Question

The gas evolved on heating $Ca{{F}_{2}}$ and $Si{{O}_{2}}$ with concentrated ${{H}_{2}}S{{O}_{4}}$, on hydrolysis gives a white gelatinous precipitate. The precipitate is:
(A)- hydrofluorosilicic acid
(B)- silica gel
(C)- silicic acid
(D)- calcium fluorosilicate

Answer 1

Hint: The given endothermic reaction of the calcium fluoride with sulphuric acid, used widely for the production of the hydrogen fluoride acid, which further reacts with the silicon dioxide to produce a gas along with water. The gas on reaction with water gives a white precipitate

Complete step by step solution:
In the reaction of calcium fluoride with concentrated sulphuric acid, heated at high temperature. It produces hydrogen fluoride and calcium sulphate, as follows:
$Ca{{F}_{2}}+{{H}_{2}}S{{O}_{4}}\to 2HF+CaS{{O}_{4}}$--------- (a)
The hydrogen fluoride formed, further reacts with silicon dioxide, $Si{{O}_{2}}$ to form silicon tetrafluoride gas and water, as follows:
$Si{{O}_{2}}(s)+4HF(aq)\to Si{{F}_{4}}(g)+2{{H}_{2}}O(l)$--------- (b)
The silicon tetrafluoride gas on further hydrolysis with water, forms the white gelatinous precipitate of hydrofluorosilicic acid, as follows:
$3Si{{F}_{4}}+2{{H}_{2}}O\to 2{{H}_{2}}Si{{F}_{6}}+S{{i}_{2}}O$--------- (c)

Therefore, on heating $Ca{{F}_{2}}$ and $Si{{O}_{2}}$ with concentrated ${{H}_{2}}S{{O}_{4}}$, and further hydrolysis of the gas evolved produces a white gelatinous precipitate of option (A)- hydrofluorosilicic acid.

Additional information:
The precipitate of hydrofluorosilicic acid is used in water fluoridation in water treatment plants, in cement hardening and as a preservative agent for woods.

Note: The reaction (a) is used commonly for the production of gaseous hydrofluoric acid and is an endothermic reaction, thus, needs continuous input of heat. Also, the calcium fluoride used in the reaction is obtained from the mineral fluorite, also known as the fluorspar. The reaction (b) takes place in the wet etching process, to decorate the glass of silicon dioxide, by etching patterns on its surface and forming silicon tetrafluoride, using the aqueous hydrofluoric acid during the reaction.