A jar of blue water in a fridge.

Place the jar in the fridge to cool down.

By Mike McRae

This activity uses water and food colouring to explore fluid systems. It’s a good idea to do this activity in a safe place outside, and to wear old clothes while you do it!

hot hazard iconSafety: This activity uses hot water, so younger scientists should ask an adult to help.


You will need

  • Clear jar or cup
  • Tap water
  • Fridge
  • Petroleum jelly
  • Food colouring

    Someone is running their finger around the rim of the jar.

    Smear a thin layer of petroleum jelly around the rim of the jar.

  • Long, clear rectangular container (can be glass or plastic)
  • Jug

What to do

  1. Pour a few centimetres of water into the jar.
  2. Add a few drops of food colouring and place the jar in the fridge to cool down.

    Someone is holding an upside-down jar tot he bottom of a flat container.

    Hold the jar firmly to the container and flip them over.

  3. When the water is cold, remove the jar from the fridge. Use your finger to smear a thin layer of petroleum jelly around the rim of the jar.
  4. Hold the jar in one hand and place the upturned container over the jar so that the floor of the container sits completely covers the opening of the jar.
  5. Holding the jar and the container firmly together, flip the jar over and place the container on a table or bench. You might need someone to help you. If you have done it correctly, the water shouldn’t leak out.

    A container of water, with dark blue water on the bottom and lighter water on top.

    What does the coloured water do when you lift the jar?

  6. Fill the jug with hot water from the tap. If the jug is a bit heavy, ask an adult to help.
  7. Pour the hot water into the container. Keep adding hot water until the level of the water in the container matches that in the jar.
  8. Holding the container steady, smoothly pull the jar up a little so that the water in the jar can escape. Try not to make too many ripples in the water. What does the coloured water do?

What’s happening?

Although both the jar and the container have water in them, the water is at different temperatures. This difference in temperature results in the two bodies of water having different densities.

The density of a fluid is the amount of mass it has per unit of volume. When two fluids of different densities mix, the fluid with the greater density will tend to sink below the less dense fluid.

Cold water has a greater density than hot water. So when the cold water is released into the hot water, instead of mixing evenly, it spreads out and forms a layer under the warmer water in the container.

Eventually you’ll notice that the water mixes completely. This is because as the two bodies of water mix, their temperatures change to an temperature somewhere between their starting temperatures. The water now has a more uniform density, and layers merge into each other.

Real-life science

Differences in densities are also important in a number of fluid systems, including the currents in Earth’s oceans, and the air in the atmosphere. Together, these two systems have a major impact on Earth’s weather and climate patterns.

In the case of the atmosphere, when a body of cold air meets a body of warm air, the cold air will flow underneath the warm air, lifting the warm air up. The boundary between the warm and the cold air is called a cold front.

Bodies of warm air often contain greater amounts of moisture. As the warm air rises above the cold air, the changes in temperature and pressure can cause the formation of clouds. Cold fronts are often associated with drops in temperature, increased winds, and rain.

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