Does warm air really go up?
Yes. Warm air and warm water spread out and get lighter for their size, so they float up. Cooler air and water sink down. You can watch it happen with colored water.
⏱ About 15 minutes. 💧 Can drip, so work on a tray. Grown-up helps: getting warm tap water.
2 pages: the experiment, then a page to write your results.
What you need
- A clear plastic container or big clear plastic cup (a see-through food container works great)
- 2 small plastic cups
- Food coloring, two colors (red and blue are easy to see)
- Warm tap water (a grown-up gets it, bath warm)
- Cold tap water with a few ice cubes
- A spoon and a tray or towel for drips
Steps
- Fill the big clear container about halfway with room-temperature tap water. Let it sit still for a minute.
- Ask your grown-up to fill a small cup with warm tap water. Add 2 drops of red food coloring.
- Fill the other small cup with icy cold water. Take out the ice, then add 2 drops of blue food coloring.
- Very slowly, with a spoon, drip some red warm water into one side of the big container. Watch where it goes.
- Now slowly drip some blue cold water into the other side. Watch where it goes.
- Do not stir! Watch for 2-3 minutes from the side, at eye level.
What you will see
The red warm water rises and spreads out across the top. The blue cold water sinks and spreads along the bottom. For a while you may see a red layer on top and a blue layer on the bottom, like a striped drink. After a long time, the colors slowly mix as the water evens out.
Why it works
When water or air warms up, its tiny pieces (called molecules) move faster and spread out. That means the same amount of space holds less stuff, so warm water or air is lighter for its size. The word for “how much stuff fits in a space” is density (DEN-sih-tee).
Lighter-for-its-size things float up, and heavier-for-its-size things sink. So warm water floats up and cold water sinks. Air works the same way. When warm and cool air or water move around like this, it is called convection (con-VEK-shun). Hot air balloons float for the very same reason.
How your house uses this
- Upstairs feels warmer. In a two-story house, warm air drifts up, so upstairs is often warmer than downstairs, especially in summer.
- The attic gets very hot. On a sunny Austin summer day, the attic is the hottest place in the house. The sun heats the roof, and warm air collects up there.
- Your heater and AC push air around. Because warm air rises and cool air sinks, rooms do not mix by themselves very well. The fan in your heating and cooling system blows air through the ducts and out the vents in the ceiling so every room stays comfy.
- Air needs a way back. Air also has to get back to the machine through the big air grille called the return, usually on a hallway wall or ceiling. That is why we keep furniture, toys and laundry away from it.
Learn more in How does heating work? and The Air Trail.
A grown-up gets the warm water from the tap and checks it first.
Try this next
- Balloon on a bottle. Stretch a balloon over the mouth of an empty plastic bottle. A grown-up sets the bottle in a bowl of warm tap water. Watch the balloon. Then move the bottle to a bowl of ice water. What happens? (Warm air spreads out and pushes the balloon up. Cool air shrinks back.)
- Paper spiral. Cut a spiral from paper (a grown-up helps with scissors) and tie a thread to the center. When the heat is running, a grown-up holds the spiral up under a ceiling vent. Does it spin? That’s the fan pushing air down. Warm air still wants to rise, so where do you think it goes after it leaves the vent?
- Upstairs, downstairs. If you have stairs and a kitchen thermometer, have a grown-up help you check the temperature at the bottom and top of the stairs. Write both numbers down.
- Bottom vs top. In the colored water lab, try warm water on the bottom first. Can you guess what happens?
Share what you found
Show your grown-up the color layers. Ask them which color they think is warm before you tell them.
2 pages: the experiment, then a page to write your results.
Keep exploring
- Keep It Cold Race: how attic insulation slows heat down.
- The Why Bank
- Hero Words: convection, density and more.
- Science Fair Ideas
- Back to the Kitchen Table Lab
