Ask a ScienceBlogger: Evaporating Water

Here is another question from Ask a ScienceBlogger. Reader Uday Panta asks:

How does water evaporate in the seas? Doesn't water evaporate at 100 C?

There were some very good responses in the comments where the question was, but I am going to answer it with some more details.

Small Particle Model

This is where we need to start - the small particle model of liquids and gases. This model treats the liquid as being made up of a lot of particles (well, obviously). If there is a gas (or liquid) at a certain temperature, then there are particles moving around at different speeds. Often it is said that temperature is a measure of the average kinetic energy of the particles in a gas. This isn't too bad of a definition, but the point is that some particles are moving fast and some are slow. They are not all going at the same speed.

Check out this great applet from the PhET simulators.


This is a snap shot of the Gas Properties simulator. The cool thing is the histogram it displays showing the distribution of both the particle energies and particle speeds.


Here is another simulator from PhET. This one shows something in different states of matter. I am just going to focus on the liquid and the gas phases for now. I am going to show a movie of this running. In this video, you will see some water in liquid form evaporating. (Go play with the simulator online too - it is pretty cool.)

So, the liquid water has some water molecules in it. The water is at a certain temperature, but some of the liquid water molecules have more energy that others. That means that some of these molecules have enough energy to break free from the other water molecules and fly free. You can see this in the above animation.

That is really the answer to the question from Uday. The water does not have to be at 100 C for some of the molecules to evaporate.

But won't the water keep evaporating? Well, this depends. First, if you keep adding energy to the water then yes it will. Watch the video again. Follow one water molecule that evaporates. Eventually, it will collide with the liquid water and become 'trapped' again. So, the water will reach an equilibrium between the water molecules that become gas the the gas molecules that become water.

It would be cool if the PhET app could show a graph of the number of gas particles vs. time and the number of liquid particles vs. time.

More like this

This would be a fine time to bring up the concept of vapor pressure. That is the partial pressure at which evaporation and condensation are in equilibrium. (Relative humidity measures how much water vapor is actually present compared with this maximum.) We say that water boils at 100 C because that is the temperature at which its vapor pressure equals the average atmospheric pressure at sea level. If normal atmospheric pressure were lower, water would boil at a lower temperature--which it does if you are in, say, Denver (1600 m above sea level) or La Paz (4000 m above sea level). Conversely, if you put it in a pressure cooker it boils at a higher temperature.

Vapor pressure also exists in the solid phase. You probably don't see this very often near the Gulf Coast, but it's a commonplace here in New England: the temperature remains below freezing for several days in a row, yet the amount of snow on the ground decreases through evaporation. If you were in a place where the atmospheric pressure were lower than the vapor pressure at the freezing point, you will not see a liquid phase: the substance will go directly from solid to gas (this is called sublimation). Carbon dioxide ("dry ice") does this because its vapor pressure at its putative melting point is significantly higher than our normal atmospheric pressure. In an environment with sufficiently high pressure, you can find liquid CO2.

By Eric Lund (not verified) on 09 Jun 2010 #permalink

The most important concept about temperature that I learned in my Physics 1 class is that temperature is an average measure of the energy of all particles involved. As soon as I got this, I was able to figure out why water can evaporate even though it's not 100C. Macro scale can sometimes only be explained by looking at the micro.

I have a son who is doing a science project about different states of matter and this simulator is the perfect thing for him right now. Anyways great post as usual ;)

These simulators are very interesting! While I tried to take your advice and watch the video, follow one molecule and watch it evaporate, I couldn't see it. I didn't see any molecules actually disappear. Regardless, this explanation makes so much sense to me now. Does this mean that each water molecule is technically a different temperature? Does the energy of that molecule directly reflect the temperature of that particular molecule?