# Water steam drops after adding ice cubes

Whenever I fill a glass with cold tap water and then add some ice cubes to it, I usually make the following observation:

After a few minutes, the glass' outer surface has (what I assume to be) a layer of (tiny) water steam drops on it.

How does this happen?

This seems to be the same kind of thing when steam condensates to water steam drops on a mirror after a hot shower in a bathroom. In that case I can perfectly see how the steam is reaching the mirror.

But I am failing to understand how the cold water from inside the glass is getting onto its surface.

## 2 Answers

After a few minutes, the glass' outer surface has (what I assume to be) a layer of (tiny) water steam drops on it. How does this happen?

It is not steam that you observe, but the condensation of water vapor in the air in contact with the glass. It is the same phenomena you observe when you see dew on the grass in the morning when night time temperatures fall overnight after a hot humid summer day.

Atmospheric air consists of nitrogen, oxygen, argon, carbon dioxide and small amounts of other gases. But it also contains a variable small amount of water vapor, which is basically gaseous $$H_{2}0$$ molecules, i.e. water in the gas phase.

Water molecules can stay in the gaseous phase in air as long as the temperature of the air is greater than the "boiling/condensation" temperature corresponding to the saturation pressure of the water vapor in air. You are undoubtedly familiar with the fact that water boils/condenses at 100 C at atmospheric pressure (approx 101.3 kPa) at the surface of the earth. 100 C is the saturation temperature corresponding to atmospheric pressure, which is the saturation pressure.

Well, the boiling/condensation temperature is only 20 C at a saturation pressure of 2.339 kPa. The partial pressure of water vapor in air is generally lower than 2.339 kPa unless the relative humidity is 100 %.

So when your glass of water is at 20 C, or approximately room temperature, and the relative humidity is less than 100 %, there will be no condensation on the surface. But when you add ice and lower the temperature of the water below the saturation temperature corresponding to the saturation pressure of the water vapor in the room, condensation will appear. We say you have reached the "dew point" of the water vapor in air.

Hope this helps.

The cold water from the inside the glass is not getting onto its surface, it just cools the glass, the glass, in its turn, cools air near it, and the moisture condenses from the air.