Question

Which of the following substances has the highest melting point?

• A. $BaO$
• B. $MgO$
• C. $KCl$
• D. $NaCl$

Answer 1

Answer: (B)

Melting points depend upon lattice energy. $\text{NaCl}$ and $\text{KCl}$ have unit charge on their ions while $\text{MgO}$ and $\text{BaO}$ have two units of charge, therefore lattice energies of $\text{MgO}$ and $\text{BaO}$ are expected to be larger than those of $\text{NaCl}$ and $\text{KCl}\text{.}$ Since, $\text{M}{\text{g}}^{2+}$ is smaller than $\text{B}{\text{a}}^{\text{2+}}\text{,}$ therefore, $\text{MgO}$ has the higher lattice energy and hence, has the highest melting point.