Question

Explain what happens when boric acid is heated.

Answer 1

On heating orthoboric acid $\left(H_{3}B{O}_3\right)$ at 370 K or above, it changes to metaboric acid $\left(HB{O}_2\right)$. On further heating, this yields boric oxide $B_2{O}_3$.

When boric acid ($H_{3}B{O}_3$) is heated at 370 K it forms metaboric acid and water.

$H_{3}B{O}_3\underrightarrow{373K}HB{O}_2+H_{2}O$

The product formed (metaboric acid) is converted into Tetraboric acid at 410 K by losing water.

$4HB{O}_2\left(Metaboricacid\right)\underrightarrow{470K}{H}_2{B}_4{O}_7\left(Tetraboricacid\right)+H_{2}O$

Tetraboric acid at red heat temperature forms boron trioxide or borax by losing water.

$H_2{B}_4{O}_7\underrightarrow{RedHeat}2B_2{O}_3+H_{2}O$

The Overall reaction is as follows:

$H_{3}B{O}_3\underset{370K}{\overset{\Delta }{\to }}\underset{Metaboricacid}{HB{O}_2}\underset{573K}{\overset{\Delta }{\to }}\underset{Tetraboricacid}{H_2{B}_3{O}_7}\underset{redhot}{\overset{\Delta }{\to }}\underset{Boricoxide}{B_2{O}_3}$