Question

When orthoboric acid $\left(H_{3}B{O}_3\right)$ is heated, the residue is:

• A. Boron
• B. Metaboric acid
• C. Boric anhydride
• D. Borax

Answer 1

The correct option is C Boric anhydride

On strong heating, first, the Boric acid ($H_{3}B{O}_3$) breaks up into Metaboric acid ($HB{O}_2$) and Water ($H_{2}O$). On further heating, the Metaboric acid breaks up into Boric oxide ($B_2{O}_3$) and Water.

$H_{3}B{O}_3\to HB{O}_2+H_{2}O$

$2HB{O}_2\to B_2{O}_3+H_{2}O$

$B_2{O}_3$ is boric anhydride.

Hence, option $C$ is correct.