Final answer:
A humidifier uses evaporation to add molecular water to the air by converting liquid water into water vapor, which increases the relative humidity.
Step-by-step explanation:
A humidifier adds molecular water to the gas phase through a process known as evaporation. Evaporation occurs when liquid water gains enough kinetic energy to overcome intermolecular attractions and enters the gas phase as water vapor. This can occur at temperatures below the boiling point of the liquid. In contrast to evaporation, condensation involves the change of state from a gas to a liquid, compression refers to the increase of pressure on a gas, and filtration involves the physical removal of particles from a substance, none of which describe how a humidifier works.