Q.(a) What are saline hydrides?

Saline (ionic) hydrides are formed when dihydrogen reacts with highly electropositive alkali metals and the heavier alkaline earth metals (Ca, Sr, Ba), producing crystalline, salt-like solids containing the discrete hydride ion, $H^-$. Because $H^-$ is a very strong Brønsted base and reducing agent, these hydrides react vigorously — often violently — with water, liberating hydrogen gas and heat: $NaH(s) + H_2O(l) \rightarrow NaOH(aq) + H_2(g)$; the heat released can be enough to ignite the hydrogen produced, causing fire. Carbon dioxide cannot be used to extinguish such a fire because saline hydrides are themselves strong reducing agents capable of reducing $CO_2$ (e.g. to carbon or carbonate/formate species), so $CO_2$ would only feed the reaction rather than smother it; a dry-powder or sand-based extinguisher must be …