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A solid expands upon heating because
the potential energy of interaction between atoms in the solid is asymmetric about the equilibrium positions of atoms
the frequency of vibration of the atoms increases
the heating generates a thermal gradient between opposite sides
a fluid called the caloric flows into the interatomic spacing of the solid during heating there by expanding it
Solution
(a)
Potential energy versus separation plot for two atoms in a solid is as shown below.
From above graph, we see that as temperature rises $\left(T_3 > T_2 > T_1\right)$, total energy of atoms also increases $\left(E_3 > E_2 > E_1\right)$
As a result, mean separation between atoms also increases $\left(r_3 > r_2 > r_1\right)$.
Hence, crystalline solids in general expands on heating.
