(a) What is meant by a chemical reaction? Explain with the help of an example.
(b) Give one example each of a chemical reaction characterised by:
1. evolution of a gas
2. change in colour
3. formation of a precipitate
4. change in temperature
5. change in state.
(a) Chemical reactions are the processes in which new substances with new properties are formed.
For example: When magnesium ribbon is heated, it burns in air to form a white powder called magnesium oxide.
(b) 1. The chemical reaction between zinc and dilute sulphuric acid.
2. The chemical reaction between citric acid and purple coloured potassium permanganate solution is characterised by change in colour (from purple to colourless).
3. The chemical reaction between potassium iodide and lead nitrate is characterised by the formation of a yellow precipitate of lead iodide.
4. The reaction between quick lime and water to form slaked lime is characterised by a change in temperature.
5. When wax is burned, then water and carbon dioxide are formed. Wax is a solid; water is a liquid whereas carbon dioxide is a gas.