Question

_____ is a colourless gas which is produced by the reaction of active metal with dilute hydrochloric acid.

Answer 1

Hint: Hydrochloric acid $\left( {HCl} \right)$ reacts with active metals like zinc, which readily donate electrons to the hydrogen atoms in the acid. On forming radicals, they come together to form ${H_2}$ gas. This is an important reaction which depicts the reactivity of metals, as well as their donating power.

Complete answer:
Hydrogen gas $({H_2})$ is a colourless gas which is produced by the reaction of active metal with dilute Hydrochloric acid.
Hydrogen gas is a colourless, odourless, tasteless gas at room temperature. It is a diatomic gas consisting of two H atoms. It has a molecular formula $({H_2})$ . Active metals are good reducing agents as they can easily lose electrons. When an active metal reacts with hydrochloric acid, the metal tends to get ionized and donates its electrons to the hydrogen ions of the acid. Once they have taken up the electrons, the hydrogen ions become atoms and combine to form hydrogen molecules. The metal takes up the place previously occupied by hydrogen atom in the solution and becomes the corresponding salt solution. This can be depicted as follows using an example:
$2Zn\, + \,2HCl \to \,2ZnCl\, + \,{H_2} \uparrow $
Here, zinc is an active metal which reacts with hydrochloric acid to form corresponding zinc salt called zinc chloride along with the liberation of hydrogen gas.

Note:
To test and find out if the gas liberated is hydrogen gas indeed, we just need to bring a burning splinter or a burning candle to the mouth of the test tube in which the active metal and hydrochloric acid has reacted. When they react, bubbles of hydrogen gas evolved will extinguish the flame with a “pop” sound, confirming that the gas evolved is indeed hydrogen gas.