Question

At a given temperature the force between molecules of a gas as a function of intermolecular distance is
A. always constant
B. always decreases
C. always increases
D. first decreases and then increases

Answer 1

The kinetic energy of gas molecules is high and tries to keep the molecules apart. The attractive forces that two molecules experience are generally electrostatic in nature and try to bring two molecules together.

Complete answer:
Real gas is different from an ideal gas because it has intermolecular forces acting between it's molecules. In ideal gases intermolecular forces do not exist and the collisions between the particles are completely elastic. The kinetic energy of the molecules and intermolecular forces determine the state of a substance (solid/liquid/gas). In a gas the average kinetic energy of the particles is high enough to overcome the forces of attraction between them so the molecules move apart when they collide. The average kinetic energy of the particles in liquid/solid is smaller than attractive forces so the molecules can stick together.
Intermolecular forces are weak van der waals type forces in a gas.
For gases the intermolecular forces are repulsive at short distances and attractive at long distances this can be explained with the help of a potential called Lennard-Jones potential. A gas molecule faces a huge repulsive potential when it is close to another molecule and it faces an attractive potential when it is some distance farther.
At a given temperature the force between molecules of a gas as a function of intermolecular distance first decreases and then increases.

So, the correct answer is “Option D”.

Note:
It has not been given to us whether the given gas is ideal or real. We talk of intermolecular forces only in the context of a real gas. So we use the behavior of intermolecular forces of real gas to determine the correct answer.