Hydrogen gas is usually liberated when an acid reacts with a metal For example: When zinc granules react with dilute sulphuric acid, then hydrogen gas is liberated and zinc sulphate solution is formed
\(Zn+H_2SO_4\rightarrow ZnSO_4+H_2\)
The evolved hydrogen gas can be tested by taking a burning candle near gas filled bubbles. If candle burns with pop sound, it confirms the evolution of hydrogen gas
