Question

Which of the following are chemical changes and which physical ? Give reason.

1. a glass bottle breaking
2. coal burning in air
3. making a cake
4. wool being knitted into a sweater

Answer 1

1. Breaking of a glass bottle is a physical change because each of the broken piece of bottle is still a glass and no new substance has been formed. In the process,  only the shape and size of bottle change.
2. Burning of coal in air is a chemical change because the burnt coal cannot be obtained back and it is an irreversible change.
3. Preparation of a cake is a chemical change as all the raw materials that go inside the mixture get cooked after baking and the process cannot be reversed.
4. Knitting sweater from wool is a physical change as we can obtain back the wool from the sweater. So, it is a reversible process.