(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 ChangeChemical 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:

  1. Water, if boiled, can be cooled and condensed.
  2. Mixing iron filings with sand is a physical change because they can be separated by a magnet.
Examples:
  1. Rusting of iron
  2. 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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Updated on: 10-Oct-2022

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