Question

Which of the following substances has the highest melting point?
A) $CaO$
B) $BiC{l_3}$
C) $KCl$
D) $Cl{O_2}$

Answer 1

Molecular composition, lattice energy, force of attraction and the presence of impurities can all affect the melting point of substances. Melting point is directly dependent on lattice energy. Again lattice energy will be greater if the charge of the ion is greater and the size of the ion is smaller.

Complete answer:
In between CaO and KCl it is evident that the charges of $C{a^{2 + }}$ and ${O^{2 - }}$ is higher than that of the ${K^ + }$ and $C{l^ - }$. so the force of attraction between $C{a^{2 + }}$ and ${O^{2 - }}$ is much greater which results in greater lattice energy for CaO than KCl. Now greater the lattice energy , greater will be melting point. So the melting point of CaO > KCl.
Now in between $BiC{l_3}$ and KCl, as $B{i^{3 + }}$ has higher charge density than ${K^ + }$ , so $BiC{l_3}$ should have greater melting point. But actually this is not the case. Here size factor is the predominating factor. Since the size of Bi > Cl, the attraction between them is not that much strong. But incase of $KCl$ size of $K^+$ and $Cl^-$ is almost the same which suggests a very strong force of attraction. so the lattice energy will be lesser for $BiC{l_3}$ . hence the melting point of $BiC{l_3}$ < KCl.
$Cl{O_2}$ exists only in the gas phase, so the force of attraction among molecules is low. It suggests lower melting point.
So the order of melting point will be –
$CaO > KCl > {\text{ }}BiC{l_3} > {\text{ }}Cl{O_2}$

So the correct answer is A).

Note: Ionic solids have high melting and boiling points. $CaO\;and\;KCl\;$ are ionic solids.
Calcium Oxide $CaO\left( {{{2572}^o}C} \right)$ has the highest melting point.
The melting points of other substances are $KCl\left( {{{770}^o}C} \right),\;BiC{l_3}\left( {{{227}^o}C} \right)$ and
$Cl{O_2}\left( {{{59}^o}C} \right)$
Since there will be more delocalized electrons per atom throughout calcium than potassium, the bond strength is stronger in calcium than potassium. Since more energy is required to break the bonds, the melting point of calcium will be higher than that of potassium.