Why does a cold cup get wet on the outside?
The air around you carries water you cannot see. When that air touches something cold, the water turns back into tiny drops. Your air conditioner does the very same thing every summer day.
⏱ About 10-15 minutes. 💧 A little wet. Grown-up helps: getting the ice.
What you need
- 2 clear plastic cups that look the same
- Ice cubes
- Tap water
- A paper towel
- A plate or tray to catch drips
- Optional: a few drops of food coloring
What most people guess
Lots of people think the water leaks through the cup. Keep that guess in mind. This lab will test it.
Steps
- Dry both cups with the paper towel, inside and out. Set them on the plate.
- Fill one cup with ice, then add cold tap water to the top. This is your cold cup.
- Fill the other cup with room-temperature tap water and no ice. This is your plain cup.
- Optional: add a drop of food coloring to the water in the cold cup.
- Wait about 10 minutes. Do not touch the cups while you wait.
- Look closely at the outside of each cup. Then gently touch the outside with one dry finger.
- If you used food coloring, look at the drops on the outside. Are they colored too?
What you will see
The cold cup gets foggy, then tiny drops of water show up on the outside. They may grow and roll down to the plate. The plain cup stays dry.
If you used food coloring, the drops on the outside are clear, not colored. So the water did not leak out of the cup. It came from somewhere else!
No drops? That’s a result too. On a very dry day, there may not be enough water in the air to make drops. Write it down and try again on a muggy day. Comparing two days is real science.
Why it works
Air always has some water floating in it as a gas you cannot see. This gas is called water vapor (VAY-per). How much water is in the air is called humidity (hyoo-MID-ih-tee).
When warm, wet air touches the cold cup, the air right next to the cup cools down. Cool air cannot hold as much water vapor, so some of it turns back into liquid drops on the cup. Gas turning into liquid is called condensation (con-den-SAY-shun). Clouds, fog and morning dew on the grass all form the same way.
How your house uses this
Inside your air conditioner there is a part called the cold coil. It is like a giant, very cold cup made of metal tubes and thin metal fins. A fan pushes warm air from your house across the coil.
- The air gets cooler, just like the air next to your cold cup.
- Water from the air turns into drops on the coil, just like the drops on your cup.
- The drops drip into a pan and run out through a pipe called the drain line.
That is why air conditioning makes a house feel less sticky. It cools the air and also pulls water out of it. In Austin summers, an AC can pull out a lot of water in one day.
Want the whole story? Read How does air conditioning work?
Use plastic cups if you are under 8, and wipe up drips so nobody slips.
Try this next
- Muggy day vs dry day. Do the lab on two different days. Which day made more drops?
- Bathroom test. After a warm shower, put your cold cup in the bathroom. Does it get wet faster?
- Salt ice. Ask a grown-up to help you add a spoon of salt to the ice water. Salt makes ice water even colder. Does frost show up on the outside?
- Cup race. Try a metal cup next to a plastic cup, both with ice water. Which one gets wet first? Metal moves heat faster.
- Window watch. On a cold winter morning, look at the inside of a window. Do you see drops? That is condensation too.
Share what you found
Show your grown-up the drops. Ask them: “Where do you think the water came from?” Then tell them the answer.
My guess before I started:
What I saw after 10 minutes:
I shared what I found.
Keep exploring
- Cool by Evaporation: water going the other way, from liquid to gas.
- Hero Words: condensation, humidity and more.
- House Mysteries: more strange things your house does.
- Science Fair Ideas: turn this lab into a project.
- Back to the Kitchen Table Lab