Chapter 6 — Changes Around Us

Meaning of change: A change happens when something becomes different in its appearance, form, size, state or composition. Changes can be tiny and slow — like colour change on a metal surface over months — or quick and obvious — like a match lighting up in seconds. Observing such changes carefully helps us understand the world.

How scientists look at change: We observe what an object or substance is like before, during and after the action that causes change. We note colour, shape, hardness, smell, temperature and whether gases or solids appear. Good observation helps us decide whether the change is physical (no new substance) or chemical (new substances form).

Examples all around: Ice melting on a sunny day, dough rising while baking, fruit turning ripe and then soft, iron turning brown in rainy weather, and milk becoming curd when left out are everyday changes. Some are useful — like cooking food — and some are harmful — like food spoilage or rusting of tools. Knowing which changes are likely to be reversed and which are permanent helps in daily tasks and safety planning.

Simple classroom activities: Try placing an ice cube on a plate and watching it melt, or dissolve a spoon of sugar in water and then evaporate the water to recover the sugar. Record what you see and how long it takes. Such activities teach students to compare and classify changes and think about why they happened.

Why it matters: Understanding changes prepares students to make safe choices at home and in the lab, to care for materials, and to learn higher science later. It builds curiosity, observational skill and the habit of recording notes carefully to support conclusions.

Definition and idea: A reversible change is one that can be undone and the original substance recovered by simple physical means. An irreversible change cannot be returned to the original state by simple physical methods because new substances are formed. This distinction is important in daily life: we reuse materials when changes are reversible and dispose or treat them differently when changes are irreversible.

Typical reversible changes: Changes of state like melting and freezing, and dissolution where a substance dissolves in a solvent, are usually reversible. For example, salt dissolved in water can be recovered by evaporating the water. Freezing water to make ice and then melting it back to water is another reversible pair.

Typical irreversible changes: Burning, rusting, and cooking are commonly irreversible. When wood burns it forms ash, smoke and gases — these are new substances and the original wood cannot be rebuilt by simple means. Cooking an egg causes proteins to change structure chemically; you cannot return the egg to its raw state.

How to test in the classroom: Perform small, safe experiments. Dissolve salt in water, then evaporate to see salt crystals reappear — reversible. Burn a small paper scrap under supervision and note ash and gases — irreversible. Record clues such as formation of gas, temperature change, colour change and whether the original can be recovered.

Why this knowledge helps: Knowing reversibility helps in recycling, food preservation and choosing separation methods. It guides us in handling materials: reversible changes may allow recovery and reuse, while irreversible changes sometimes produce waste or need special disposal.

What is melting? Melting is the change of a substance from solid to liquid caused by heating. The particles of the solid gain energy, move more freely and break their fixed positions to become a liquid. Every pure substance melts at a definite temperature called its melting point. For example, pure ice melts at 0°C under normal atmospheric pressure.

What is freezing? Freezing (solidification) is the reverse process where a liquid turns into a solid when it loses heat. In freezing, particles slow down and lock into fixed positions forming a solid crystalline structure. Water freezes at 0°C under normal conditions. Freezing is the method used to make ice and preserve food because cold temperatures slow down chemical changes.

Observation during change: When a solid melts, its shape becomes undefined and it takes the shape of its container while keeping nearly the same volume. During melting and freezing the temperature of the substance often remains constant at the melting/freezing point until the entire substance has changed its state. This is because heat energy is used to change the state rather than increase temperature at that point.

Effects of impurities and pressure: Adding impurities usually lowers the melting point; for example, adding salt to ice lowers its melting temperature. Pressure can also affect melting points: for most solids increasing pressure raises the melting point, but for water ice the relation is slightly different. These facts are used in practical applications like making ice cream quickly by mixing salt and ice.

Practical examples and safety: Butter melts on heat, chocolate softens in warm hands, and water is frozen to make ice cubes in refrigerators. When experimenting with heating or cooling, always follow safety instructions: use tongs, avoid direct flame on plastics, and supervise children when hot liquids are involved.

Evaporation explained: Evaporation is the slow change of a liquid into vapour that happens at the surface of the liquid and at temperatures below the boiling point. Molecules at the surface sometimes have higher energy and escape into the air as vapour. Factors that increase evaporation include higher temperature, bigger surface area, low humidity and moving air which carries away vapour.

