THERMAL EXPANSION

Most substances expand with increase in temperature and contract with decrease in temperature. Such expansion is called thermal expansion. 

When the distance between constituent particles of solids is more than some definite distance, r0, the force between them is attractive in nature, which becomes repulsive when the distance is less than r₀. There is no net force when the distance is r0. 

Thus at a definite temperature, the molecules oscillate about their mean position between r_min (< r₀) to r_max (> r₀) such that r₀ - r_min < r_max - r₀. As a result potential energy of the oscillating molecule is less at a distance r_min than when it is at a distance r_max. 

This symmetry (also called anharmonicity) of intermolecular potential energy versus intermolecular distance is responsible for thermal expansion that results in the average intermolecular distance between molecules,

r = (r_min + r_max) / 2 increasing with temperature.