Phase changes

The states of matter depend on the of its intermolecular forces (IMF) and kinetics energies (KE) of its particles -either molecules, ions, or atoms. The chemical identities of the particles in a liquid determine the types and strength of intermolecular attraction possible.

The change of energy in a system determines the phase changes. To change from a certain state to another state of matter, we must add enough energy to overcome the intermolecular forces that holding the particles together.

Figure 1. The phase changes.

Melting The energy required to break the intermolecular forces and increase the motion of the particles during solid to liquid transition is called heat of fusion, or enthalpy of fusion (ΔH_fus). In chemistry, melting is known as fusion.

Vaporation The degree of particle movement keeps increasing as the temperature rises. The motion of the particles becomes more rapid and spread apart as the particles move into gaseous states. At this point, even less intermolecular forces keep the molecules together. The transition from liquid to gas is called evaporation. The energy involved in this phase changes is called heat of vaporization, or enthalpy of vaporation (ΔH_vap).

Sublimation Some solids can skip the liquid phase and change to the gaseous state directly. The transition from solid to gas is called sublimation. The energy required in sublimation is called heat of sublimation, or enthalpy of sublimation (ΔH_Sub).

Generally, the heat of sublimation is a lot larger than the heat of vaporation, with the heat of fusion as the smallest value. This is because all the intermolecular forces in the gaseous states are severed, while in liquid some are intact, and in solid most are unbroken. It requires more energy to sever all intermolecular forces between the particles.