Dr. Universe: Why do mirrors fog up when you breathe on them? -Zinnia, 7, Richmond, Virginia
Dear Zinnia,
That’s a great observation. When you breathe out, you let a couple of different things into the air.
Not only do you breathe out carbon dioxide, but you also breathe out teeny tiny droplets of water. These water droplets are so small we can’t see them with our eyes.
Scientists actually have a name for these little droplets of water in the air: water vapor. You may remember from our question about the states of matter that there are all kinds of different gases, liquids and solids in our world. Water vapor is a kind of gas.
My friend Cigdem Capan, a physics instructor at Washington State University, said one big factor that can help water move between these different states of matter is temperature.
When you breathe on a mirror, you are helping water move from a gas state to a liquid state. The surface of the mirror is a lot colder than the water vapor that comes from your warm human body. If you breathe on a mirror, you can easily feel that heat releasing into the air.
As water vapor in your breath reaches the mirror’s cool surface, the vapor droplets come together to form a liquid. When this happens, you can see thousands of super tiny liquid droplets form on the mirror: the fog.
Scientists also call this transition from a gas to a liquid, condensation. It’s the same process that helps form big, fluffy clouds in the sky, tiny drops of morning dew, or the water droplets on the outside of your cool water glass.
“If you are wearing eyeglasses and you are wearing a face mask, you can also see the glass fog up,” Capan said.
That’s condensation, too. While you may not always be able to see the water vapor from your breath, when the temperatures drop it is a bit easier to observe this condensation in action.
It’s been pretty cold here in the Northwest, so I’ve noticed this happen when I go outdoors. As we breathe into the chilly air, the warm water vapor condenses into tiny droplets of liquid water—and even some solid water, or ice—that form a kind of miniature cloud. It’s pretty fun to watch.
Whether you fog up the cool air, a window or your glasses, you may have also noticed that the moisture doesn’t stick around forever.
Try breathing on the surface of a glass mirror or windowpane and watch what happens. Eventually, the liquid droplets disappear from the mirror. Why do you think that might be?
Share your ideas with your friends or family, and see if you can work together to figure out where those water droplets go. If you need a hint, do a little bit of research on how puddles dry up or investigate the water cycle on our planet. Tell us what you discover at Dr.Universe@wsu.edu
Sincerely,
Dr. Universe
Know a kid with a science question?
With help from my friends at Washington State University, we’re investigating tough and smart questions from curious kids around the world.