Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
What is a Change?
Everything around us keeps changing — water freezes, food gets cooked, plants grow, iron rusts. These changes happen in our daily lives.
Types of Changes
1. Reversible Changes
A change that can be reversed to get back the original substance.
| Change | How to Reverse |
|---|---|
| Melting of ice → water | Cool it → ice again |
| Dissolving sugar in water | Evaporate water → sugar back |
| Folding paper | Unfold it |
| Stretching rubber band | Release it |
| Melting wax | Cool it → solid wax |
Key idea: No new substance is formed. The change is only in the form or state.
2. Irreversible Changes
A change that cannot be reversed. The original substance cannot be recovered.
| Change | Why Irreversible |
|---|---|
| Burning paper | Ash formed — paper cannot be recovered |
| Cooking an egg | Egg white becomes solid permanently |
| Rusting of iron | Rust is a new substance (iron oxide) |
| Ripening of fruit | Cannot become unripe |
| Growing of a plant | Cannot become a seed again |
| Breaking a glass | Cannot be unbroken |
Key idea: Usually a new substance is formed.
3. Slow Changes
Changes that take a long time to complete.
Examples: Rusting of iron, growth of a tree, changing of seasons, ageing of a person.
4. Fast Changes
Changes that happen quickly.
Examples: Burning of paper, bursting of a cracker, lighting of a matchstick.
5. Periodic Changes
Changes that happen at regular intervals and repeat.
Examples: Rotation of Earth (day and night), revolution of Earth around the Sun (seasons), heartbeat, pendulum swing.
6. Non-periodic Changes
Changes that happen without a fixed pattern or interval.
Examples: Occurrence of earthquake, a child falling ill, cloudburst.
7. Desirable Changes
Changes that we want to happen.
Examples: Germination of seeds, ripening of fruits, cooking of food.
8. Undesirable Changes
Changes that we do not want to happen.
Examples: Rusting of iron, souring of milk, tooth decay.
Can All Reversible Changes Be Described as Physical?
Most reversible changes are physical changes (change in shape, size, state — no new substance formed).
Most irreversible changes involve a chemical change (new substance is formed).
However: Some physical changes are hard to reverse (e.g., breaking a glass is physical — no new substance — but irreversible in practice).
Interesting Examples to Analyse
| Change | Reversible or Irreversible? | Reason |
|---|---|---|
| Melting of ice | Reversible | Water → cool → ice |
| Burning wood | Irreversible | Ash formed; cannot become wood |
| Stretching spring | Reversible | Spring returns to original shape |
| Making curd from milk | Irreversible | Curd cannot be converted back to milk |
| Dissolving salt in water | Reversible | Evaporate water → salt back |
| Inflating a balloon | Reversible | Deflate → balloon returns to original |
| Cooking rice | Irreversible | Cooked rice cannot become raw rice |
Exam Tips
- The most common question: "Give two examples each of reversible and irreversible changes" — always mention whether a new substance is formed in your answer.
- Melting of wax is reversible; burning of wax is irreversible — a common tricky comparison.
- Rusting is both slow and irreversible — both properties may be asked separately.
- Periodic vs non-periodic is a direct definition + example question worth 2 marks.