$(a)$ 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.

(N/A) The rate of formation of hydrogen gas bubbles is slow with calcium (less violent) in comparison to that with potassium (more violent). The rate of hydrogen gas evolution is faster in the case of potassium,which generates enough heat to ignite the hydrogen gas.
$Ca + 2H_2O \rightarrow Ca(OH)_2 + H_2 \uparrow$
(The reaction is less vigorous,so the hydrogen gas does not catch fire.)
$(b)$ $(i)$ $Mg$ (Magnesium) or $Mn$ (Manganese).
$(ii)$ $Cu$ (Copper),$Hg$ (Mercury),$Ag$ (Silver),or $Au$ (Gold).
$(iii)$ $Fe$ (Iron),$Zn$ (Zinc),or $Al$ (Aluminium).