When part of a liquid or gas is heated, it expands and becomes less dense. The warmer, less dense liquid rises upwards, and the cooler liquid falls to take its place. This cycle of a liquid or gas rising and falling is called a convection current.

We set up a very simple convection current demonstration using hot and cold water with food colouring to show the movement of warm water through cold water.

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Convection Current Demonstration

You’ll need

Tall glass or vase

A smaller glass or cup

Hot water

Cold water

Food colouring

Instructions

Fill the tall glass or jar with cold water.

Fill the smaller container with hot ( but not boiling water ) and add a few drops of food colouring.

Carefully place the small container into the container with the cold water.

Watch what happens to the warmer, coloured water.

The hot, coloured water rises upwards and collects at the top of the cold water. It then cools and sinks downwards. Eventually, all the water will be at the same temperature.

How does convection work?

Particles of warm water move more quickly and spread out. They rise upwards through denser cooler water which sinks to the bottom where it warms up. Eventually, all the liquid is at the same temperature.

The warm water rises as when liquids and gases are heated, they expand. This means they take up more room but have the same mass, so their density is less than when they are cool. Substances with lower densities float on substances with higher densities.

Convection current in heated water

Extra Challenge

Repeat the activity with cold water instead of hot and watch what happens to the coloured water.

You can see that the cold water isn’t moving upwards like the warm water.

How is heat transferred?

Heat can be transferred in three different ways. Conduction, convection and radiation.

Conduction

Conduction of heat occurs when vibrating particles pass any extra vibrational energy to nearby particles. The more energy the particles have, the hotter the object gets. An example of this type of heat transfer is when metal pans are heated on a hob. Heat travels through the pan. If the pan handle is also metal, it will get hot too. This is why metal pans often have plastic or wooden coverings on their handles. Plastic and metal are not good conductors of heat.

Convection

Convection of heat is when warmer molecules of a liquid or gas move from a warmer to a cooler area taking the heat with them.

Water being heated in a pan is an example of convection. This is the type of heat transfer we demonstrated above.

Another way to demonstrate convection is with a spinning convection snake.

Radiation of heat is when heat is radiated to the surrounding area by heat waves. Particles are not involved in this kind of heat transfer.

The heat from the sun travelling through space is an example of heat transfer by radiation. This type of heat is transferred by waves.

Diagram showing conduction, convection and conduction

All three kinds of heat transfer can be seen in the campfire and pan example above.

Heat travels by radiation from the campfire to the metal pan. Heat travels through the pan’s metal by conduction to warm the lower layers of water. The water is then heated by convection as the less dense warmer water rises through the cooler water to create a convection current!

Remember – heat is only transferred if there is a temperature difference.

Science concepts

Density

Conduction

Convection