(a) Give the main difference between physical changes and chemical changes.(b) Which of the following are chemical changes and which physical? Give reason.(i) a glass bottle breaking(ii) coal burning in air(iii) making a cake(iv) wool being knitted into a sweater
(a) Important differences between a physical and a chemical change are given below:
| Physical Change | Chemical Change |
1.No new substance is formed.
| 1. A new substance is formed.
|
2. It is a temporary change.
| 2. It is a permanent change.
|
| 3. Physical change is reversible.
| 3. A chemical change is usually irreversible.
|
| 4. No heat energy is evolved or absorbed.
| 4. Heat energy is evolved or consumed in a large amount.
|
Examples: - Water, if boiled, can be cooled and condensed.
- Mixing iron filings with sand is a physical change because they can be separated by a magnet.
| Examples: - Rusting of iron
- Burning of paper
|
(b) (i) Breaking of a glass bottle is a physical change because each of the broken pieces of the bottle is still a glass and no new substance has been formed. In the process, only the shape and size of the bottle change.
(ii) Burning of coal in the air is a chemical change because the burnt coal cannot be obtained back and it is an irreversible change.
(iii) Preparation of a cake is a chemical change because all the raw materials that go inside the mixture get cooked after baking and the process cannot be reversed.
(iv) Knitting a sweater from wool is a physical change because we can obtain back the wool from the sweater. So, it is a reversible process.
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