Hydrolysis of ${\mathrm{SiF}}_4$ in alkaline medium produces

The most characteristic reaction of silicon tetrafluoride is that with water, by which gelatinous silica and hydrofluosilicic acid (H2SiF6) are formed. The first reaction is one of hydrolysis: 

${\mathrm{SiF}}_4 + 3{\mathrm{H}}_2\mathrm{O} = {\mathrm{H}}_2{\mathrm{SiO}}_3 + 4\mathrm{HF};$
but this hydrolysis is not perfect, for silicon tetrafluoride combines with hydrogen fluoride to form hydrofluosilicic acid, H2SiF6, which can exist in aqueous solution: 

$2{\mathrm{SiF}}_4 + 4\mathrm{HF} = 2{\mathrm{H}}_2{\mathrm{SiF}}_6;$

consequently the complete reaction is 

$3{\mathrm{SiF}}_{4 }+ 3{\mathrm{H}}_2\mathrm{O} = {\mathrm{H}}_2{\mathrm{SiO}}_3 + 2{\mathrm{H}}_2{\mathrm{SiF}}_6.$
${\mathrm{SiF}}_4+4{\mathrm{H}}_2\mathrm{O}\to \mathrm{Si}(\mathrm{OH}{)}_4+4\mathrm{HF}\text{ and }{\mathrm{SiF}}_4+2\mathrm{HF}\to 2{\mathrm{H}}^{+}+{\left[{\mathrm{SiF}}_6\right]}^{2-}$
Hence it will be understood that when silicon tetrafluoride is passed into a concentrated solution of hydrogen fluoride, hydrofluosilicic acid is formed without separation of silicic acid.