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, hydrogen gas is usually liberated. This is a general reaction represented by the chemical equation:

Metal + Acid → Salt+Hydrogen gas

Here’s an example of this reaction using hydrochloric acid (HCl) and zinc (Zn) as the metal:
Zn + 2HCl → ZnCl2 + H₂

In this reaction:

» Zn is the metal (zinc),
» HCl is the acid (hydrochloric acid),
» ZnCl₂ is the salt (zinc chloride), and
» H₂ is the liberated hydrogen gas.

Testing for the Presence of Hydrogen Gas:

To test for the presence of hydrogen gas, you can use the “pop” test or the hydrogen gas collection method.

1. Pop Test:

» Collect the gas produced during the reaction by inverting a test tube filled with water over the reaction vessel.
» The hydrogen gas will rise and displace the water in the test tube.
Ignite a matchstick and quickly bring it near the open end of the test tube.
If hydrogen gas is present, you will hear a distinctive “pop” sound as the hydrogen ignites.

2. Hydrogen Gas Collection:

» Conduct the reaction in a test tube or a gas collection apparatus.
Displace water from the test tube or collection apparatus with the hydrogen gas produced during the reaction.
» Collect the gas over water to ensure that any impurities in the gas are dissolved.
» Test the collected gas by bringing a flame near the mouth of the test tube. If hydrogen is present, it will burn with a pop sound.

Both methods are simple and effective ways to confirm the liberation of hydrogen gas during the reaction between an acid and a metal. It’s important to note that these tests should be conducted with caution, as hydrogen gas is flammable.