Chapter 6 — Changes Around Us

Change means any alteration in the appearance, state, composition or position of a substance or object. In our surroundings many changes happen all the time. Broadly, changes are classified into two types:

• Physical changes: These change only the form or state of a substance but not its chemical identity. Examples: melting, freezing, evaporation, condensation, dissolving, cutting and breaking. Many physical changes are reversible.
• Chemical changes: These produce one or more new substances with properties different from the original ones. Signs of chemical change include colour change, formation of gas (bubbles), formation of a solid (precipitate), change in temperature (heat produced or absorbed) and irreversibility in usual conditions. Examples: rusting, burning, cooking, digestion.

How to tell which change has occurred: look for changes in colour, temperature, smell, formation of gas or solid, and whether the original material can be recovered by simple physical methods. Factors that cause change include heat, mixing, mechanical force, and biological processes (growth, ripening).

At a simple level we also talk about change with time — for example temperature of water rising while heating, or amount of water decreasing while evaporating. Representing such changes on a graph helps to understand the process clearly.

What is a change? A change is any process in which a substance or object alters its appearance, shape, state or composition. Changes around us are of two main kinds: physical (reversible) changes and chemical (usually irreversible) changes.

1. Physical changes (often reversible)

• Definition: Changes in which no new substance is formed. Only physical properties such as shape, size or state change.
• Typical features: change of state (solid <-> liquid <-> gas), change of shape (stretching, bending), mixing/dissolving (when original substance can be recovered).
• Examples: melting of ice, freezing of water, evaporation, condensation, dissolving sugar in water, cutting paper.
• Reversibility: many physical changes are reversible by simple physical processes (e.g., evaporation, cooling, filtration).

2. Chemical changes (usually irreversible)

• Definition: Changes in which one or more new substances are formed with properties different from the original.
• Typical features: change in colour, evolution of gas, formation of a precipitate, temperature change (heat produced or absorbed), emission of light or sound.
• Examples: burning of paper, rusting of iron, cooking food, ripening of fruit (biochemical), digestion.
• Reversibility: generally irreversible by simple physical means because composition has changed; reversing requires chemical reactions.

How to tell them apart (simple tests):

• If you can get back the original substance by heating, cooling, evaporating or filtering, it is likely a physical (reversible) change.
• If a new substance appears, often accompanied by gas, colour change, temperature change or precipitate, it is likely a chemical (irreversible) change.

Summary classification (simple):

• Physical change = no new substance formed; properties may change but composition stays same; often reversible.
• Chemical change = new substance(s) formed; different properties; usually irreversible.

Important note: Some changes can be slow or fast, and some processes may combine physical and chemical aspects (for example, heating a mixture that later reacts chemically). Observe indicators (gas, colour, heat, precipitate) to decide which type occurred.

Definition: A reversible change is a physical change in which a substance can be brought back to its original form by reversing the conditions (for example, by cooling, evaporating, or releasing force). No new substance is formed during a reversible change.

Key features:

• No chemical bonds are broken or new ones formed — the change is physical.
• The original material can be recovered by reversing the condition that caused the change (e.g., heat, pressure, or force).
• Mass of the whole system remains conserved.

How to identify a reversible change: If you can return the product to the original substance by simply changing temperature, pressure or removing a solvent, it is reversible. Example checks: cooling a liquid to see if it solidifies back; evaporating the solvent to recover dissolved solute.

Short experiments/demonstrations:

• Melting ice in a beaker and then freezing the water again to form ice — shows melting and freezing are reversible.
• Dissolve sugar or salt in water and then evaporate the water to recover the dry sugar/salt — shows dissolving can be reversible.
• Stretch a spring or elastic band and release it — it returns to original shape (elastic deformation is reversible within limits).

Difference from irreversible change (brief): In irreversible changes (like burning paper, rusting, or cooking an egg) new substances form and the original substance cannot be recovered by simple physical means.

What are irreversible changes?
Irreversible changes are changes in which the original substance cannot be restored by simple physical means. In these changes new substances are usually formed or the original material is permanently altered.

Key features

• New substances may be formed (chemical change).
• Physical appearance and properties change permanently (shape, texture, state).
• Energy is often released or absorbed (for example, heat in burning).
• Usually not reversible by simple mechanical or physical processes (heating, cooling, filtering).

How to recognise an irreversible change

• Change in colour that cannot be reversed (rusting of iron).
• Evolution of gas (bubbling that produces a new gas).
• Change in temperature (exothermic or endothermic reaction).
• Formation of a new solid (precipitate) or permanent hardening (cement setting).

Why they are irreversible
Irreversibility often happens because chemical bonds are broken and new bonds form to make new substances. Restoring the original substance would require another chemical reaction, often with different conditions and reagents.

Simple examples to visualise the idea
When wood burns, it turns to ash and gases. You cannot get back the original wood simply by collecting the ash. When milk turns into curd, bacteria change some components of milk chemically; you cannot convert curd back into the original milk.

Connection to conservation laws
The law of conservation of mass still holds: total mass of reactants equals total mass of products. However, some products may be gases that escape, so measured mass of the remaining material can appear to decrease.

