Where did the puddle go?

About this video

After a morning shower there is a big puddle on the path. By the afternoon the sun is out and the puddle is gone, though nobody mopped it up. The water turned into a gas and went into the air. A liquid slowly turning into a gas at its surface is called evaporating, and water as a gas is called water vapour. Water vapour is invisible: you can't see it, but the air around you always holds some.

Evaporating happens at any temperature, but it goes faster when it is warm, when the wind blows and when the water is spread out thin. That is why washing dries fastest on a warm, windy day.

Heat water in a pan and at 100 °C (212 °F) at sea level it boils: bubbles form inside the water, rise and burst. The bubbles are not air; they are bubbles of water vapour. Look closely and there is a clear gap just above the water. That gap is the invisible water vapour. The white cloud above it is something else: tiny drops of liquid water, formed when the vapour met the cooler air.

Water vapour turning back into liquid water is called condensing, and it happens when the vapour touches something cold. Take a cold drink out of the fridge on a warm day and drops soon cover the outside of the glass. The water didn't leak through the glass: it came from the air. The mist on a bathroom mirror after a hot shower is condensing too. Either way, the water is never lost. It only changes state.

Transcript

1. 0:00In the morning, a puddle. By the afternoon, it's gone. Nobody mopped it up.
2. 0:09The water turned into an invisible gas, water vapour, and went into the air. Warmth, wind and spreading it thin make it faster.
3. 0:19Heat water to a hundred degrees and it boils. The bubbles are water vapour. The clear gap is vapour. The white cloud is tiny drops.
4. 0:30A cold glass collects drops. They didn't leak through. They came from the air.
5. 0:39Evaporating goes one way. Condensing comes back.
6. 0:46The water is never lost. It only changes state.