Light and catalysts: light may start or speed changes (fading, photosynthesis); catalysts help some chemical changes occur faster without being consumed.

How to tell cause vs factor

Cause is the action that produces change (e.g., heating a wax candle causes it to melt). Factor is a condition that modifies the rate or extent of that change (e.g., higher room temperature makes the candle melt faster).

Summary

To understand any change, identify the cause (what action or agent started the change) and the factors (conditions) that affect how quickly or how completely the change happens.

What it means: Observation is carefully watching and noting how things change. Investigation is planning and doing experiments to find out why and how those changes happen. Together they help us understand whether a change is physical (reversible) or chemical (usually irreversible).

Steps for observation and investigation

• Ask a question (e.g., How fast does ice melt at room temperature?).
• Make a hypothesis (a possible answer).
• Plan a controlled experiment: identify variables — independent (what you change), dependent (what you measure), and controlled (what you keep same).
• Observe carefully and record data (qualitative notes and quantitative measurements).
• Repeat the experiment to check results and draw conclusions.

Types of changes and how to recognise them

• Physical changes: change in shape, size, state or appearance (cutting, bending, melting, dissolving). Usually no new substance is formed and change is often reversible. Observation: mass/identity remains same if system is closed.
• Chemical changes: new substance(s) form (burning, rusting, cooking, digestion). Indicators: colour change, gas evolved (bubbles, smell), temperature change (heat released/absorbed), formation of precipitate, and generally not easily reversible.

How to record observations – use clear tables, note time, temperature, mass, appearance, and any smells or sounds. Use drawings or photos for visual change. Quantitative data (numbers) lets you plot graphs to see trends; qualitative notes (words) help identify type of change.

Important points for Class 6 – Always control one variable at a time, repeat trials, compare before-and-after states, and use simple instruments (thermometer, balance, stopwatch) for reliable measurements. Consider whether any gas/ash escaped (mass may change if system is not closed).

Overview
Separation techniques are physical methods used to separate the different components of a mixture without changing their chemical identities. Reversibility refers to whether a change can be undone to recover the original substance(s). Most separation methods deal with physical (reversible) changes; chemical (irreversible) changes form new substances and usually cannot be undone easily.

Common Separation Techniques (what they separate, how they work, and typical uses)

• Handpicking — Picking visible different items (pebbles from rice). Best for large, distinct components.
• Sieving — Passing mixture through a mesh to separate particles by size (flour from husk). Used for grains, sand screening.
• Winnowing — Using wind or air flow to blow away lighter particles (chaff from grain). Used in agriculture.
• Sedimentation and Decantation — Letting heavier particles settle, then pouring off the liquid (muddy water → let settle and decant). Used in water treatment.
• Filtration — Passing a mixture through filter paper/cloth to separate solids from liquids (tea leaves from brewed tea). Good for insoluble solids in liquids.
• Evaporation — Evaporating solvent to recover dissolved solids (salt from seawater). Useful for soluble solids in liquids.
• Crystallization — Concentrating a solution and allowing crystals of the solute to form (pure salt or sugar crystals). Produces purified solids.
• Magnetic separation — Using magnets to pull out magnetic materials (iron filings from sand).
• Centrifugation (brief mention) — Spinning mixtures to force denser components outward to separate fine suspensions (used in labs and washing machines).

Reversibility of Changes

• Reversible changes — Physical changes where original material(s) can be recovered by simple physical means. Examples: melting/freezing (ice ↔ water), dissolving and then evaporating solvent (sugar in water → sugar recovered by evaporation), stretching a rubber band (if it returns to original shape).
• Irreversible changes — Chemical changes where new substances form and original materials cannot be recovered by simple physical means. Examples: burning paper, cooking an egg, rusting iron, baking a cake.
• Important point — Some processes that look irreversible may be reversed by complex chemical or industrial methods, but for classroom purposes irreversible changes are those where the original substances cannot be obtained back by simple means.

How separation techniques relate to reversibility
Separation methods operate on mixtures (physical combinations) so the components are not chemically changed; therefore the processes used to separate them are usually reversible in principle (for example, a sugar solution can be separated into water and sugar by evaporation). In contrast, when a chemical change occurs (new substance formed), separation techniques cannot restore the original substances.

Tips for choosing a technique
Decide based on physical properties: particle size (sieve), density (sedimentation/centrifuge), solubility (filtration + evaporation/crystallization), magnetic properties (magnet), and volatility (distillation/evaporation).

Summary
Understand the mixture type and properties of components to select the proper separation method. Recognize whether a change is physical (often reversible) or chemical (usually irreversible).

Introduction
Changes around us are of two main types: physical (no new substance formed) and chemical (new substance formed). Both types have useful applications in daily life and can also cause harmful effects. Knowing precautions helps us use the useful changes safely and avoid or reduce the harmful ones.

Applications

• Cooking and food preparation: Chemical changes (e.g., baking, roasting) make food edible and tasty; physical changes (e.g., melting butter) help in mixing and texture.
• Medicine and hygiene: Making medicines often involves chemical reactions; antiseptics and soaps use chemical/physical changes to remove germs.
• Industry and construction: Making cement, glass, alloys, and polymers involves controlled chemical and physical changes to give desired properties.
• Preservation and fermentation: Fermentation (a chemical change) produces bread, curd, and vinegar; controlled spoilage gives useful products.
• Energy and fuels: Combustion (a chemical change) of fuels gives heat and light used in homes, vehicles, and industries.

Effects (Useful and Harmful)

• Useful effects: Food becomes edible (cooked), metals are made into alloys for strength, medicines fight disease, fermentation gives bread and yogurt.
• Harmful effects: Rust (corrosion) damages structures and machines; burning fossil fuels produces smoke and gases causing air pollution; food spoilage causes wastage and illness.

Precautions

• Prevent rust and corrosion: Keep metal dry, paint or oil metal surfaces, use galvanization or coatings.
• Safe cooking and heating: Use proper ventilation to avoid inhaling smoke; never leave cooking unattended; keep flammables away from heat.
• Food safety: Store perishable food in refrigerators, cover food to prevent contamination, follow expiry dates.
• Handling chemicals and cleaning agents: Read labels, do not mix household chemicals (e.g., bleach and ammonia), use gloves and protective gear if needed.
• Reduce pollution: Use cleaner fuels, avoid open burning of waste, plant trees to improve air quality.
• Education and regular maintenance: Teach children safe practices (no playing with fire or chemicals); regularly check and repair appliances and structures.

Summary
Understanding which changes are useful and which are harmful allows us to make good use of physical and chemical changes while taking simple precautions to protect health, property, and the environment.