Water in the Air Investigation 6

CONDENSATION Water can exist in three phases or states: solid ice, liquid water, and gaseous vapor. When water changes from gas to liquid, it is called condensation Water vapor condenses to form liquid water. Dew on cups of ice water is water that condensed from gas to liquid.

Evaporation The process by which liquid water becomes a gas called water vapor.

Evaporation Cont. For evaporation to occur, heat energy has to transfer to the water. The heat comes from the surrounding environment (water in cloth, cloth, thermometer, and air) The heat energy was absorbed by the molecules in the liquid water. The extra energy caused the molecules to speed up and evaporate. The water vapor in the air increases when water evaporates from the surface.

Humidity Water vapor in the air is called humidity. The air is humid when there is a lot of water vapor in the air (makes you feel sticky). The amount of water vapor in the air changes as the temperature changes. The warmer the temp, the more water vapor air can hold. The colder the temp, the less water vapor it can hold.

Humidity cont. When air contains as much water vapor as it can, it is saturated. It takes a lot of water vapor to saturate warm air, but only a little bit of water vapor to saturate cold air.

Relative Humidity Measured in percentages Compares the amount of water vapor in the air to the amount of water vapor needed to saturate the volume of air. 50% humidity means that the air contains 50% of the amount of water vapor needed to saturate the air at today’s temperature.

Relative Humidity Example. 1Kg of air at 25°C can hold 20 g of water vapor. If it is holding 10 g of water vapor, the relative humidity is 50%.

Water Vapor A part of air, it gets into the air in several ways: Evaporation of liquid water when it absorbs energy Transpiration of water vapor from plants. All terrestrial animals exhale water vapor while breathing. Fire, internal combustion engines, and volcanic eruptions all add water vapor to the atmosphere.