Question

Consider the sequence of oxides : $Na _{2} O , SiO _{2}$, $P _{4} O _{10}$. Which factor decreases from $Na _{2} O$ to $SiO _{2}$ and also from $SiO _{2}$ to $P _{4} O _{10}$ ?

• A. Covalent character
• B. Melting point
• C. pH when mixed with water
• D. Solubility in aqueous alkali

Answer 1

Answer: (C)

pH when mixed with water

Answer 2

$Na _{2} O$ is a basic oxide and reacts with water to give $NaOH$ so that its $pH$ is higher than both $SiO _{2}$ and $P _{4} O _{10}$. Both $SiO _{2}$ and $P _{4} O _{10}$ are acidic oxides but $SiO _{2}$ is insoluble while $P_{4} O_{10}$ reacts with water to give $H _{3} PO _{4}$. Thus, for $P _{4} O _{10}, pH$ when mixed with water is lower than that of $SiO _{2}$.