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?
Hydrogen gas is usually liberated when an acid reacts with a metal. The general reaction can be summarised as: Acid + Metal → Salt + Hydrogen gas.
For example, when zinc granules are added to dilute sulphuric acid, hydrogen gas is evolved with effervescence, and zinc sulphate is formed.
To test for the presence of hydrogen gas, pass the evolved gas through a soap solution to form gas-filled bubbles. Then bring a burning candle near the bubble. If the gas is hydrogen, it burns with a pop sound.
Explanation
The textbook states that all acids generate hydrogen gas on reacting with metals, as seen in Activity 2.3. The metal displaces hydrogen atoms from acids as hydrogen gas and forms a salt. The soap bubble test with a burning candle is the standard method to identify hydrogen gas.