In given reaction gas liberated is/are

In which of the following reactions hydrogen gas is liberated ?

The chemical reaction in which heat is liberated are called ……….. reactions .

Zinc on reaction with steam liberates _______ gas.

In the electrolytic decomposition of water, the volume of gas liberated at the cathode is twice that of the gas liberated at the anode. Explain.

Ammonium dichromate on heating liberates a gas. The same gas will be obtained by :