Physical and Chemical Changes — Class 7 Science
This chapter explains how matter changes in our daily life. Some changes only alter appearance or state; others form entirely new substances. Understanding these changes helps you in chemistry, environment studies, and everyday decisions — from cooking and cleaning to preserving materials. Below you will first find clear and exam-friendly notes, followed by a full set of questions & answers including 7 MCQs (with choices), 7 very short, 7 short and 7 long answer questions — perfect for NCERT exam preparation and school tests.
Notes: Physical and Chemical Changes
Definition — Physical Change: A physical change alters the form, appearance or state of a substance but does not produce a new substance. Common physical changes include changes of state (melting, freezing, evaporation, condensation), cutting, breaking, bending and dissolving.
Definition — Chemical Change: A chemical change (chemical reaction) produces one or more new substances with properties different from the original ones. Chemical changes often show signs such as color change, gas release, precipitate formation, temperature change, or emission of light or odor.
Key Differences (Simple)
- New substance: Chemical change → yes. Physical change → no.
- Reversibility: Physical changes are usually reversible (e.g., water → ice → water). Chemical changes are often hard to reverse without another chemical reaction (e.g., rusting).
- Energy change: Chemical reactions commonly involve noticeable energy change (heat released or absorbed); physical changes may involve small energy changes (like melting) but no new chemical energy.
- Examples: Physical — melting ice, dissolving salt; Chemical — burning wood, digestion, rusting.
Indicators of a Chemical Change
- Color change (when no dye was added) — e.g., iron rusting.
- Gas evolution — bubbles that appear due to a chemical reaction.
- Precipitate formation — a solid forming when two solutions mix.
- Temperature change — reaction becomes warmer (exothermic) or cooler (endothermic).
- Change in smell or light emission — sometimes a new odor or light appears.
Types of Simple Chemical Reactions (Class 7 level)
- Combination (synthesis): Two or more substances combine to form one (A + B → AB).
- Decomposition: A substance breaks into simpler substances (AB → A + B).
- Displacement: A more reactive element displaces a less reactive element from its compound.
- Precipitation: Two solutions react to form an insoluble solid (precipitate).
Conservation of Mass
Antoine Lavoisier’s principle — mass is conserved in a chemical reaction. In a closed system, the total mass before a reaction equals the total mass after. This is a fundamental idea: atoms are rearranged, but none are lost in ordinary chemical reactions.
Energy Changes: Exothermic and Endothermic
- Exothermic: Releases heat to surroundings (e.g., many combustion reactions, some oxidation processes).
- Endothermic: Absorbs heat from surroundings (e.g., photosynthesis, some decomposition reactions).
Everyday Examples and Why They Matter
- Cooking: Baking an egg or cake involves chemical change — new substances are formed and the change is mostly irreversible.
- Rust & Corrosion: Iron exposed to moisture and oxygen forms rust (iron oxide) — a chemical change that weakens metals.
- Cleaning: Dissolving soap in water is physical; chemical reactions in detergents remove grease by chemical action.
- Preservation: Pickling, drying, refrigeration slow or prevent chemical changes that spoil food.
Safe Classroom Demonstrations (Teacher-supervised / simple)
- Melting ice in a beaker — a physical change (state change).
- Dissolving salt or sugar in water — physical change; can be recovered by evaporation.
- Mixing baking soda and vinegar in an open container — produces bubbles (CO₂) and shows gas evolution — indicator of chemical change (use safe quantities and adult supervision).
Important note for safety: Avoid open flames in classroom notes if not supervised; do not perform hazardous chemical experiments at home. Focus on observation and safe demonstrations.
How to Distinguish Quickly (Exam Tip)
- If a new substance appears (smell, color, precipitate, gas) → likely chemical.
- If only shape, size or state changes (and the material’s composition stays same) → physical.
- Ask: can I get the original back by simple physical means (evaporation, freezing, filtering)? If yes → physical; if no → likely chemical.
Questions & Answers
7 MCQs (choose the correct option)
1. Melting of ice is an example of a:
(a) chemical change (b) physical change (c) both (d) no change
Answer: (b) physical change — only the state changes, no new substance is formed.
2. Which of the following is NOT an indicator of a chemical change?
(a) formation of gas (b) change in shape (c) color change (d) temperature change
Answer: (b) change in shape — shape change alone is usually physical.
3. Rusting of iron produces:
(a) iron oxide (b) iron chloride (c) iron carbonate (d) iron sulfate
Answer: (a) iron oxide — a new substance formed by chemical reaction with oxygen.
4. Which reaction is exothermic?
(a) photosynthesis (b) melting ice (c) combustion of petrol (d) evaporating water
Answer: (c) combustion of petrol — it releases heat (energy).
5. Dissolving sugar in water is:
(a) chemical change (b) physical change (c) both (d) irreversible chemical change
Answer: (b) physical change — sugar molecules disperse but remain chemically the same.
6. The law that mass is neither created nor destroyed in a chemical reaction is given by:
(a) Newton (b) Lavoisier (c) Boyle (d) Mendeleev
Answer: (b) Lavoisier — conservation of mass.
7. Formation of a precipitate indicates:
(a) physical change (b) chemical change (c) change of state (d) nothing
Answer: (b) chemical change — a new insoluble solid is formed.
7 Very Short Answer Questions
Q1: What is meant by a physical change?
A: A change in which only appearance or state changes but no new substance forms — e.g., ice melting.
Q2: Give one example of an endothermic process.
A: Photosynthesis is endothermic — it absorbs energy (sunlight).
Q3: What is a precipitate?
A: A solid that forms when two solutions react chemically and the solid separates out.
