Physical and Chemical Changes
1. Physical Change
A change that ALTERS the PHYSICAL properties of a substance WITHOUT changing its CHEMICAL composition.
Characteristics:
- NO new substance is formed
- Usually REVERSIBLE
- Temporary change
- Change in PHYSICAL properties (shape, size, state, colour)
- Molecular composition REMAINS the same
Examples of Physical Changes:
| Change | What Happens | Reversible? |
|---|---|---|
| Melting of ice | H₂O(s) → H₂O(l) | YES — freezing |
| Boiling of water | H₂O(l) → H₂O(g) | YES — condensation |
| Cutting paper | Paper changes shape | NO (practically) |
| Dissolving sugar in water | Sugar molecules spread | YES — evaporation |
| Crushing a can | Shape changes | NO |
| Stretching a rubber band | Shape changes | YES |
2. Chemical Change
A change in which a NEW substance is formed with DIFFERENT chemical properties.
Characteristics:
- ONE or more NEW substances are formed
- Usually IRREVERSIBLE (or hard to reverse)
- Permanent change
- Change in CHEMICAL composition
- Energy CHANGE (heat, light, or sound may be released/absorbed)
Examples of Chemical Changes:
| Change | New Substance(s) | Irreversible? |
|---|---|---|
| Burning of wood | Ash, CO₂, water vapour | YES |
| Rusting of iron | Iron oxide (Fe₂O₃·xH₂O) | YES |
| Cooking of food | New food compounds | YES |
| Digestion of food | Simpler nutrients | YES |
| Fermentation of grapes | Alcohol (ethanol) | YES |
| Photosynthesis | Glucose + oxygen | Effectively YES |
3. Comparison Table
| Aspect | Physical Change | Chemical Change |
|---|---|---|
| New substance formed | NO | YES |
| Reversibility | Usually REVERSIBLE | Usually IRREVERSIBLE |
| Energy change | SMALL or none | OFTEN significant |
| Molecular composition | SAME | CHANGED |
| Examples | Melting, dissolving, boiling | Burning, rusting, cooking |
| Nature | TEMPORARY | PERMANENT |
4. Recognising Chemical Changes — Clues
| Clue | Example |
|---|---|
| Change in COLOUR | Iron rusts (brown), paper burns (black) |
| Evolution of GAS | Baking soda + vinegar → CO₂ |
| Formation of PRECIPITATE | AgNO₃ + NaCl → AgCl (white precipitate) |
| Change in TEMPERATURE | Exothermic (heat released) or endothermic (heat absorbed) |
| Evolution of LIGHT or SOUND | Burning magnesium (dazzling white light) |
| Change in ODOUR | Cooking, burning produce new smells |
5. Reversible and Irreversible Changes
Reversible Changes
Changes that can be REVERSED by changing the conditions.
Examples: Melting → freezing, boiling → condensing, dissolving → evaporating, stretching a spring.
Irreversible Changes
Changes that CANNOT be reversed easily or at all.
Examples: Burning, rusting, cooking, growth of a plant, making of curd from milk.
'Most physical changes are REVERSIBLE (though not all — cutting paper is not easily reversible). Most chemical changes are IRREVERSIBLE.'
6. Periodic and Non-Periodic Changes
| Periodic Changes | Non-Periodic Changes |
|---|---|
| Occur at REGULAR intervals | Occur IRREGULARLY |
| Example: Day and night, seasons | Example: Earthquake, volcanic eruption |
7. Desirable and Undesirable Changes
| Desirable Changes | Undesirable Changes |
|---|---|
| Changes we WANT to happen | Changes we DON'T want |
| Example: Ripening of fruit, milk to curd | Example: Rusting of iron, spoilage of food |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Melting is a chemical change' | Melting is a PHYSICAL change — only the STATE changes, not the chemical composition |
| 'Dissolving sugar in water is a chemical change' | Dissolving is PHYSICAL — sugar can be RECOVERED by evaporation. No new substance formed |
| 'All irreversible changes are chemical' | Some physical changes (cutting paper) are ALSO irreversible |
| 'Burning produces no new substances' | Burning is a CHEMICAL change — ash, smoke, and gases are NEW substances |
ICSE Exam Focus (5–6 marks)
- 2-mark questions: Define physical and chemical changes with one example each
- 3-mark questions: Distinguish between physical and chemical changes
- 4-mark questions: Classify given changes with reasons
- 5-mark questions: Explain why a given change is physical or chemical
Self-Test
Q1. Classify: Melting of wax. A1. PHYSICAL change. Only the STATE changes (solid to liquid). The chemical composition (wax) remains the SAME. Wax can be remoulded and used again.
Q2. Classify: Burning of a candle. A2. BOTH! Melting of wax: PHYSICAL. Burning of wax vapour: CHEMICAL (new substances — CO₂, H₂O, soot).
Q3. Give two examples of chemical changes. A3. (1) Rusting of iron — iron reacts with oxygen and moisture to form iron oxide. (2) Cooking of food — new chemical compounds are formed.
Q4. Why is rusting considered a chemical change? A4. Rusting produces a NEW substance (iron oxide, Fe₂O₃·xH₂O) with DIFFERENT properties. It is IRREVERSIBLE. The iron CANNOT be recovered easily.
Q5. What are the clues that indicate a chemical change has occurred? A5. Change in colour, evolution of gas, formation of precipitate, change in temperature, evolution of light, change in odour.
Q6. Is the dissolution of salt in water a chemical change? Explain. A6. NO — it is a PHYSICAL change. Salt can be RECOVERED by evaporating the water. The salt's chemical composition (NaCl) remains unchanged.