Boiling versus evaporation: Boiling is rapid vaporisation that occurs at the boiling point and happens throughout the liquid (not just the surface). In contrast, evaporation is slower and only at the surface. Boiling produces bubbles from inside the liquid, while evaporation does not.

Condensation: Condensation is the reverse of evaporation. It is the change of vapour into liquid when the vapour cools or comes into contact with a colder surface. Examples are droplets on the outside of a cold drink glass and dew on grass in the morning. Clouds and fog form when tiny droplets condense from water vapour in air.

Everyday importance: Evaporation keeps us cool by sweat; sweating removes heat from the body. Drying clothes and puddles drying after rain are due to evaporation. Condensation is important in the water cycle and in making distilled water: steam is condensed to get pure water. In industry and daily life, controlling evaporation and condensation helps preserve food, cool machines and collect water in arid areas.

Simple experiments: Put equal amounts of water in two shallow dishes; place one in sunlight and one in shade. The sunny dish will lose water faster. Boil water and hold a cold metal plate above the steam to see drops forming from condensation — a clear demonstration of both processes.

What is sublimation? Sublimation is the direct change of a solid into a gas without passing through the liquid stage. Not all solids sublimate; only those that can produce vapour at room temperature or under reduced pressure. Common examples include camphor and naphthalene balls, which slowly disappear as they give off vapour over time.

What is deposition? Deposition is the reverse process where a gas changes directly into a solid without becoming a liquid first. Frost formation is a good example: water vapour in cold air turns directly into ice crystals on surfaces such as windows or leaves, forming white frost rather than droplets of water first.

Why sublimation happens: Sublimation takes place when particles in a solid gain enough energy to escape into the gas phase rather than forming a liquid. It is common for substances with relatively high vapour pressure at ordinary temperatures or when the pressure above the solid is low. Sublimation is also easier in warm, dry conditions that favor evaporation of vapour away from the solid surface.

Uses and safety: Sublimation is used to purify solids in laboratories by heating a solid to sublimate the pure substance which then condenses on a cooler surface as purified crystals. In households, mothballs (naphthalene) sublimate and release vapour that drives away insects, but breathing these vapours repeatedly can be harmful so use in ventilated areas. Iodine sublimation is used to show purple fumes in a demonstration and then deposit as crystals on a cold surface.

Simple classroom activity: Place a small amount of camphor in a petri dish with a cover. Over days it will shrink as it sublimates. Observe frost forming overnight on a cold surface for deposition. Discuss why the solid vanished without producing a puddle and why tiny crystals formed directly from gas.

What is dissolving? Dissolving is the process where a solute (such as salt or sugar) mixes uniformly with a solvent (such as water) to form a solution. The particles of the solute become surrounded by solvent molecules and spread out so they cannot be easily seen or separated by simple filtering. Solutions are homogeneous mixtures with the same composition throughout.

Types of solutions: Solutions may be solid in liquid (salt in water), gas in liquid (carbon dioxide in soda), liquid in liquid (alcohol in water), or solid in solid (brass is a mixture of copper and zinc). The solvent is usually the substance present in larger amount.

Solubility and saturation: Solubility describes how much solute can dissolve in a given amount of solvent at a specific temperature. A saturated solution contains the maximum amount of dissolved solute; any extra will remain undissolved. Solubility often increases with temperature for solids, while the solubility of gases in liquids usually decreases as temperature rises.

Factors affecting dissolving: Stirring, increasing temperature and reducing particle size (powdering) help a solute dissolve faster. Stirring brings fresh solvent into contact with the solute; powdered solids dissolve faster due to larger surface area. For gases, higher pressure increases solubility in liquids.

Practical uses and recovery: Dissolving is used in cooking, medicine and cleaning. To recover a dissolved solid, we can evaporate the solvent (e.g., evaporating water to get salt crystals). Distillation separates liquids by boiling and condensing vapour; filtration separates insoluble substances. Understanding solutions helps in daily tasks like making drinks and in industries that require mixing or purification.

