Question

Question: Which of the following has the highest melting point?
A. \[BeC{l_2}\]
B. \[LiCl\]
C. \[AlC{l_3}\]
D. \[NaCl\]

Answer 1

Hint: The melting point is the temperature of any substance related to its average kinetic energy where energy is the ability to perform work. Since the average kinetic energy of the substance remains the same during phase change, the temperature will remain the same too.

Complete step by step answer:
The phase changes gradually while melting or boiling any substance, the spacing between molecules starts to increase and the force of attraction decreases. This increases potential energy while kinetic energy remains constant. Thus, the temperature stays constant during the course of phase change since the added heat is used to increase only potential energy. Therefore, heat is not manifested outside which is why temperature stays constant.
Therefore, the melting point is directly proportional to the force of attraction between the atoms.
The higher the force of attraction higher will be the melting point. For the ionic compounds, the attraction force is greater than the covalent compound. As sodium and chlorine have strong ionic bonds therefore the force of attraction in \[\;NaCl\] is highest as well the boiling point of \[\;NaCl\] is highest among others.

So, the correct answer is D.

Note:
We can say that during phase change the energy given or taken in is referred to as latent heat which is used only to form bonds between molecules or break the bonds between molecules. This heat does not affect the average kinetic energy of the substance and it remains constant during melting or boiling.