Physical properties :

$\rightarrow$ Orthoboric acid, $\mathrm{H}_{3} \mathrm{BO}_{3}$ is a white crystalline solid, with soapy touch.

$\rightarrow$ It is sparingly soluble in water but highly soluble in hot water.

– It is also formed by the hydrolysis (reaction with water or dilute acid) of most boron compounds (halides, hydrides, etc.).

– It has a layer structure in which planar $\mathrm{BO}_{3}$ units are joined by hydrogen bonds as shown in figure.

Chemical properties :

– It can be prepared by acidifying an aqueous solution of borax.

$\mathrm{Na}_{2} \mathrm{~B}_{4} \mathrm{O}_{7}+2 \mathrm{HCl}+5 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{NaCl}+4 \mathrm{~B}\left(\mathrm{OH}_{3}\right)$

$\rightarrow$ Boric acid is a weak monobasic acid. It is not a protonic acid but acts as a Lewis acid by accepting electrons from a hydroxyl ion :

$\mathrm{B}(\mathrm{OH})_{3}+2 \mathrm{HOH} \rightarrow\left[\mathrm{B}(\mathrm{OH})_{4}\right]^{-}+\mathrm{H}_{3} \mathrm{O}^{+}$

$\rightarrow$ On heating, orthoboric acid above $370 \mathrm{~K}$ forms metaboric acid, $\mathrm{HBO}_{2}$ which on further heating yields boric oxide, $\mathrm{B}_{2} \mathrm{O}_{3}$.

$\mathrm{H}_{3} \mathrm{BO}_{3} \stackrel{\Delta}{\longrightarrow} \mathrm{HBO}_{2} \stackrel{\Delta}{\longrightarrow} \mathrm{B}_{2} \mathrm{O}_{3}$