Summary
Irreversible changes are permanent changes involving the formation of new substances or permanent structural change. They contrast with reversible changes, where the original state can be restored by simple means.

Physical change: A change in which the physical properties of a substance (shape, size, state, texture) change but no new substance is formed. Physical changes are usually reversible. Examples: melting, freezing, cutting, dissolving (if no chemical reaction).

Characteristics of physical changes

• No new substance is produced.
• Mass remains the same (in a closed system).
• Often reversible (e.g., freezing ⇄ melting).
• Only physical properties such as shape, size, state or appearance change.

Chemical change: A change in which one or more new substances with different properties are formed. Chemical changes are usually not easily reversible. Examples: burning, rusting, digestion, fermentation.

Characteristics and indicators of chemical changes

• New substances formed with different properties (colour, smell, taste, texture).
• May be accompanied by emission or absorption of energy (heat, light).
• May produce gas (bubbles), precipitate (solid from two solutions), change in colour, or change in temperature.
• Mass is conserved overall (law of conservation of mass) in a closed system, though measurable mass may change in an open system (e.g., gas escapes).

How to distinguish: Look for formation of a new substance, permanent colour change, formation of gas or precipitate, and energy change. If only state, shape or size changes and the original substance can be recovered, it is a physical change.

Simple classroom experiments: melting ice and refreezing (physical); mixing vinegar and baking soda to produce gas (chemical); leaving iron in water/air to get rust (chemical); dissolving salt in water and evaporating to recover salt (physical).

Summary: Physical changes alter form; chemical changes alter identity. Conservation of mass applies to chemical reactions if the system is closed.

What is a chemical change? A chemical change (chemical reaction) is a process in which one or more substances (reactants) are transformed into new substances (products) having different properties. Chemical changes usually cannot be easily reversed.

Indicators of a chemical change — signs that a chemical change has occurred. A single indicator may not prove a chemical change by itself, but two or more together give strong evidence.

• Color change: A permanent color change often shows new substances have formed (e.g., iron turning brown when it rusts).
• Gas production (bubbling/effervescence): Sudden bubble formation (not due to boiling) indicates a gas is produced (e.g., vinegar + baking soda produces CO₂).
• Formation of a precipitate: When two clear solutions mix and an insoluble solid (precipitate) appears (e.g., mixing silver nitrate and sodium chloride gives white AgCl).
• Temperature change: The reaction may release heat (exothermic) or absorb heat (endothermic) — e.g., burning wood releases heat; mixing quicklime (CaO) with water releases heat.
• Change in smell (odor): New smells can indicate new substances (e.g., rotten eggs smell when proteins decompose producing H₂S).
• Light or sound: Some reactions emit light (e.g., burning magnesium gives bright light) or sound (explosive reactions).
• Change in properties / irreversibility: New chemical properties (different reactivity, melting point, etc.) and changes that are difficult to reverse (e.g., burning paper) point to chemical change.

Important note: Some physical changes can show one of these signs (e.g., boiling produces bubbles but is physical). Use several indicators together to conclude a chemical change.

What this topic covers
Simple Experiments and Activities under 'Changes Around Us' help students observe and classify changes as physical (reversible) or chemical (often irreversible). These activities use everyday materials to show melting, dissolving, evaporation, rusting, burning, coagulation (curdling) and gas-producing reactions.

• Melting and Freezing (Physical change): Place ice cubes in a beaker at room temperature. Observe melting to water. Put water in a freezer to see it freeze again. Observation: shape and state change but composition unchanged — reversible.
• Dissolving (Physical change): Dissolve sugar/salt in water and then evaporate water to get the solute back (e.g., salt crystals). Observation: solute appears to disappear but returns on evaporation — reversible.
• Evaporation and Concentration: Leave a bowl of water in sun and measure mass decrease with time or mark water level. Observation: liquid decreases as vapour — physical change; dissolved solids remain.
• Rusting of Iron (Chemical change): Keep an iron nail in moist air (or in salt water) and another kept dry. Over days, the wet nail forms rust (brown flaky oxide). Observation: new substance (rust) forms; change is usually irreversible.
• Burning a Candle / Paper (Chemical change): Observe flame, heat, light and formation of ash and smoke. Observation: new substances (ash, gases) formed; irreversible.
• Making Curd from Milk (Chemical/biological change): Add a little curd to warm milk and keep it undisturbed. Milk becomes curd due to bacterial action — irreversible chemical change.
• Baking Soda + Vinegar (Gas-forming chemical reaction): Mix small amount of baking soda in vinegar in a bottle and watch fizzing (CO2 gas). Observation: bubbles show gas formation and a new solution forms — chemical change.
• Stretching, Cutting, Mixing Colours (Physical changes): Tearing paper, cutting clay, mixing milk colours — shape or appearance may change without new substances forming; these are physical/reversible in many cases.

How to use these activities in class: For each activity ask students to record what they see (appearance, temperature, mass, gas/colour change). Ask: Can we get back the original substance? Is heat involved? Do new substances form? These questions help classify the change.

