When a metal reacts with an acid, usually hydrogen gas is liberated. For example, zinc reacts with dilute sulphuric acid as follows:
Zn H2SO4 -----> ZnSO4 + H2
Zitc Sulphuric acid Zinc sulphite Hydrogen
The hydrogen gas evolved can be tested by passing the gas through soap solution and then bringing a burning candle near the soap bubble filled with the gas. The soap bubble burst and the hydrogen gas burns with a pop sound.
