Final answer:
The expansion of water when it changes from a liquid state to a gas state is a result of the increase in kinetic energy of the water molecules, allowing them to overcome intermolecular forces and move farther apart.
Step-by-step explanation:
The expansion of water when it changes from a liquid state to a gas state can be explained using the particle theory. The kinetic energy of the water molecules increases as the temperature rises, allowing them to overcome the intermolecular forces holding them together. This increase in kinetic energy leads to an increase in molecular spacing as the water molecules move farther apart, resulting in the expansion of water.