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?

When an acid reacts with a metal it liberates Hydrogen gas.
Metal + Acid \(\rightarrow \) Salt + Hydrogen gas.

For example:

\( Zn(s) + 2HCl(aq) → ZnCl_2(aq) + H_2(g) \)

To test the presence of hydrogen \(H_2 \)gas:
Bring a burning splinter near to the mouth of the test tube where \(H_2 \) gas is released, the matchstick bums with a pop sound.

Fig. Hydrogen production and testing the presence of it.