Physical and Chemical Changes
1. What Is a Change?
A CHANGE is an ALTERATION in the physical or chemical properties of a substance.
Changes are classified into:
- Physical changes
- Chemical changes
2. Physical Change
A PHYSICAL CHANGE is a change in which the SUBSTANCE remains the SAME but its PHYSICAL STATE or PROPERTIES change.
Characteristics of Physical Change
- NO new substance is formed.
- Usually REVERSIBLE.
- Only physical properties change (shape, size, state, colour in some cases).
- No change in chemical composition.
- Mass remains CONSERVED.
Examples of Physical Changes
| Example | What Changes | What Stays Same |
|---|---|---|
| Melting of ice | Solid → Liquid | Still H₂O |
| Boiling of water | Liquid → Gas | Still H₂O |
| Cutting paper | Shape and size | Still paper |
| Dissolving sugar in water | Solid → Solution | Sugar is still sugar |
| Stretching a rubber band | Shape | Same rubber |
| Magnetising iron | Magnetic property | Still iron |
3. Chemical Change
A CHEMICAL CHANGE is a change in which one or MORE new substances are formed.
Characteristics of Chemical Change
- ONE or MORE new substances are formed.
- Usually IRREVERSIBLE (under ordinary conditions).
- Change in CHEMICAL composition.
- Energy change (heat, light, sound may be released or absorbed).
- Change in colour, odour, or gas evolution may occur.
Examples of Chemical Changes
| Example | New Substances Formed | Observations |
|---|---|---|
| Burning of wood | Ash, CO₂, water vapour | Heat, light, ash |
| Rusting of iron | Iron oxide (Fe₂O₃.xH₂O) | Reddish-brown coating |
| Cooking of food | New compounds | Change in taste, smell |
| Digestion of food | Simpler substances | Energy release |
| Photosynthesis | Glucose, oxygen | In presence of sunlight |
| Curdling of milk | Curds (lactic acid) | Sour taste, semi-solid |
4. Comparison: Physical vs Chemical Change
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance | NO | YES |
| Change in composition | NO | YES |
| Reversibility | Usually reversible | Usually irreversible |
| Energy change | May occur (heat) | Usually significant (heat/light) |
| Examples | Melting, boiling, cutting | Rusting, burning, cooking |
| Mass change | No change | Conserved (Law) |
| Can be reversed by | Physical means | Chemical means |
5. Reversible and Irreversible Changes
Reversible Changes
Changes that can be REVERSED to get back the original substance.
Examples:
- Ice ↔ Water (melting ↔ freezing).
- Water ↔ Water vapour (evaporation ↔ condensation).
- Stretching a spring.
Irreversible Changes
Changes that CANNOT be reversed.
Examples:
- Burning of paper (ash cannot become paper again).
- Rusting of iron.
- Growing of a plant.
- Breaking of a glass.
6. Rusting — A Chemical Change
RUSTING is the corrosion of IRON in the presence of OXYGEN and WATER/MOISTURE.
Conditions for Rusting
Rusting requires BOTH:
- OXYGEN (from air).
- WATER or MOISTURE.
Equation: 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃.xH₂O (hydrated iron oxide — rust).
Prevention of Rusting
| Method | How It Works |
|---|---|
| Painting | Forms a barrier to air and moisture |
| Oiling/greasing | Coats the iron surface |
| Galvanisation | Coating with ZINC (zinc corrodes first) |
| Electroplating | Coating with another metal (chromium, nickel) |
| Alloying | Making stainless steel (iron + chromium) |
Experiment to Show Rusting Requires Both Air and Water
Take three test tubes:
- Iron nail in AIR + WATER → RUSTS.
- Iron nail in BOILED WATER (no air) + oil layer → Does NOT rust.
- Iron nail with DRIED air (anhydrous CaCl₂ absorbs moisture) → Does NOT rust.
Conclusion: Both AIR (oxygen) and WATER are necessary for rusting.
7. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Distinguish physical vs chemical changes | 2-3 marks | Very High |
| Examples of each type | 2 marks | High |
| Rusting (conditions and prevention) | 3 marks | Very High |
| Reversible vs irreversible | 2 marks | Medium |
| Energy changes | 1-2 marks | Low |
Common Mistakes
- Saying 'melting of wax is a chemical change' — it is PHYSICAL (wax is still wax, only state changes). BUT burning of wax is CHEMICAL.
- Thinking all irreversible changes are chemical (breaking glass is physical but irreversible).
- Forgetting that rust requires BOTH water and oxygen.
- Writing 'galvanisation' as 'galvanization' — both acceptable, but know the process.
Self-Test (5 Questions)
Q1. Which is a chemical change? (1 mark)
- A) Melting of ice
- B) Cutting of wood
- C) Rusting of iron
- D) Dissolving of salt
Q2. 'Is boiling of water a physical or chemical change?' (1 mark)
Q3. Name TWO conditions necessary for rusting. (2 marks)
Q4. Give TWO methods to prevent rusting. (2 marks)
Q5. 'Why is burning of wood considered a chemical change?' (2 marks)
Answers
A1. C) Rusting of iron (new substance — iron oxide — is formed). A2. Physical change (water changes from liquid to gas, but it is still H₂O). A3. Oxygen (from air) and Water (or moisture). A4. Painting, oiling/greasing, galvanisation, electroplating, alloying (any two). A5. Burning of wood produces NEW substances: ash, carbon dioxide, and water vapour. The original wood cannot be recovered. This makes it a chemical change.
