According to the molecular theory of matter, objects absorbing thermal energy have molecules with increasing kinetic energy. This causes the object to expand. All three states of matter undergo thermal expansion. Solids expand in a linear fashion, in an areal fashion (superficial), and in a volumetric (cubical) fashion. Liquids and gases undergo only volumetric expansion. The equations for these thermal expansions are listed below.
| Phase of Matter | Type of Expansion | Equation |
| Solid | Linear | ΔL= L0 α Δ T |
| Solid | Areal | ΔA = A0 2α Δ T |
| Solid | Volumetric | ΔV = V0 3α Δ T |
| Liquid | Volumetric | ΔV = V0 β Δ |
| Gas | Volumetric | PV = nRT |
- Δ L, Δ A; Δ V represent the change in length, area, and volume, respectively
- α and β are coefficients of expansion
- L represents length
- Δ T represents change in temperature
- n represents moles of gas
- R represents the ideal gas constant