Question

About 5 mL of dilute hydrochloric acid is taken in a test tube. To this, a few pieces of granulated zinc are added. Which of the following is true?

• A. It is a physical change and hydrogen gas is evolved
• B. It is a chemical change and zinc dissolves
• C. It is a physical change and zinc dissolves
• D. It is a chemical change, zinc dissolves and hydrogen gas is liberated

Answer 1

The correct option is D

It is a chemical change, zinc dissolves and hydrogen gas is liberated

Explanation for the correct option:

(D) It is a chemical change, zinc dissolves and hydrogen gas is liberated:

When 5 mL of dilute Hydrochloric acid $\left(\mathrm{HCl}\right)$ is taken in a test tube and a few pieces of granulated Zinc $\left(\mathrm{Zn}\right)$ are added to it, the following chemical reaction takes place.

• $\underset{\left(\mathrm{Zinc}\right)}{\mathrm{Zn}\left(\mathrm{s}\right)}+\underset{(\mathrm{Dil}.\mathrm{Hydrochloric}\mathrm{acid})}{2\mathrm{HCl}\left(\mathrm{aq}\right)}\to \underset{(\mathrm{Zinc}\mathrm{chloride})}{{\mathrm{ZnCl}}_2\left(\mathrm{aq}\right)}+\underset{\left(\mathrm{Hydrogen}\right)}{{\mathrm{H}}_2\left(\mathrm{g}\right)}$
• As Zinc is more reactive than Hydrogen $\left(\mathrm{H}\right)$, it is replaced by Zinc from hydrochloric acid and zinc dissolves to produce Zinc chloride $\left({\mathrm{ZnCl}}_2\right)$ and Hydrogen $\left({\mathrm{H}}_2\right)$ gas is liberated.
• As new substances i.e.; zinc chloride and hydrogen gas are produced with different chemical properties and composition when compared with the starting substances, it is called a chemical change.

Chemical change: A change in which one or more new substances are formed and there is a change in the chemical composition of the substance undergoing the change.

• It is a permanent change.
• It is an irreversible change as the substance undergoing change can not be brought back to its original state.

Explanation for the incorrect options:

(A) It is a physical change and hydrogen gas is evolved: Hydrogen gas is evolved during the reaction but it is not a physical change as new substances are formed with different chemical composition and chemical properties.

Physical change: A change in which no new substance is formed and there is no change in the chemical composition of the substance undergoing the change.

• It is a temporary change.
• It is a reversible change as most of the times, the substance can be brought back to its original state.

(B) It is a chemical change and zinc dissolves: It is a chemical change and Zinc also dissolves but liberation of Hydrogen gas is not mentioned in this option.

(C) It is a physical change and zinc dissolves: It is not a physical change.

Hence, option (D) is correct.