Question

while the substance is a liquid at what temperature do the molecules of the substance have the greatest average kinetic energy ​

Answer 1

The molecules of the liquid will be having the greatest average kinetic energy when the temperature of the liquid reaches its boiling point.

Explanation:

Average kinetic energy is the measure of kinetic energy exhibited by the molecules of any substance. It is directly proportionate to the temperature of the substance. According to kinetic molecular theory,

                           $k=\frac{3 R T}{2 N_{A}}$

As the molecules in solids are mostly closely packed, so they will attain the least kinetic energy of molecules in solid phase. Similarly, the molecules in liquid are slightly loosely packed and on increasing the temperature, the molecules will attain more energy to increase their neighbouring distances.

And when that temperature reaches the boiling point of that liquid substance, the molecules had attained the maximum amount of energy to increase its average kinetic energy to the maximum limit and transferring the liquid molecules to gaseous state.

As the molecules in gas phase are very loosely packed. The starting of the change in the state of molecules from liquid to gas phase on reaching the boiling point indicate the maximum average kinetic energy at that point.