Final answer:
Liquid water changes into the next phase called vapor when enough energy is added, which is a process known as vaporization or more specifically evaporation at temperatures below the boiling point.
Step-by-step explanation:
When enough energy is added to liquid water, it will change into the next phase called vapor. This transition is known as vaporization. At temperatures below the boiling point, the formation of vapor from a liquid is specifically called evaporation. Therefore, the correct answer to the question is 'vapor', which reflects the material in the gas phase when the gas phase is not the stable phase.