The Amazing Water Trick – Investigating Density

density of hot and cold water

This fun science trick demonstrates that hot and cold water have different densities.

The demonstration reminded my kids that lower density liquids float above higher density liquids. They learned that hot water molecules move faster and further apart from each other than cold water molecules.

The trick also allowed us to revisit the topic of surface tension and to build on the air pressure science we learned last term.

What you need

  • 2 identical jars or drinking glasses (eg babyfood or salsa jars)
  • hot and cold water
  • red and blue food colouring
  • index cards or similar
  • scissors
  • a large shallow dish to catch spills

What you do

1. Cut a square of card big enough to completely cover the mouth of a jar.

2. Fill one jar with cold water. Add a drop of blue colouring and stand the jar in the baking pan.

2. Fill the second jar with very hot water. Add a drop of red colouring.

3. Slowly add more hot water to the red jar until you see a bulge of water over the rim. (Take the opportunity to talk about surface tension here.)

4.  Lay the square of card on top of the jar of hot water and tap it gently.

5. Ask your students to predict what will happen when you quickly turn the jar upside down. Then invert the jar, and discuss how air pressure and surface tension keep the card in place and the water in the jar.

6. Put the upside-down red jar on top of the blue jar.  Ask your students to predict what will happen when you slowly pull out the card, then do so.

fun water experiments

Atmospheric pressure holds the card and water in place (left)

What happens

When you remove the card between the two jars, the hot red water stays in its jar on top and the cold blue water stays on the bottom.

fun water experiments

Hot water floating on top of cold water

Reverse the jars

7. Repeat the experiment the other way round. Turn the jar of cold water upside-down and put it on top of the hot water jar. Again, ask students to predict what will happen when you remove the card.

What happens

When you remove the card, the water quickly turns purple, as the cold and hot water mix together.

fun water experiments

Hot and cold water mix

The scientific explanation

When water is heated, the water molecules move faster. They bounce off one another and move further apart. The extra space between the molecules means the same volume of water weights slightly less and is less dense than cold water.

When we put the hot water on top, the less dense hot water floats on top of the denser cold water. The red water and blue water stay in their respective jars and do not mix.

When we put the cold jar on top, the more dense cold water immediately sinks to the bottom and the two colours mix.

We talked about a previous demonstration in which we created a column of liquids with different densities, and reminded ourselves by pouring water into a glass of oil and watching the oil float to the surface.

fun water experiments

Oil floating on water

More fun water experiments

The Tiger Chronicle recently did a bunch of fun water experiments. I especially like the supercooled water video.

All Things Beautiful explored density as part of their science investigations that all students should do before high school series.

I found the Amazing Water Trick at Exploratorium.

fun water experiments

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Fun water experiments

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