Question

_____________ acid is used in the extraction process of amorphous silicon to remove unreacted silica.

• A. Hydrochloric
• B. Sulphuric
• C. Nitric
• D. Hydrofluoric

Answer 1

The correct option is D Hydrofluoric

Finely powdered silica (sand or quartz) is mixed with magnesium powder and heated in a fire- clay crucible. Magnesium oxide and silicon are formed. The product is washed with dilute hydrochloric acid to dissolve magnesium oxide, a basic impurity. Then it is washed with hydrofluoric acid to remove unreacted silica. The powder left behind is silicon its amorphous form.

$Si{O}_2+2Mg⟶Si+2MgO$

Hydrochloric acid is used to dissolve magnesium oxide, a basic impurity. Hydrofluoric acid is used to remove silica by washing.