Safety note: Do not burn materials unattended; handle hot objects carefully; use adult supervision for chemical reactions.

What are reversing changes? Some physical changes can be undone — these are called reversible changes. Examples: melting of ice (can be reversed by freezing), dissolving sugar in water (can be reversed by evaporating water). Other changes are irreversible (cannot be undone) — for example burning paper, rusting of iron, or cutting a fruit.

Why do we need recovery techniques? In everyday life and in industries we often need to separate and recover substances from mixtures (for reuse, purification, or safety). Different techniques are used depending on the nature of the mixture: particle size, solubility, magnetic properties, boiling points, etc.

Common recovery techniques

• Filtration: Separates an insoluble solid from a liquid using a porous medium (filter paper, cloth). Example: separating sand from water.
• Decantation / Sedimentation: Letting heavy solids settle (sedimentation) then pouring off the clear liquid (decantation). Example: separating mud from water after it settles.
• Sieving: Uses a mesh to separate particles of different sizes. Example: removing pebbles from flour or sifting sand.
• Magnetic separation: Uses magnets to remove magnetic materials (iron filings) from mixtures. Example: separating iron nails from a mixture of sand and nails.
• Evaporation: Evaporating the solvent to recover a dissolved solid. Example: obtaining salt from salt water by heating and evaporating the water; sugar recovery from sugarcane juice by evaporation and crystallization.
• Crystallization: Concentrating a solution then letting the solute form crystals as it cools or the solvent evaporates. Used to obtain pure solid crystals (e.g., salt or alum crystals).
• Distillation: Heating a liquid mixture to boil and condense components separately based on different boiling points. Example: obtaining pure water from seawater (simple distillation).
• Condensation: Converting vapor back to liquid (used in distillation and water recovery from steam).
• Centrifugation (basic idea): Spinning mixtures so heavier particles move outward and separate from lighter ones (used in laboratories and to separate cream from milk).

Which changes are reversible? Physical changes that do not produce new substances are usually reversible by changing conditions (temperature, pressure, or removing a solvent). Reversible examples: freezing/melting, evaporation/condensation, dissolving/evaporation.

Which changes are irreversible? Chemical changes that form new substances are usually irreversible by simple physical methods. Example: burning, rusting, digestion.

Important idea — Conservation of mass: In a closed system the total mass before and after a change remains the same. Recovery methods rearrange or separate components but the total mass of components is conserved (allowing for losses during practical processes).

Safety and practical tips: Use proper heat sources and protective gear when evaporating or distilling. Avoid open flames with flammable liquids. Choose the gentlest effective method to reduce loss of material.

Changes around us are of two main kinds: physical changes (no new substance formed) and chemical changes (a new substance forms). Both kinds have many applications and benefits in daily life, but some changes can be harmful, so safety rules are important.

Applications of physical changes

• Melting and freezing: making ice cubes, melting butter for cooking, and freezing foods to preserve them.
• Evaporation and condensation: drying clothes, using evaporation to cool (sweating), and condensation in water collection (distillation).
• Dissolving: making solutions like sugar in tea, medicines in water, and saltwater for cooking.
• Cutting, tearing and shaping: tailoring clothes, cutting paper, and shaping wood or metal (these change shape/size but not chemical identity).

Applications of chemical changes

• Cooking: raw ingredients undergo chemical changes to become edible and safer (e.g., eggs, rice).
• Combustion as energy source: burning wood, coal or gas for cooking and heating.
• Fermentation: making curd, bread and idli/dosa batter using microbes.
• Making medicines and materials: many medicines and plastics are produced by chemical reactions.

Benefits

• Preservation and safety: freezing, drying and adding salt/sugar slow down decay and keep food safe to eat.
• Comfort and convenience: heating, cooling, and materials processing (e.g., sewing, cutting) make life easier.
• Health and production: chemical changes enable medicines, fuels and useful materials.
• Resource use: recycling and controlled chemical processes recover useful materials and reduce waste.

Safety and precautions

• Avoid inhaling smoke and fumes from burning — use hoods and ventilated areas when cooking or heating substances.
• Store chemicals (cleaning agents, medicines) safely out of reach of children and follow instructions on labels.
• Prevent accidents in heat-related changes: use oven mitts, keep flammable items away from flames, turn off appliances after use.
• Prevent food-borne illness: cook food thoroughly, refrigerate quickly, and avoid using spoiled ingredients (chemical/biological changes cause spoilage).
• Prevent corrosion/rusting: keep metals dry, paint or oil metal surfaces, and store tools properly.
• First aid basics: for burns cool with running water, for cuts clean and cover the wound, and seek medical help for serious injuries.

How to decide when a change is safe or useful

• Ask if a new substance is formed (smell, color change, gas, temperature change) — that may be a chemical change.
• Use protective steps for processes that release heat, gas or fumes (gloves, goggles, ventilation).
• Use controlled methods (timers, thermometers, correct containers) to get the benefit while reducing risk.

Understanding these applications, benefits and safety precautions helps us use changes around us wisely and avoid harm.