What causes the change of state?

Answer 1

A simplistic summary of a very complex process would be:

As heat is added to a solid material matrix, the kinetic energy of its molecules becomes greater than the intermolecular forces that hold those molecules. With this a change in phase occurs.

For example, in a solid crystal, the molecules are held rigidly in position by the intermolecular forces. As the heat of the matrix increases, the molecules become more energetic and the movement around their equilibrium positions becomes faster and larger. Eventually, their enhanced motion permits them to create spaces in the crystal that other molecules can move into - the solid has now become a liquid (a different phase).

As the heat increases even more, the increased kinetic energy of the molecules allows them to completely overcome any intermolecular forces and atmospheric pressure, and the material begins to become a gas and escapes the matrix.

Cooling (removing heat) from a gas to allow condensation and then crystallization is the opposite process.

Answer 2

Changes in state (such as from solid to liquid or liquid to gas) occur due to a transfer of energy to or from the substance. This energy can be in the form of heat or pressure. When energy is added, particles in the substance gain enough kinetic energy to overcome the forces holding them together, resulting in a change of state.