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

Hydrogen gas is liberated when an acid reacts with a metal.

For example, when zinc metal reacts with dilute sulphuric acid, hydrogen gas is evolved and salt zinc chloride is formed as: 

$Zn+H_{2} SO_{4}\rightarrow \ Zn( SO_{4})_{2} \ +2H_{2} \uparrow $

It can be tested by bringing a burning candle near the gas. The candle continues burning with a pop sound.