Blog

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!

You will need

What to 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.