OS11 Floating Hot Water on Cold Water

If you pour water into water, it mixes up and has more volume. That is what usually happens. But if you are really up to the challenge, you can make hot water sit on top of cold water.

jar of hot water, jar of cold water

Question: How can hot water be made to sit on cold water?


3 Jars



Food coloring

Microwave or stove

Eye dropper


Half fill a jar with water

Add ice until some ice remains floating in the water

Half fill another jar with water

Heat this water close to boiling in the microwave (use a pan on the stove)

Add a drop of food coloring

food coloring in hot water

Sometimes, when you heat water on the stove, you can look down into the pan and see the water moving. These currents are in the jar of hot water and carry the food coloring around.

How does the food coloring spread through this water?

Stir to finish mixing the coloring into the water

Half fill a jar with ice cold water with no ice in it

This is the hard part: You will use the eye dropper to add the hot water to the jar of cold water. Do this by sliding the water down the side of the jar. Continue adding water until the hot layer is 1.5 cm thick.

hot water on cold water

The hot water tries to stay up on top of the cold water. Right underneath the hot water is an area cooling off with a layer warming up so some of the food coloring goes down into these layers creating the lighter colored layer.

Observe the hot water layer every fifteen minutes for an hour


Describe how the food coloring moves through the hot water

Describe what the hot water layer does

At the start

In fifteen minutes

hot water mixing with cold water

Cold water has currents in it like the hot water does. That lowest layer of food coloring is getting pulled into these currents.

In thirty minutes

In forty-five minutes

In an hour


Note: Density is how much stuff is in a certain volume. Something with less density will float on top of something less dense.

Is hot water more or less dense than cold water? Why do you think so?

Is this difference very much? Why do you think so?

What happens to the temperature of the hot water over the hour?

What will happen to the density of the hot water over the hour?

What will happen to the food coloring over time? Why do you think so?

food coloring is mixed

The hot water cools off. The cold water warms up. The water currents carry the food coloring around until all of the water turns blue. Some of the currents are still visible in the center.

What I Found Out:

The drop of food coloring split into many long streamers in the hot water. They slowly moved around the jar as they sank toward the bottom. The streamers spread out through the water.
The drops of hot water went into the cold water a little bit then rose to the top. The layer of hot water spread across the cold water. The color was darker on the top of the hot water layer.
I stopped adding hot water when my layer was 1.5 cm thick. Gradually a layer of lighter blue water spread under the hot layer. It got almost as thick as the hot layer started out. Streamers sank toward the bottom. A center core of blue went from the hot layer to the bottom. Then the blue spread all through the water.
The hot water must be less dense than the cold water because it stays on top. The densities must be very similar because I had to be so careful not to mix them when I put the hot water in and a layer at the edge does mix a little.
As the jar sits on the table, the hot water cools off. The cold water warms up. As they get closer together in temperature, their densities get closer and the hot and cold water mix. Then the food coloring will spread throughout the water.