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.