Question

When dilute hydrochloric acid is added to granulated zinc placed in a test tube, the observation made is:
A) The surface of metal turns shining.
B) The reaction mixture turns milky.
C) The odour of chlorine is observed.
D) A colourless and odourless gas evolves with bubbles.

Answer 1

Hint: We know that zinc is a metal. Metals participate in oxidation and reduction reactions when they are treated with appropriate reagents. To solve this we must know what happens when zinc reacts with hydrochloric acid

Complete step by step answer:
We know that the atomic number of zinc is 30. The electronic configuration of zinc is $1{s^2}2{s^2}2{p^6}3{s^2}3{d^{10}}4{s^2}$.
The outermost shell of zinc i.e. 4s shell contains two valence electrons. Zinc has a tendency to lose the two electrons and gain a +2 oxidation state. Thus, zinc undergoes oxidation by losing electrons and forms ${\text{Z}}{{\text{n}}^{2 + }}$ ion.
We know that hydrochloric acid $\left( {{\text{HCl}}} \right)$ is a strong acid. Hydrochloric acid donates a proton and accepts an electron from the metal.
When dilute hydrochloric acid is added to granulated zinc placed in a test tube, zinc metal is converted to zinc chloride and hydrogen gas is evolved in the reaction.

The reaction of zinc with hydrochloric acid is as follows:
${\text{Zn}} + {\text{2HCl}} \to {\text{ZnC}}{{\text{l}}_{\text{2}}} + {{\text{H}}_{\text{2}}}$
In the reaction we can see that a zinc chloride salt is formed and hydrogen gas is evolved. The evolved hydrogen gas is colourless and odourless. From the solution of zinc chloride, hydrogen gas evolves as bubbles.
Thus, when dilute hydrochloric acid is added to a test tube containing granulated zinc we can see that a colourless and odourless gas evolves with bubbles.
Thus, when dilute hydrochloric acid is added to granulated zinc placed in a test tube, the observation made is a colourless and odourless gas evolves with bubbles.

Thus, the correct option is (D) a colourless and odourless gas evolves with bubbles.

Note: The reaction of zinc with hydrochloric acid is a single displacement reaction. In the reaction, zinc metal displaces hydrogen gas. It is a rapid and vigorous reaction as zinc metal reacts very quickly.