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Question
Which of the following are chemical changes and which physical ? Give reason.
- a glass bottle breaking
- coal burning in air
- making a cake
- wool being knitted into a sweater
Solution
- 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.
- Burning of coal in air is a chemical change because the burnt coal cannot be obtained back and it is an irreversible change.
- 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.
- Knitting sweater from wool is a physical change as we can obtain back the wool from the sweater. So, it is a reversible process.
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