Q4: Mention one sign of chemical change.
A: Evolution of gas or formation of bubbles without boiling.
Q5: Can dissolving be reversed?
A: Yes — many dissolutions (e.g., salt in water) can be reversed by evaporation.
Q6: What happens during rusting?
A: Iron reacts with oxygen and moisture to form iron oxide (rust), a chemical change.
Q7: Is tearing a paper physical or chemical?
A: Physical — the paper’s composition remains the same; only its shape changes.
7 Short Answer Questions
Q1: Explain with one example how you can tell a physical change has occurred.
A: If a substance changes its state (solid to liquid) without changing its chemical identity, it is physical. For example, water turning to steam on heating is a physical change — water molecules remain H₂O.
Q2: Why is burning of wood considered a chemical change?
A: Burning changes wood into different substances such as ash, smoke and gases. Light and heat are produced; the composition is completely altered — therefore it is a chemical change.
Q3: How does the law of conservation of mass help chemists?
A: It tells chemists that mass is conserved during reactions, so they can balance chemical equations and predict how much product will be formed from given reactants.
Q4: Describe an instance where a chemical change is useful in daily life.
A: Cooking food is a chemical change; proteins denature and new flavors form, making food edible and nutritious. This irreversible process is essential for our diet.
Q5: What is difference between evaporation and boiling?
A: Evaporation is a slow process that occurs at the surface of a liquid at any temperature. Boiling occurs throughout the liquid at its boiling point and forms bubbles.
Q6: Give an example where a physical change is often mistaken for chemical change.
A: When milk sours it may curdle and smell different — this involves chemical change (fermentation) but people sometimes think it’s just physical because it looks like clumping.
Q7: Can a chemical change be reversed? Give an example if yes.
A: Some chemical changes can be reversed by further chemical reactions (e.g., formation of ammonia and its breakdown), but many are difficult to reverse by simple physical means (e.g., burning).
7 Long Answer Questions
Q1: Discuss in detail the differences between physical and chemical changes with examples and indicators.
A: Physical changes only alter appearance, size or state without producing new substances; e.g., cutting paper, melting ice, dissolving sugar in water. These changes are usually reversible. Chemical changes produce new substances with different properties; indicators include gas evolution, color change, temperature change, precipitate formation or smell. Examples: rusting of iron (iron oxide forms), burning of wood (ash, smoke), baking a cake (new substances and irreversible change). Energy changes are common in chemical reactions — exothermic or endothermic — whereas physical changes involve less dramatic energy changes tied to state changes. The distinction is important in industry, cooking and conservation of materials.
Q2: Explain the conservation of mass and give a simple classroom-friendly example to illustrate it (observational description only).
A: Conservation of mass states mass is neither created nor destroyed in ordinary chemical reactions. In classroom observation, if a reaction occurs in a closed container, the mass before and after remains the same, because no material escapes. For example, mixing two solutions in a sealed bottle and observing no change in total mass illustrates the principle. This supports the idea that atoms are only rearranged to form new substances.
Q3: Describe exothermic and endothermic reactions, and give two examples of each. Explain how students might notice the energy change.
A: Exothermic reactions release energy to the surroundings, often as heat or light; examples include some combustion reactions and many oxidation reactions (like rusting releases small energy). Students notice exothermic reactions because the container or surroundings become warmer. Endothermic reactions absorb energy; examples include photosynthesis and the dissolving of some salts that cool the solution. In the classroom, temperature change (measured by a thermometer or felt by touch with teacher supervision) indicates energy flow. Observing energy changes helps classify reactions and understand their practical uses.
Q4: How are chemical reactions important in industry and everyday life? Give specific cases and their benefits.
A: Chemical reactions are central in manufacturing medicines, fertilizers, plastics, and fuels. For example, reactions to produce ammonia are critical for fertilizer production, helping food supply. Photosynthesis (chemical process) supports life by producing oxygen and food in plants. In everyday life, fermentation (chemical change) makes bread and yogurt, while controlled combustion provides energy for heating and transport. Understanding reactions allows for safer storage, better materials and innovations like water purification.
Q5: Explain with examples how you would identify that a precipitate has formed and why this is important in chemistry.
A: When two clear solutions mix and a cloudy solid separates, that solid is a precipitate. For example, mixing solutions of silver nitrate and sodium chloride produces white silver chloride solid. Precipitation helps chemists separate substances, test for ions and purify products. Precipitates are often filtered off and analyzed to determine what elements or compounds are present.
Q6: Some changes are slow and others are fast. Discuss factors that affect the rate of chemical change and give examples.
A: Rate of chemical change depends on temperature (higher speeds up reactions), concentration of reactants (more frequent collisions), surface area (powdered substances react faster), presence of catalysts (substances that speed up reactions without being consumed) and pressure for gases. For example, powdered sugar dissolves and reacts faster than sugar cubes; heating speeds up cooking reactions. These factors are used in industry to control production and in the lab to optimize experiments.
Q7: Describe an everyday situation where both physical and chemical changes happen together. Explain the sequence of changes.
A: Cooking food often shows both. When you boil vegetables, physical change occurs (softening and change of state of water), while chemical changes happen too (starch breaks down, proteins denature, flavor compounds form). The sequence: heat (physical) alters texture, then chemical reactions change taste and nutritional properties; some chemical changes are desirable (flavor), while others (overcooking) can reduce nutrients. Understanding this helps in cooking methods to balance taste and nutrition.
Study tip: revise the indicators and examples — they are often asked in exams. Use the MCQs for quick practice and long answers to prepare your written answers.
0 Comments