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Chemistry · Chapter 1: Introduction to Chemistry
Physical and Chemical Changes
Physical and chemical changes with everyday examples, signs of a chemical change and the differences between the two.
Key termRememberExampleWatch out
01Concept
Matter changes all the time: ice melts, milk turns into yoghurt, a matchstick burns. Chemists divide these changes into two kinds.
Physical change
A physical change is a change in the form, shape or state of a substance, in which no new substance is formed. It can usually be reversed.
- Ice melting into water, and water freezing into ice.
- Sugar dissolving in tea. (Boil the water away and the sugar is left behind.)
- Ghee melting in a hot pan.
- Breaking glass, cutting paper, folding cloth.
- Water evaporating from wet clothes.
Chemical change
A chemical change is a change in which one or more new substances are formed, with new properties. It is usually difficult to reverse.
- Burning gas in a stove: methane + oxygen → carbon dioxide + water.
- Iron rusting: iron + oxygen + water → rust (hydrated iron oxide).
- Milk turning into yoghurt (dahi): bacteria change milk sugar into lactic acid.
- Cooking a roti or frying an egg.
- Mangoes ripening and food going bad.
How to tell them apart
| Physical change | Chemical change |
|---|---|
| No new substance is formed | A new substance is formed |
| Usually easy to reverse | Usually hard to reverse |
| Only small energy changes | Energy is often given out or taken in (heat, light) |
| Mass of the substance stays the same | Total mass of all substances stays the same, but the substances change |
| Melting ice, dissolving salt | Burning, rusting, cooking |
Signs that a chemical change has happened:
- Colour change: a cut apple turns brown
- Gas given off (bubbles): baking soda with lemon juice
- Heat or light given out: burning wood
- A solid forms in a liquid (a precipitate)
- A new smell: food going bad
02Real World Examples
Rust near the sea
In coastal cities like Karachi, salty, damp air makes iron rust faster. Painting and galvanising (zinc coating) stop this chemical change.
Making dahi at home
A spoon of old yoghurt added to warm milk starts a chemical change: bacteria turn milk sugar into lactic acid.
Ice in summer
Ice from an ice factory melts in a cooler: a physical change. Freeze the water again and you get ice back.
Baking soda in pakoras
Baking soda gives off carbon dioxide when heated, so pakoras puff up: a chemical change.
03Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.Define physical change and chemical change with two examples each.Show answer
Answer
- Physical change: no new substance is formed. Melting ice, dissolving sugar.
- Chemical change: a new substance is formed. Burning gas, rusting of iron.
Q2.Is dissolving salt in water a physical or chemical change? Give a reason.Show answer
Answer
A physical change, because no new substance is formed. The salt can be recovered by evaporating the water.
Q3.Write four signs of a chemical change.Show answer
Answer
- Colour change
- Gas given off (bubbles)
- Heat or light given out
- A solid (precipitate) forms or a new smell appears
Q4.Why is the burning of a candle both a physical and a chemical change?Show answer
Answer
The wax melts (physical change) and then the wax vapour burns to form carbon dioxide and water (chemical change).
Long questions
Q5.Differentiate between physical and chemical changes with examples from daily life.Show answer
Answer
| Physical change | Chemical change |
|---|---|
| No new substance forms | A new substance forms |
| Usually reversible | Usually irreversible |
| Small energy change | Energy often given out or taken in |
| Melting ghee, drying clothes, breaking glass | Cooking roti, making dahi, rusting of iron |