Condensation

Aavi, Thirav and their friends make lemonade with cold water and ice cubes in a glass tumbler. A few minutes later, tiny water droplets appear on the outside of the tumbler and join into bigger drops. Take a cold bottle out of the fridge and it does the same. Where does this water come from? Did it seep through the glass?

You have seen drops like these before. Dew forms on plants, mostly in the morning. Boil water in a half-filled pot covered with a steel plate, and drops collect on the inside of the plate. When water vapour in the air touches a cold surface, it forms water droplets. The change of water vapour into liquid water is called condensation.

Aavi and Thirav test their idea:

• They cover a tumbler of ice water with a small steel plate and weigh it on a digital balance every five minutes for 30 minutes. The reading goes up as droplets form outside.
• They repeat it with the water level marked on the tumbler. The level inside does not go down, yet extra water collects on the outside.

So the water is not seeping out of the tumbler. It comes from the water vapour in the air, which condenses on the cold glass. That is also why the tumbler gets heavier.

Machines called Atmospheric Water Generators (AWG) use the same idea: they cool humid air so that its water vapour condenses, and collect it as drinking water.