What are mixtures? A mixture is a combination of two or more substances where each substance keeps its own properties and there is no chemical bonding between them. Mixtures can be homogeneous (uniform like saltwater) or heterogeneous (visible different parts like sand and pebbles). Since components are not chemically joined, we can often separate them by physical methods.

Common methods of separation: Filtration separates insoluble solids from liquids using filter paper or cloth — for example, removing sand from water. Evaporation recovers a dissolved solid by heating the solution until the solvent disappears, leaving crystals behind, as in obtaining salt from seawater. Distillation separates liquids with different boiling points by heating and condensing vapour; it is used to purify water or separate alcohol from a mixture.

Other useful methods: Magnetic separation takes advantage of magnetism to remove iron filings from sand. Decantation carefully pours off a liquid from a settled solid or an immiscible liquid. Sieving separates particles by size. Centrifugation forces solids to settle quickly by spinning. Paper chromatography separates coloured substances in inks by allowing a solvent to carry pigments up filter paper at different speeds.

Choosing the method: To select the right separation technique look at the properties of components: solubility, particle size, boiling points or magnetic behaviour. For example, to separate a mixture of iron filings, sand and salt: first use a magnet to remove iron, then add water to dissolve salt and filter to get sand, and finally evaporate filtrate to recover salt.

Classroom activity idea: Give students a mixed sample of rice, sand and iron filings and ask them to plan stepwise separation using sieving, magnet and water dissolution. This fosters logical thinking and understanding of material properties.

What is combustion? Combustion, or burning, is a chemical reaction in which a substance reacts with oxygen to release heat, and often light. In rapid combustion we see flames; slower forms of oxidation, like rusting, do not show flames but are still chemical changes where oxygen combines with a material. Combustion is an important source of heat in daily life but must be controlled for safety.

Requirements for burning (fire triangle): Burning needs three things: fuel (the material that burns), oxygen (from air) and heat (to start the reaction). This is called the fire triangle. Removing any one of these stops burning — for instance, covering a small flame with a lid cuts off oxygen and the flame goes out.

Products and types: Complete combustion of clean fuels produces carbon dioxide and water vapour and releases maximum heat. Incomplete combustion, when oxygen is limited, produces carbon monoxide (a poisonous gas), soot and less heat. Solid residues like ash are formed when some materials burn. Always notice the smells, smoke and residues — they help identify what happened.

Everyday uses and hazards: Combustion is used for cooking on stoves, warming homes, and in engines. Hazards include burns, fires that spread and inhalation of toxic gases. Keep flammable materials away from open flames, use stoves with care, and ensure good ventilation while using fuels indoors.

Classroom demonstration and safety: Under teacher supervision, light a candle and observe the flame, smoke and melted wax. Note that melting wax is a physical change while burning wax is chemical. Teach basic precautions: use tongs and stands, keep water or sand ready to extinguish small fires, and never inhale smoke intentionally. Learn simple first aid for burns such as cooling the area with running water and seeking adult help for serious cases.

What is rusting? Rusting is a chemical reaction in which iron reacts with oxygen and water to form iron oxide, a reddish-brown substance called rust. Rusting weakens the metal, spoils its appearance and may cause machines or structures to fail if left unchecked. Corrosion is a broader term for chemical deterioration of metals and other materials by reactions with their environment.

Conditions that speed rusting: Moisture and oxygen are the main causes. Salt or acidic conditions make rusting faster because they allow ions to move more freely and reactions to occur more rapidly. For example, objects near the sea rust faster due to salty air and moisture.

How to prevent rusting: Protect iron from water and oxygen by painting, oiling, greasing or galvanising (coating with zinc). Stainless steel contains chromium which forms a protective layer and resists corrosion. Keeping metal dry, avoiding contact with salt, and regular maintenance increase the life of tools and structures.

Simple experiment: Place three identical iron nails in three jars: one in dry air, one in clean water and one in salt water. Observe over days and weeks; the nail in salt water will rust fastest, showing the effect of dissolved salts. Record changes in colour and surface texture to support conclusions.

Real life importance and care: Rusting increases repair costs for vehicles, bridges and household items. Learning to prevent rust by suitable coatings and storage helps save money and improves safety. In industry, engineers select materials and protective methods to reduce corrosion in buildings, ships and machinery.

