Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

(N/A) Hydrogen gas $(H_2)$ is usually liberated when an acid reacts with a metal.
Example: When zinc granules react with dilute sulfuric acid $(H_2SO_4)$,zinc sulfate $(ZnSO_4)$ and hydrogen gas are produced.
The chemical equation is:
$Zn(s) + H_2SO_4(aq) \to ZnSO_4(aq) + H_2(g) \uparrow$
Test for Hydrogen gas:
To test for the presence of hydrogen gas,pass the gas produced through a soap solution. Bubbles are formed in the soap solution,which contain hydrogen gas. When a burning candle is brought near these soap bubbles,the gas burns with a 'pop' sound,which confirms the presence of hydrogen gas.