Episode 8: Hot Water Rising

Read the full transcript of Episode 8: Hot Water Rising

Before you start

These experiments have not been specifically safety tested for home use but we believe them to be safe if the instructions are followed. Adult supervision or direction is recommended as appropriate. All experiments are carried out at your own risk.

What you’ll need

• Four identical see through bottles. If you can only find two, just do it twice!
• Jug of cold water from your cold tap
• Jug of hot water from your hot tap (run until it is hot, not just warm)
• Food colouring. Two colours would be great, one is fine
• Teaspoon
• Funnel/large straw or just steady hands
• Scraps of thin card. If you don’t have thin card, paper works better than thick card
• Sink or waterproof tray in case it all goes wrong

What to do

In this activity you’ll be tipping bottles of water upside down and then trying to balance them. There will be some spillages along the way but we don’t want any breakages so we recommend doing this in a bowl, sink or using plastic instead of glass. Finally the demo needs hot water, but this should never be boiling or hot enough to burn, out of the tap works just fine!

1. Choose different food colouring for your hot and cold water.
2. Add a small amount of food colouring to your water jugs. Give each jug a good stir with a spoon.
3. Fill two bottles all the way to the top with cold water and two to the top with hot water. Use the funnel if you need to.
4. Put one hot water bottle and one cold into your drip tray/sink.
5. Put the card over the top of your cold water bottle. Holding the card in place, flip the bottle over and then balance it on top of your hot water bottle.
6. Do the same with your hot bottle, balancing it on top of the cold water bottle.
7. Carefully remove the cards separating the bottles, making sure that the bottles stay balanced.  Get some help for this bit if you can!

You now have two experiments going at the same time.

• Be amazed as the water mixes extremely quickly in one set of bottles.
• Be surprised as the water forms perfect layers that don’t mix at all in the other.

What to talk about

• Why do you think some things float and others sink?
• Why does rice rise and fall when you cook it?

What’s going on?

If you’ve been swimming, you may have noticed that the water near the surface is warmer than the water deeper down. This is because the warm water is less dense and it floats on the cold water – in the same way that a cork floats because it is less dense than water. 

So, why is the warm water less dense? Water is made of molecules (H₂0). The temperature of the water is a measure of how fast they move and its density tells you about how tightly the molecules are squished together. If we could zoom in far enough we would see the water molecules wobbling and moving. The higher the temperature, the more they move and the greater the average distance between them. The molecules in water at 60°C (which feels hot) are more spread out than the molecules in water at 10°C (which feels cold) making the warm water less dense.

What about our experiment? In the pair of bottles with hot water at the bottom, the cold water sinks and the hot water rises so that it floats on top. In the process the two mix. In the pair of bottles with the cold water on the bottom there is no movement because the cold water has nowhere to sink to, so everything stays in perfect layers.

The movement in any liquid or gas due to a temperature difference is called convection. It is the same process that makes the grains of rice rise and fall when you cook them; rice is carried upwards by hot water rising and downwards as the water at the surface cools and sinks. Convection is also the process that makes smoke rise out of chimneys and how your central heating warms up a whole room, not just the air next to the radiator!

What next?

Once you’ve perfected balancing bottles, you could raid your kitchen cupboards for some more convection experiments.

• Try adding coloured hot water to an empty spice jar (the ones with the sprinkle top) and drop it to the bottom of a bowl of cold water. You can watch the hot water rise out of the spice jar in front of your eyes.
• Have a couple of different temperatures of water, from ice cold through warm to hot and mix a different colour into each. Try the bottle experiment again, with the different temperatures of water.  How big does the difference in temperature need to be before convection starts to occur and the colours mix?

Did you know?

Even though it has a pretty dramatic effect, the difference in density between cold water at 10°C and warm water at 60°C is less than 2%.

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