Defining the two types: A physical change alters the form or appearance of a substance but not its chemical identity. Examples include cutting, bending, breaking and change of state like melting. A chemical change produces one or more new substances with different properties; examples include burning, rusting and cooking where new materials form.

Signs of chemical change: Look for clues: a colour change, formation of gas (bubbles), formation of a solid from two liquids (precipitate), change in temperature without external heating, or production of a new smell. If the original substance cannot be recovered by simple means, the change is likely chemical.

Signs of physical change: Physical changes often show only a change in shape, size or state. No new substance appears and the change can usually be reversed by simple actions such as cooling, evaporating or reshaping. For example, frozen water becomes ice and then melts back to water — same substance throughout.

Class tests and examples: Mix vinegar and baking soda to see bubbling and gas release — chemical change. Dissolve sugar in water and then recover sugar by evaporation — physical change. Burn a wood stick; ash and smoke show a chemical change. Breaking glass shows a physical change as no new substance forms yet shape changes.

Using observations to decide: Use more than one clue because not every chemical change shows all signs. Record temperature, colour and if any solid or gas appears. Discuss whether the change can be reversed and what methods would be needed to try. This habit trains careful observation and reasoning in young scientists.

Heat and materials: Heat often changes the state and properties of materials. It can melt solids to liquids (like ice melting), evaporate liquids to gases (boiling water), soften plastics and expand metals. Heating can also speed up chemical reactions, cause decomposition or change colour. For example, clay hardens and changes colour when fired in a kiln, and metals may become soft and bendable at high temperatures.

Heat and living things: In living organisms, heat affects proteins and cells. Moderate heat can kill harmful microbes and make food safe to eat by changing its chemical structure (cooking). But excessive heat can damage tissues — burns in humans and wilting in plants. Freezing damages cells in some cases by forming ice crystals that break cell membranes.

Practical household applications: Cooking, boiling water to sterilise, drying clothes by evaporation, ironing clothes to remove wrinkles (heat softens fibres), and making glass or metal objects by melting are everyday uses of heat. Refrigeration slows down heat-driven chemical changes to preserve food, while heating speeds processes when needed.

Safety and prevention: Safe use of heat requires precautions: use mitts for hot vessels, keep flammable items away from open flames, provide ventilation to avoid buildup of dangerous gases like carbon monoxide, and follow instructions when using chemicals that react with heat. In case of burns, cool with running water and seek help for serious injuries.

Energy and environment: Using heat efficiently saves fuel and reduces pollution. Covering a pot while cooking reduces heat loss and saves energy, and insulating houses keeps heat in during winter. Understanding how heat affects materials and living things guides safe and economical choices at home and in schools.

Food spoilage: Food spoils when microorganisms (bacteria, fungi) grow on it or when chemical processes change its taste, smell, colour or texture. Spoilage makes food unsafe to eat. Warm, moist conditions and exposure to air speed up spoilage because microbes grow well and chemical reactions occur faster. For example, milk left in room temperature sours due to bacteria producing acids.

Methods of preservation: Cooling and freezing slow down microbial growth and chemical reactions, extending food life. Drying removes water so microbes cannot multiply. Salting and sugaring draw water out of food and create environments unfriendly to microbes. Pickling uses acid (vinegar) and often salt to preserve vegetables. Canning uses heat to kill microbes and seals the food away from new contamination.

Practical advice: Store perishables in refrigerators, keep dry foods in airtight containers, and follow expiry dates. Avoid tasting food that smells bad or looks mouldy. When preserving, use clean utensils and follow safe recipes to avoid poisoning. Vacuum packing and preservatives are modern methods used in shops and factories.

Recording observations in experiments: When studying food changes, record what you do and what you observe in a table: 'What was done', 'Before', 'During', 'After', 'Time taken' and 'Conclusion'. For example, place apple slices in different conditions (exposed to air, in lemon juice, wrapped) and note colour changes over time. Use repeated trials and careful notes to make reliable conclusions.

Class activity and learning: Observe bread or fruit in different conditions to learn how temperature and cover affect spoilage. Discuss why one sample spoils faster and how preservation methods worked. Learning to record observations and draw conclusions helps students think like scientists and apply knowledge to keep food safe and reduce waste.