CC11 — Physical & Chemical Changes

📖 CC11 · CDS General Science — Chemistry ★ High Priority

The ability to distinguish a physical change from a chemical change is fundamental to all of chemistry. CDS tests this concept directly — often with tricky examples like dissolving salt (physical) vs rusting of iron (chemical). Learn the indicators and master the examples.

📌 CDS Focus: Physical change = no new substance formed; reversible in most cases (melting, boiling, cutting). Chemical change = new substance formed; usually irreversible (burning, rusting, digestion, fermentation). Exothermic vs endothermic reactions. Indicators of chemical change (colour, gas evolution, precipitate, temperature change). These are direct CDS MCQ areas.

1. Physical vs Chemical Changes

Fig. 1 — Physical vs Chemical Change: Key Differences with Examples

2. Indicators of a Chemical Change

Fig. 2 — Five Signs That a Chemical Change Has Occurred

3. Exothermic vs Endothermic Reactions

🔥 Exothermic Reactions (Release Heat)

Energy released to surroundings; temperature of surroundings rises
Products have lower energy than reactants
Examples: combustion (burning wood, coal); respiration (glucose + O₂ → CO₂ + H₂O + energy); neutralisation (acid + base → salt + water + heat); rusting of iron; dissolution of NaOH in water
ΔH is negative (heat is given out)

⛴ Endothermic Reactions (Absorb Heat)

Energy absorbed from surroundings; temperature of surroundings falls
Products have higher energy than reactants
Examples: photosynthesis (CO₂ + H₂O + sunlight → glucose + O₂); dissolving ammonium chloride in water (cooling effect); thermal decomposition (CaCO₃ → CaO + CO₂); electrolysis of water
ΔH is positive (heat is absorbed)

📝 CDS PYQs — Physical & Chemical Changes

Q1. Which of the following is a chemical change? CDS PYQ

(a) Melting of wax(b) Dissolving sugar in water(c) Burning of candle wax(d) Evaporation of water

✔ Answer: (c) Burning of candle wax

Burning is a chemical change — wax (hydrocarbon) reacts with oxygen to produce CO₂ and H₂O, which are new substances. The wax cannot be recovered. Melting and evaporation are physical changes (state changes; same substance). Dissolving sugar in water is physical — sugar can be recovered by evaporation. This is one of the most tested CDS concept questions.

Q2. Rusting of iron is an example of: CDS PYQ

(a) Physical change only(b) Chemical change (slow oxidation)(c) Physical change, reversible(d) Neither physical nor chemical

✔ Answer: (b) Chemical change (slow oxidation)

Rusting is a chemical change — iron reacts with oxygen and water to form iron oxide (Fe₂O₃·nH₂O), which is a completely different substance. It is irreversible under normal conditions. Rusting is a slow oxidation reaction (corrosion). It requires BOTH oxygen AND water (moisture). This is distinguished from a physical change where no new substance forms.

Q3. Photosynthesis is an example of: CDS PYQ

(a) Exothermic reaction(b) Endothermic reaction(c) Physical change(d) Decomposition reaction

✔ Answer: (b) Endothermic reaction

Photosynthesis: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. It absorbs light energy from the sun to build glucose from CO₂ and H₂O — making it an endothermic reaction. Respiration is the reverse — it releases energy (exothermic). This pairing is a repeated CDS concept question.

Q4. Which of the following is NOT an indicator of a chemical change? ⚡ Tricky

(a) Gas evolution(b) Change in colour(c) Change in size/shape(d) Formation of precipitate

✔ Answer: (c) Change in size/shape

Change in size or shape is an indicator of a physical change (e.g., cutting iron, shaping clay). The indicators of a chemical change are: gas evolution, change in colour, formation of precipitate, change in temperature/energy, and change in odour. All involve formation of a new substance or release/absorption of energy.

🧠 Quick Memory Chart — CC11

⚛ Physical Change

No new substance formed
Usually reversible
Same chemical composition
Melting, boiling, cutting, dissolving
Sugar in water: physical (recover by evaporation)

⚛ Chemical Change

New substance(s) formed
Usually irreversible
Different chemical properties
Burning, rusting, digestion, fermentation
Cooking egg: chemical (irreversible)

🔥 Exo vs Endo

Exo: releases heat (combustion, respiration)
Endo: absorbs heat (photosynthesis)
5 indicators: colour, temp, gas, precipitate, odour
Rusting: slow oxidation (chemical)
Melting: physical (reversible)

📝 Practice Exercise

E1. Which of the following is a physical change?

(a) Burning of coal(b) Rusting of iron(c) Melting of ice(d) Fermentation of grapes

E2. Dissolution of NaOH in water is:

(a) Endothermic(b) Exothermic(c) Neither(d) Reversible physical change

E3. Which of these is a chemical change?

(a) Magnetising a steel rod(b) Dissolving common salt in water(c) Digestion of food(d) Freezing of water

E4. Respiration in organisms is:

(a) Endothermic(b) Exothermic(c) Photochemical(d) Neither

Answers:
E1 → (c) Melting of ice [state change; H₂O remains H₂O; reversible] | E2 → (b) Exothermic [NaOH dissolves with release of heat; solution becomes warm] | E3 → (c) Digestion of food [enzymes break down food into simpler molecules; new substances formed; irreversible] | E4 → (b) Exothermic [glucose + O₂ → CO₂ + H₂O + energy; ATP produced; heat released]

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