Explain what happens when boric acid is heated.

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Solution

On heating orthoboric acid $(H_{3} B O_{3})$ at 370 K or above, it changes to metaboric acid $(H B O_{2})$ . 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.
$H_{3} B O_{3} 373 K - -- \to H B O_{2} + H_{2} O$
The product formed (metaboric acid) is converted into Tetraboric acid at 410 K by losing water.
$4 H B O_{2} (M e t a b o r i c a c i d) 470 K - -- \to H_{2} B_{4} O_{7} (T e t r a b o r i c a c i d) + H_{2} O$
Tetraboric acid at red heat temperature forms boron trioxide or borax by losing water.
$H_{2} B_{4} O_{7} R e d H e a t - ------ \to 2 B_{2} O_{3} + H_{2} O$
The Overall reaction is as follows:

$H_{3} B O_{3} Δ - -- \to 370 K H B O_{2} M e t a b o r i c a c i d Δ - -- \to 573 K H_{2} B_{3} O_{7} T e t r a b o r i c a c i d Δ - --- \to r e d h o t B_{2} O_{3} B o r i c o x i d e$

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