Question:

The gas evolved on heating CaF2 and SiO2 with concentrated H2SO4, on hydrolysis gives a white gelatinous precipitate. The precipitate is:

silicic acid

hydrofluorosilicic acid

silica gel

calciumfluorosilicate

Reactions: H2SO4 + CaF2 → Ca(HSO4)2 + 2HF
2HF + SiO2 → SiF4 + 2H2O
SiF4 + 2H2O → H2[SiF6]
H2SiF6 is hydrofluorosilicic acid.