Warm Rises, Cold Sinks: A Colorful Science Experiment on Convection for Kids

This is an introduction to the scientific principle of convection—”the movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in a transfer of heat.” A complicated sounding proposition, but one that is easy and fun to demonstrate. Once your child has seen this process at work for themselves it will come in handy for answering many “why” questions about weather and other science topics.

Materials:

• Blue and red food coloring
• Spoon
• A large, clear container that comfortably holds at least 8 cups of water
• About 4 cups of room-temperature water
• 2 cups of water chilled in the refrigerator or freezer (but not frozen)
• 2 cups of water heated on the stove or in the microwave (but still safe to touch)

Explain to your child how warm liquids (like water) or gases (like air) rise while colder liquids or gases sink, and that you will be doing a scientific demonstration (How grown-up that sounds!) to see that for yourselves. Tell them this is called convection. They may not remember that, but it is planting an important seed for later. Next time this topic comes up (and it surely will), you can remind them what it’s called.

Set the large container on the kitchen counter or somewhere about at eye level for your child. Add about 4 cups of room-temperature water. (NOTE: Tap water is not necessarily room temperature. You may want to prepare this beforehand and let it sit for an hour or two.)

Take your 2 cups of previously chilled water and add a few drops of blue food coloring. Let your child stir thoroughly.

Take your 2 cups of warmed water and add a few drops of red food coloring. Let your child stir thoroughly.

Carefully pour the blue water onto one side of the large container and pour the red water onto the opposite side. Pour slowly!

Watch the magic happen as the red water spreads and rises to the top, and the blue water sinks and covers the bottom. Be careful not to disturb the container and check back every few minutes. Eventually, there will be a middle layer that starts to mix into purple. Bonus color mixing lesson there!

Teaching moments:

Convection is a very helpful principle for a young child to understand because it relates to so many questions they ask and so many interesting inventions. Here are a few:
Natural phenomenon:
- Clouds
- Thunderstorms
- Rain
- Wind
- Ocean currents

Inventions
- Hot air balloons
- Air conditioners
- Convection ovens
- Hot water heaters

Other easily observable examples
- Steam rising from hot cocoa
- Water boiling on the stove
- Cold air spilling to the floor when the freezer is opened