In the compound cah2 calcium has an oxidation number of 2+ and hydrogen has an oxidation number of

The oxidation number of H is -1.

Sum of the oxidation numbers in each element = charge of the complex

CaH₂ has 1 Ca atom and 2H atoms. The charge of the complex is zero. Let’s say Oxidation number of H is "a".

Then,

    (+2) + 2 x a = 0

        +2  + 2a  = 0

                  2a = -2

                    a = -1

Hence, the oxidation number of Hydrogen atom in CaH₂ is -1

c, liquids evaporate, creating a gas