Colourful convection

Show that if hot water is below cold water, they mix, but if the situation is reversed they do not. Students can use their knowledge of floating and sinking to explain this. An introduction to convection.

Equipment

• 2 trays
• Red and blue food colouring
• Stirrer
• Water from hot tap (eg in a thermos flask)
• Water from the cold tap at room temperature (eg in a jug)
• Four large identical jars or bottles
• 2 pieces of card (the lids used for foil containers work well)
• A cork (optional)

Preparation & safety

Practise the demonstration in a large sink or basin before performing it in front of the class. If you use glass jars/bottles ensure that they can be set up and taken down safely without danger of breakages. Alternatively, use plastic containers.

Procedure

1. Place two jars on a tray.
2. Put a few drops of red dye in one jar and fill it with hot water. Make sure that the water reaches the open mouth of the jar.
3. Put a few drops of blue dye in another jar and fill it with cold water. Make sure that the water reaches the open mouth of the jar.
4. Place the piece of card over the mouth of the hot (red) water jar and press firmly in place. Turn this jar upside down and place it directly on top of the jar of cold (blue) water.
5. Carefully slide the card out from between the two bottles so that their mouths are in contact.
6. Repeat steps 1-5 for the other tray, but this time place the blue jar on top of the red.

Discussion prompts

• What will happen if I remove the card?
• Why do some things float and others sink?
• Why does the water not mix when the hot water is on top?

Teaching notes

Students should be familiar with the idea of objects (eg a cork) floating in water if their density is less than that of water. Extend this idea to liquids floating by explaining that water expands slightly when you warm it. The density of the cold water (998 kg m^-3 at 20°C) is a little higher than that of the hot water (988 kg m^-3 at 50°C). If they imagine a small volume of hot water surrounded by cold water it will rise up to the surface and float just like a cork would if it were submerged underwater and then released.

On the first tray the hot water is on top. There is very little mixing because the hot water is already floating. On the second tray, the hot water is below and so it rises and cold water flows downwards to replace it and the two mix.

Provide a simple diagram of the arrangement of particles in the two jars. Emphasise that the density of water decreases when you warm it because the (average) space between the particles increases. The particles themselves do not expand.

Learning outcome

Students explain why hot water rises and cold water sinks in terms of differences in the distance between particles that make up the water.

This experiment was safety-checked in March 2020.