(a) When calcium metal is added to water, the gas evolved does not catch fire but the same gas evolved on adding potassium metal to water catches fire. Explain why?
(b) Name a metal for each case : (i) It displaces hydrogen gas from nitric acid. (ii) It does not react with any physical state of water. (iii) It does not react with cold as well as hot water but reacts with steam.

Explanation

(a) Reaction of potassium with water is so violent and exothermic that the evolved hydrogen catches fire.
`2K(s) + 2H_2O (l) to 2KOH (aq) + H_2(g)`
The reaction of calcium with water is less violent. The heat evolved is not sufficient for hydrogen to catch fire.
`Ca(s) + 2H_2O(l) to Ca(OH)_2(aq) + H_2(g)`
(b) (i) Magnesium (ii) Copper (iii) Aluminium