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  4. When sand is heated with hydrofluoric acid and a wet rod is brought in contact with vapours evolving, a white deposit is due to

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Q. When sand is heated with hydrofluoric acid and a wet rod is brought in contact with vapours evolving, a white deposit is due to

VMMC MedicalVMMC Medical 2004

Solution:

Sand, on heating with HF, give silicon terafluoride vapours, which form silicic acid $ \text{(}{{\text{H}}_{4}}\text{Si}{{\text{O}}_{4}}\text{),} $ on coming in contact with water. $ Si{{O}_{2}}+4HF\xrightarrow{{}}Si{{F}_{4}}+2{{H}_{2}}O $ $ 3\,Si{{F}_{4}}+4{{H}_{2}}O\xrightarrow{{}}2{{H}_{2}}Si{{F}_{6}}+\underset{\text{while}}{\mathop{{{H}_{4}}Si{{O}_{4}}}}\, $