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

Hydrogen gas is usually liberated when an acid reacts with a metal. To test the presence of this gas, take few pieces of zinc granules and add 5 ml of dilute H₂SO₄. Shake it and pass the gas produced, into a soap solution. The bubbles of the soap solution get formed. These soap bubbles contain hydrogen gas.

To test the evolved hydrogen gas, a candle is brought near the soap bubbles, the gas burns producing a pop sound confirming the presence of hydrogen gas.