# How does temperature affect the kinetic energy in gases, liquids, and solids?

Nov 3, 2016

Temperature affects the kinetic energy in a gas the most, followed by a comparable liquid, and then a comparable solid.

The higher the temperature, the higher the average kinetic energy, but the magnitude of this difference depends on the amount of motion intrinsically present within these phases.

IN GASES

In general, the average kinetic energy increases at higher temperatures for gases. Since gases are quite compressible, the effects of higher or lower temperature are significant.

barK = barK_T + barK_R + barK_"vib" + cancel(barK_"electronic")^("small")

• They can all move translationally, which clearly means the average translational kinetic energy (${\overline{K}}_{T}$) increases at higher temperatures.

• Diatomic and polyatomic gases can rotate, which also contributes to its average rotational kinetic energy (${\overline{K}}_{R}$).

• Diatomic gases can vibrate by stretching their bonds, and polyatomic gases can vibrate by stretching and bending their structures, contributing to the average vibrational kinetic energy (${\overline{K}}_{\text{vib}}$).

• The average electronic kinetic energy (${\overline{K}}_{\text{elec}}$) due to electronic transitions is usually negligible because electronic energy levels are large relative to rotational and vibrational energy levels for molecules in their ground state.

IN LIQUIDS

For liquids and solids, it is much simpler.

Since liquids have intermolecular forces binding them together, temperature really only affects the strength of those intermolecular forces, since those forces are restricting the effects of the change in average kinetic energy.

As temperature increases, the average kinetic energy of the liquid molecules increases until the intermolecular forces break. You won't often see noticeable changes in the volume or looseness of the liquid, since they are fairly incompressible.

When the intermolecular forces break and we get to the boiling point, that's when you can have more freely-moving particles, but even then, anything still in the liquid form is still fairly incompressible.

IN SOLIDS

For solids, the rigid nature of the lattices the particles are in restricts their kinetic energy from affecting much of the average motion in the solid.

As temperature increases, the average kinetic energy increases, but we will see hardly any obvious difference in volume or shape. When we approach the melting point, the lattice energies break and allow the particles to move slightly more freely, but still leaving them fairly incompressible.

For solids, temperature changes, in the absence of induced phase changes, usually just manifests itself as temperature changes, and nothing else.