Question

Aqueous solution of which of the following acids cannot be kept in the glass bottle?
A. ${\text{HI}}$
B. ${\text{HF}}$
C. ${\text{HCl}}$
D. ${\text{HBr}}$

Answer 1

Silicon dioxide is a constituent of glass. The acid which reacts with silicon dioxide in glass cannot be stored in a glass bottle. To solve this we must know which of the given acids reacts with silicon dioxide.

Complete answer:
Glass is composed of silicon dioxide $\left( {{\text{Si}}{{\text{O}}_2}} \right)$ and sodium oxide $\left( {{\text{N}}{{\text{a}}_{\text{2}}}{\text{O}}} \right)$. Glass is also known as water glass or soluble glass. The chemical name of soluble glass is sodium silicate.
The silicon dioxide present in glass reacts with hydrofluoric acid $\left( {{\text{HF}}} \right)$. The fluoride ion of hydrofluoric acid attracts the silicon atom of silicon dioxide. As a result, a strong bond is formed between fluorine and silicon atoms. The bond between silicon and oxygen atoms weakens and thus the structure lattice of glass is disturbed.
The glass surface thus dissolves in the hydrofluoric acid.
The reaction of silicon dioxide in glass with hydrofluoric acid is as follows:
${\text{Si}}{{\text{O}}_{\text{2}}} + {\text{4HF}} \to {\text{Si}}{{\text{F}}_{\text{4}}} + {\text{2}}{{\text{H}}_{\text{2}}}{\text{O}}$
${\text{Si}}{{\text{F}}_{\text{4}}}$ is not a solid at room temperature.
Another reaction of silicon dioxide in glass with hydrofluoric acid is as follows:
${\text{Si}}{{\text{O}}_{\text{2}}} + {\text{4HF}} \to {{\text{H}}_2}{\text{Si}}{{\text{F}}_{\text{6}}} + {\text{2}}{{\text{H}}_{\text{2}}}{\text{O}}$
${{\text{H}}_2}{\text{Si}}{{\text{F}}_{\text{6}}}$ formed in the reaction is known as hydrofluorosilicic acid and it is used in polishing of glass. This reaction is known as etching of glass.
Thus, aqueous solution of hydrofluoric acid $\left( {{\text{HF}}} \right)$ cannot be stored in the glass bottle.

**Thus, the correct option is (B) ${\text{HF}}$.

Note:**
Hydrofluoric acid is a hydrogen halide. It is solid in state and generally is not a solvent in nature. Its reaction with silicon dioxide is an exception. Hydroiodic acid, hydrochloric acid and hydrobromic acid do not react with silicon dioxide.