Which is the anhydride of orthosilicic acid?
A.${\text{Si}}$
B.${\text{SiO}}$
C.${\text{Si}}{{\text{O}}_2}$
D.${\text{Si}}{{\text{O}}_3}$

Hint: Orthosilicic acid is a chemical compound which has the formula ${\text{Si}}{\left( {{\text{OH}}} \right)_{\text{4}}}$ . In chemistry, the term ‘anhydride’ refers to the chemical compound obtained by the elimination of water from another compound, either in practice or in principle.

Complete step by step answer:
Orthosilicic acid is synthesized using non – aqueous solutions. It is a compound of silicon, hydrogen and oxygen.
Orthosilicic acid can be prepared only as an unstable solution in water. This is because the molecules of orthosilicic acid easily condense with one another to form polymeric chains, rings, sheets or three dimensional networks which constitute the structural unit of silica gel and many other minerals which have very low solubility in water.
Now, an anhydride can be said to be the compound which is obtained by the removal of the elements of water from a particular acid.
Let us understand what anhydride is with the help of an example. We know that sulphuric acid is a mineral acid containing the elements hydrogen, sulphur and oxygen and has the chemical formula ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ . Thus, the removal of water, ${{\text{H}}_{\text{2}}}{\text{O}}$ from sulphuric acid will give sulphur trioxide with the chemical formula ${\text{S}}{{\text{O}}_3}$ . Hence, sulphur trioxide is said to be the anhydride of sulphuric acid.
${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}} \to {\text{S}}{{\text{O}}_3} + {{\text{H}}_{\text{2}}}{\text{O}}$
Now, we have orthosilicic acid which has the formula ${\text{Si}}{\left( {{\text{OH}}} \right)_{\text{4}}}$ or ${{\text{H}}_4}{\text{Si}}{{\text{O}}_{\text{4}}}$ . The elimination of water from orthosilicic acid will give silicon dioxide or silica which has the chemical formula ${\text{Si}}{{\text{O}}_2}$ .
\[{\text{Si}}{\left( {{\text{OH}}} \right)_{\text{4}}} \to {\text{Si}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}}\]
So, silica or ${\text{Si}}{{\text{O}}_2}$ is the anhydride of orthosilicic acid.

Hence, the correct option is C.

Note:
Orthosilicic acid is present in seafood, seawater, vegetables, whole grains and some beverages.
Orthosilicic acid is regarded as a dietary form of silicon, which is a mineral involved in the formation of bone and collagen.
Orthosilicic acid is used for treating certain medical conditions like arthritis, Alzheimer’s disease, stroke, high cholesterol etc.
It is also found to boost skin and hair health. It protects the skin from negative effects of ageing and also preserves dental health.