Silica is insoluble in

 ${\text{ 1) SiO}}_2+6\mathrm{HF}\to \underset{ \mathrm{Fluosilisicacid} }{{\mathrm{H}}_2{\mathrm{SiF}}_6}+2{\mathrm{H}}_2\mathrm{O}$

SiO₂ is soluble in HF due to the formation of fluorosilicic acid and hence HF is used for ETCHING process

${\text{ 2) SiO}}_2+2\mathrm{NaOH}\to {\mathrm{Na}}_2{\mathrm{SiO}}_3+{\mathrm{H}}_2\mathrm{O}$

$\begin{array}{ccc} \mathrm{Acid} & \begin{array}{c} (\mathrm{aq}) \\ (\mathrm{Base}) \end{array}& \begin{array}{c} \mathrm{sodiumsilicate} \\ (\mathrm{Waterglass}) \end{array}\end{array}$

$\text{ 3) }\underset{ \mathrm{Acid} }{{\mathrm{SiO}}_2}+\underset{\underset{\left(\mathrm{aq}\right)}{( \mathrm{Base} )}}{2\mathrm{KOH}}\to {\mathrm{K}}_2{\mathrm{SiO}}_3+{\mathrm{H}}_2\mathrm{O}$
SiO₂ is soluble in base like NaOH , KOH due to the formation of soluble silicates
SiO₂ donot soluble in any other solvents
SiO₂ is only soluble in HF(acid) and insoluble in other acids