Hot water, cold water: the convection jars
Convection is easy to see with coloured water. With a grown-up, fill one jar with warm tap water (like a warm bath, never hot) coloured red, and an identical jar with cold water coloured blue. Put a card on one jar and set the other on top. Test one: cold water on top of warm. When the card is slid out, the denser cold water sinks and the warm water rises, so the colours mix. Test two: warm on top of cold. The warm water is less dense and stays on top, so the layers stay separate. Convection happens when a warm fluid is below a cold one. The energy that drives it came from the heat in the warm water, which started as the Sun’s energy or the heater’s energy.
| Test | Top jar | Bottom jar | Result |
|---|---|---|---|
| 1 | cold | warm | colours mix |
| 2 | warm | cold | layers stay apart |
Convection jars. With a grown-up, do the two jar tests over a tray, with bath-warm water only (never boiling) and food colouring. Predict first, draw the result, and say which order makes a convection current. Do not drink the coloured water. Wipe spills at once.