Chapter Overview & Weightage
Physical and Chemical Changes is Chapter 6 in CBSE Class 7 Science (NCERT). It introduces one of the most fundamental distinctions in chemistry — the difference between changes that alter only the physical form of matter (physical changes) and those that produce new substances with different properties (chemical changes).
This chapter carries 6–8 marks in CBSE Class 7 annual exams. Identifying physical vs chemical changes, and understanding why rusting is a chemical change, are the most tested topics. One 3-mark question and one 1-mark identification question are typical.
What this chapter covers:
- Definition and characteristics of physical changes
- Definition and characteristics of chemical changes
- Examples from daily life: rusting, burning, dissolving, melting
- Galvanisation as protection against rusting
- Crystallisation as a method of obtaining pure substances
Key Concepts You Must Know
Physical Changes
A physical change is a change in the physical properties of a substance (shape, size, colour, state) without changing its chemical composition.
Key characteristics:
- No new substance is formed
- Change is generally reversible — the original substance can be recovered
- Chemical composition remains the same
Examples:
- Melting ice → water (H₂O remains H₂O)
- Dissolving sugar in water (sugar can be recovered by evaporation)
- Tearing paper (still paper, just smaller pieces)
- Boiling water → steam
- Bending a wire
Chemical Changes
A chemical change is a change in which new substances with different properties are formed.
Key characteristics:
- New substance with new properties is formed
- Change is generally irreversible (cannot easily get back the original substances)
- Chemical composition changes
- Often accompanied by: heat/light production, gas evolution, colour change, smell change, precipitate formation
Examples:
- Burning wood → ash + CO₂ + water (cannot un-burn wood)
- Rusting of iron → iron oxide (Fe₂O₃)
- Cooking food
- Curdling of milk
- Baking a cake
Important Concept — Signs of Chemical Change
- Change in colour
- Change in smell
- Evolution of gas (bubbles)
- Change in temperature (exothermic or endothermic)
- Formation of a new substance
- Formation of precipitate
Rusting — The Key Chemical Change Example
Rusting is the chemical reaction of iron with oxygen and water in the air:
Iron → Iron oxide (rust) — a completely different substance. Rust is brittle, reddish-brown, and has no metallic properties.
Why is rusting a chemical change?
- New substance (iron oxide) is formed
- Properties are completely different (iron is shiny and strong; rust is flaky and weak)
- The process is irreversible (you cannot un-rust iron)
Preventing Rusting — Galvanisation
Galvanisation is the process of coating iron with a thin layer of zinc to prevent rusting.
Why zinc? Zinc is more reactive than iron — it reacts with oxygen and water preferentially, protecting the iron underneath (called sacrificial protection). Even if the zinc coating is scratched, it continues to protect nearby iron areas.
Applications: galvanised iron sheets for roofing, galvanised pipes, dustbins.
Crystallisation
Crystallisation is the process of forming pure crystals of a substance from its solution by slow cooling or evaporation.
Example: Salt crystals from seawater (evaporation), copper sulphate crystals from copper sulphate solution (cooling).
This is a physical change — no new substance is formed; the salt is recovered from solution.
Solved Previous Year Questions
PYQ 1 — Classify as Physical or Chemical
Q: Classify the following as physical or chemical changes: (a) Cutting vegetables, (b) Burning candle, (c) Mixing sand and water, (d) Milk turning sour. (CBSE Class 7 pattern)
Solution:
- (a) Cutting vegetables → Physical (no new substance; reversible in principle — same chemical composition)
- (b) Burning candle → Chemical (wax is converted to CO₂ and water; irreversible; new substances formed)
- (c) Mixing sand and water → Physical (sand can be filtered out; no new substance formed)
- (d) Milk turning sour → Chemical (lactic acid is formed; new substance; different smell and taste; irreversible)
PYQ 2 — Rusting
Q: Why is rusting considered a chemical change? State two ways to prevent rusting. (CBSE — 2 marks)
Solution:
Why chemical: New substance iron oxide (rust) is formed with completely different properties (colour, brittleness, no metallic sheen). The change is irreversible.
Two ways to prevent:
- Painting (oil paint prevents moisture and oxygen contact)
- Galvanisation (zinc coating provides sacrificial protection)
- (Also acceptable): Applying grease or oil; storing in dry conditions
PYQ 3 — Crystallisation
Q: What is crystallisation? Why is it called a physical change? (CBSE — 2 marks)
Solution:
Crystallisation is the process of forming crystals by slowly cooling a hot saturated solution or by evaporation of solvent from a solution.
It is a physical change because:
- No new substance is formed — we get back the same substance (e.g., copper sulphate crystals from copper sulphate solution)
- The process is reversible — dissolve the crystals in water and you get the solution back
- The chemical composition of the substance does not change
Difficulty Distribution
| Difficulty | Topic | Marks |
|---|---|---|
| Easy (40%) | Classify as physical/chemical; identify sign of chemical change | 1 mark each |
| Medium (40%) | Explain rusting; state preventions; define crystallisation | 2–3 marks |
| Hard (20%) | Detailed explanation of a chemical change with equation; compare physical vs chemical | 4–5 marks |
Expert Strategy
The reversibility rule is a useful shortcut: if you can easily reverse the change to get back the original substance, it’s likely a physical change. If you can’t reverse it, it’s likely a chemical change. BUT reversibility is not the defining criterion — new substance formation is. Burning is chemical even if theoretically you could react CO₂ with water to get back carbohydrates (photosynthesis does this, but it’s a different process).
For board exams, always justify your classification with a reason. “Burning candle — chemical change, because wax is converted to CO₂ and water vapour, which are new substances with different properties, and the change is irreversible.” This level of explanation earns full marks; just writing “chemical” earns one mark at most.
Common Traps
Trap 1 — Dissolving is always physical: Usually yes — dissolving sugar in water is physical (recoverable). But dissolving a metal in acid is chemical (new substances: salt + hydrogen gas; irreversible). Always check if a new substance is formed.
Trap 2 — Reversible = physical change (always): Not always. Melting is reversible and physical. But the direction of reversal doesn’t determine the type. Some chemical changes appear reversible (e.g., heating CuSO₄·5H₂O removes water → anhydrous CuSO₄; adding water back restores it). The key test is: is a new substance formed?
Trap 3 — Colour change always means chemical change: Sometimes a physical change involves a colour change — e.g., iodine (solid, purple-black) turning to a gas (violet vapour) on heating. This is sublimation — a physical change. A colour change is a SIGN of chemical change, but alone it’s not conclusive.