Answer: crushing a solid into a powder
Explanation: A physical reaction is defined as the reaction in which a change in shape, size takes place. No new substance gets formed in these reactions.
A chemical reaction is defined as the reaction in which a change in chemical composition takes place. A new substance is formed in these reactions.
Indicators of a chemical change are:
1. Energy is released or absorbed
2. Formation of precipitate
3. Change in color
4. Evolution of gas