ASK A SCIENTIST
Question: How does water evaporate?
Answer: Water is a remarkable substance. It is quite uncommon in the universe, but there is so much of it on Earth that it can be easy to overlook how special it is. The way that one water molecule (the smallest piece you can get that is still water - much smaller than you can see with a typical microscope) interacts with other water molecules is key to answering your question. Although the particles in all liquids have some degree of attraction for one another, water provides an example of a molecule that is really good at sticking together with other water molecules, which affects how water behaves compared with other liquids.
In order to answer your question about how water evaporates, let's first consider what is going on. Evaporation is the term we use to describe the process of particles at the surface of a liquid breaking free from the liquid and becoming a gas. It may look like the liquid water 'disappears', but it is really going from a liquid we can see to an invisible gas. In order for the water molecules to break free, they must have enough energy to overcome the attraction they have for each other since the water vapor molecules are much farther apart than the liquid water molecules. This is how evaporation happens, by molecules moving fast enough to break free from the attraction of other liquid particles at the surface. The rate of evaporation depends on temperature, which measures how fast the typical water molecule is moving. Evaporation happens faster at higher temperatures since the average molecules are moving faster. Whatever the average speed, some water molecules will move faster than average while others will move slower than average. Only the fastest moving molecules will have enough energy to become a gas by escaping at the liquid's surface. We have all probably seen water boiling at some point. Boiling refers to the same change of liquid to vapor, which we call a phase change, but here it happens anywhere in the liquid, not just at the surface. This means that even an average molecule has enough energy to become water vapor. This is why a pot of liquid water brought to a boil on the stove will completely 'disappear' into the vapor phase much more quickly than if the same pot of water was left at room temperature to evaporate.
Now that you understand a little more about evaporation, here's an experiment you can try at home. First be sure to get permission from an adult; better yet, ask an adult to be your assistant for the experiment. Place a drop of water in a small plastic cup. In another cup, place a drop of rubbing alcohol, which you should be able to find at any pharmacy or grocery store. Now watch each drop and keep track of how long it takes for one of the drops to 'disappear' - to become a gas. Use your results to explain which liquid is better at sticking together. Don't forget to throw the cups away and wash your hands after your experiment.
Ask a Scientist appears Thursdays. Questions are answered by faculty at Binghamton University. Teachers in the greater Binghamton area who wish to participate in the program are asked to write to Ask A Scientist, c/o Binghamton University, Office of Communications and Marketing, PO Box 6000, Binghamton, NY 13902-6000 or e-mail email@example.com. Check out the Ask a Scientist Web site at askascientist.binghamton.edu. To submit a question, download the submission form(.pdf, 460kb).