Question

The gas evolved on heating $CaF_2$ and $SiO_2$ with concentrated $H_2SO_4$, which on hydrolysis gives a white gelatinous precipitate. The precipitate is:

• A. Hydrofluorosilicic acid
• B. Silica gel
• C. Silicic acid
• D. Calcium fluorosilicate

Answer 1

Answer: (A)

Reactions:
$H _{2} SO _{4}+ CaF _{2} \rightarrow Ca \left( HSO _{4}\right)_{2}+ H _{2} F _{2}$
$2 H _{2} F _{2}+ SiO _{2} \rightarrow SiF _{4}+2 H _{2} O$
$SiF _{4}+ H _{2} O \rightarrow H _{2}\left[ SiF _{6}\right]$
$H _{2} SiF _{6}$ is hydrofluorosilicic acid.