Final answer:
When water on a sidewalk surface turns to gas, it undergoes evaporation.
Step-by-step explanation:
When water on the surface of a sidewalk turns from a liquid to a gas, it undergoes evaporation. Evaporation is the process in which a liquid is converted to a gas below its boiling temperature. The sun heats the water and provides enough energy for water molecules to escape into the atmosphere as water vapor.