Slowing down molecules
In stormy weather, raindrops trickle down the outside of the window. But
sometimes, in very cool weather, the inside of the window is cloudy
and wet. Where does the film of water on the inside of the window come
from?
The cloudy film of water comes from water vapor [(vay]{.smallcaps}
puhr)—water molecules mixed with the air inside the house. The water
collects on the windows when the glass is cool.
Water vapor is a gas. The molecules of water vapor are as warm as the
air around them in the house, so they move very fast. But when the
molecules hit the cool glass in the window, they lose heat. As the
molecules grow cooler, they slow down and move closer together. When
they are moving slowly enough, they condense (kuhn [dehns),]{.smallcaps}
or turn into tiny drops of liquid.
Sometimes, when the weather is very cold, the glass in the window gets
much colder than the air inside the house. Then the molecules of water
vapor lose even more heat when they touch the glass. They slow down much
more and move much closer together. When they get close enough to pull
hard on each other, they freeze. Then the window is covered with
frost—thin, feathery bits of solid ice.
