Physical and Chemical Changes - Class 7 Science (CBSE)
Based on the 2025-26 NCERT syllabus for Class 7 Science. This chapter helps students distinguish between physical and chemical changes and understand important chemical processes like rusting and crystallisation.
1. Why this chapter matters
Every day, matter around us undergoes changes -- ice melting, iron rusting, food cooking. Understanding which changes are physical and which are chemical helps us predict and control these processes. In CBSE exams, this chapter contributes 6-8 marks.
2. Physical changes
A physical change is a change in which the chemical composition of a substance does NOT change. The substance remains the same, only its physical state or appearance changes.
Characteristics
- No new substance is formed.
- Change is usually reversible.
- Mass of the substance remains the same.
- Examples: melting of ice, boiling of water, cutting paper, dissolving sugar in water, stretching a rubber band.
3. Chemical changes
A chemical change is a change in which one or more new substances with different chemical properties are formed.
Characteristics
- One or more new substances are formed.
- Change is usually irreversible.
- Energy change (heat, light, or sound) often occurs.
- Examples: rusting of iron, burning of paper, cooking of food, digestion of food.
4. Comparison: physical vs chemical change
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance formed | No | Yes |
| Reversibility | Usually reversible | Usually irreversible |
| Energy change | May occur (melting absorbs heat) | Often occurs (burning releases heat) |
| Composition | Same | Changes |
| Examples | Melting, boiling, dissolving | Rusting, burning, cooking |
| Mass | Conserved | Conserved |
| Can be detected by | Change in state, shape, size | Change in colour, odour, composition |
5. Rusting of iron
Rusting is a chemical change where iron reacts with oxygen and moisture to form iron oxide (rust).
Iron + Oxygen + Water = Iron oxide (rust)
Conditions for rusting
Both air (oxygen) and water (moisture) are necessary for rusting. If either is absent, rusting does not occur.
Experiment to show conditions
- Nail in dry air (with calcium chloride): No rust.
- Nail in boiled water (no air): No rust.
- Nail in ordinary water (both air and water): Rust forms.
Effects of rusting
Rust is a reddish-brown, flaky substance that weakens iron structures. It eats away the metal over time.
6. Preventing rusting
Galvanisation
Galvanisation is the process of coating iron with a layer of zinc to prevent rusting.
- The zinc layer acts as a barrier, preventing oxygen and moisture from reaching the iron.
- Even if the zinc layer is scratched, it still protects the iron (zinc corrodes instead of iron).
- Uses: Iron sheets for roofing, buckets, pipes.
Other methods of rust prevention
- Painting or oiling: Creates a barrier against air and moisture.
- Greasing: Applied to moving parts (bicycle chains, machine parts).
- Alloying: Mixing iron with other metals (stainless steel = iron + chromium + nickel).
7. Crystallisation
Crystallisation is a physical process used to obtain pure crystals of a substance from its solution.
Example: Obtaining pure copper sulphate crystals
- Dissolve impure copper sulphate in hot water to form a saturated solution.
- Filter the solution to remove insoluble impurities.
- Allow the solution to cool slowly.
- Pure blue crystals of copper sulphate form.
Applications of crystallisation
- Purification of salt from sea water (salt pans).
- Preparation of alum, sugar crystals.
- Pharmaceutical industry for pure medicines.
8. Worked examples
Example 1: Classify the following as physical or chemical change: (a) Burning of wood, (b) Melting of wax.
(a) Chemical change -- new substances (ash, gases) are formed. (b) Physical change -- wax melts and can be solidified again; no new substance.
Example 2: Why does an iron nail kept in a closed plastic bag not rust?
Both air and water are needed for rusting. The closed plastic bag prevents moisture (and air) from reaching the nail.
Example 3: What is galvanisation? How does it prevent rusting?
Galvanisation is coating iron with zinc. The zinc layer prevents oxygen and water from reaching the iron. If scratched, zinc corrodes first, protecting the iron.
Example 4: Is crystallisation a physical or chemical change? Why?
Crystallisation is a physical change. No new substance is formed. The substance retains its chemical properties, only the crystal shape changes.
9. Common mistakes and how to fix them
| Mistake | Fix |
|---|---|
| Thinking all irreversible changes are chemical | Some physical changes are irreversible too (e.g., breaking glass) |
| Believing rust is a protective layer | Rust flakes off, exposing more iron to rust |
| Saying galvanisation uses copper | Galvanisation uses ZINC, not copper |
| Confusing crystallisation with evaporation | Crystallisation forms crystals; evaporation removes liquid |
| Thinking dissolution is always chemical | Dissolving sugar is physical (reversible, no new substance) |
10. CBSE exam focus
| Question type | Marks | Frequency |
|---|---|---|
| Distinguish physical and chemical change | 2-3 marks | 1 question |
| Conditions for rusting | 2-3 marks | 1 question |
| Methods of rust prevention | 2 marks | 1 question |
| Crystallisation process | 3 marks | 1 question |
| Galvanisation explanation | 2 marks | Occasional |
11. Self-test
- State whether the following are physical or chemical changes: (a) Formation of curd from milk (b) Evaporation of water (c) Rusting of iron
- What are the essential conditions for rusting?
- How does painting prevent rusting?
- What is crystallisation? Give one application.
- Differentiate between physical and chemical changes (any four points).
- Why is stainless steel preferred over iron for kitchen utensils?
12. Answer key
- (a) Chemical change. (b) Physical change. (c) Chemical change.
- Both oxygen (air) and water (moisture) are essential.
- Paint forms a barrier that prevents air and moisture from reaching the metal surface.
- Crystallisation is the process of forming pure crystals from a solution. Application: obtaining sugar crystals from sugarcane juice.
- See comparison table in section 4.
- Stainless steel does not rust because it is an alloy of iron, chromium, and nickel. It is resistant to corrosion.
13. Quick revision
- Physical change: no new substance, usually reversible.
- Chemical change: new substance formed, usually irreversible.
- Rusting: iron + oxygen + water = iron oxide.
- Rust prevention: painting, oiling, greasing, galvanisation, alloying.
- Galvanisation: coating iron with zinc.
- Crystallisation: physical method to obtain pure crystals.
- Burning, cooking, digestion are chemical changes.
- Melting, boiling, dissolving are physical changes.
