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Q & A: cool water evaporates

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Q:
Why does water evaporate if you leave it outside, even if it is not hot outside? Why does the surface area affect how fast the water evaporates? Is it because more is exposed to the heat or exposed to the air?
- Joshua (age 11)
K.B.Polk Elementary, Dallas, Texas
A:
There's an awful lot of room for water molecules in the air. Any that happen to fly up in the air are very unlikely to return once they wander a bit away from the starting place. Unless there are already a lot of water molecules in the air, the flow will be one way, from the liquid to the gas. In order to keep there from being net evaporation, there must be a flow back from the air to the water.
The concentration of water molecules in the air is often given by the relative humidity. At 100% relative humidity, the flow of molecules from a cup of water into the air will just balance the flow from the air to the cup, if theyíre both at the same temperature. Itís true that the rate at which the water molecules leaves goes up quickly as the water gets hotter and the rate at which the molecules returns only goes up a little as the air gets hotter. So the evaporation is much faster when the water is hotter, and 100 % relative humidity means a higher concentration of water in the air when itís hot than when itís cold. So thatís probably where the impression arises that water shouldnít evaporate when itís cold.

One interesting case of cold water evaporating occurs in your freezer. Ice cubes directly evaporate, although we use the word "sublimate" instead. Even though ice is a solid, its molecules are rattling around and occasionally fly off the surface, just as they do in a liquid.

All the evaporation occurs at the surface, so thatís a big reason why the rate depends on the surface area. Itís true that the evaporation cools the water, which would reduce the evaporation rate if no heat flowed in. As you say, heat can flow into the exposed surface, but it can also flow in from the container. So I bet thatís a less important reason for the surface area to matter.

Mike W.

(republished on 07/24/06)

Follow-Up #1: cold evaporation

Q:
why we have evaporation in cold water i.e. from water gullies in winter and from ice when we take a piece out of freezer and why this increases when we low the tempreture???
- Pedram
Tehran
A:
As we discussed in the previous answer, when water is cold, even frozen, the molecules are still rattling around. Occasionally, one gets enough energy to break loose and fly off the surface. The colder the water or ice, the less often this happens.

You raise an interesting point when you ask why evaporation happens more at lower temperature.  Generally speaking, it doesn't. However, if a particular weather pattern sometimes brings very dry cold air and sometimes moist warm air, you could get more net evaporation in the cold. The reason would be that in these particular conditions the warm weather would allow more water to flow back from the air to the liquid. The net evaporation rate depends on the difference between the rate molecules leave the surface and the rate they come in from the gas.

Mike W.

(published on 08/23/09)

Follow-Up #2: water to vapor

Q:
The books teach us that water changes phase into a gas at 100 degrees C. But that's not true because cool water in a saucer also evaporates. Why do the books not include this in the section that covers phase change? Is phase change at standard temperatures a hoax? Can you explain?
- Alistair
South Africa
A:
Since that question confused almost all of us when we first learned about phase changes, I'm very glad you asked.

The 100įC phase change occurs for pure water at a fixed pressure of 1 atm.  Above 100įC, molecules leaving the liquid outnumber those going from the gas to the liquid. The gas grows until all the liquid is gone, and the container (at fixed pressure) expands. Below 100įC the opposite happens.

If you kept the container at a lower pressure, the same thing would happen but at a lower temperature.

What's different about the saucer case? There the pressure of pure water in the gas is much less than 1 atm. The total pressure is 1 atm, but most of it comes from air molecules. So evaporation will beat condensation at much lower temperatures than 100įC.

Notice that if what I'm saying is right, then whether the amount of water in the saucer grows or shrinks should depend not only on the temperature but also on how much water vapor is in the nearby air. At a certain water vapor concentration (called 100% relative humidity) the evaporation and condensation just balance.

Mike W.

(published on 11/06/10)

Follow-up on this answer.