What it shows
Hot water is lighter than cold water, so a drop of colour in hot water rises in a plume while colour in cold water sinks. This movement of a fluid carrying heat is convection and drives sea breezes and boiling in a pot.
The science
When water is heated, it expands and its density falls, so it floats upward through cooler, denser water. Cooler water sinks to take its place, so a loop of moving water called a convection current carries heat around. In convection the heated matter itself moves, unlike conduction. The same idea makes warm air rise from a heater, and makes the land breeze and sea breeze.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two tall clear glasses
- Water, hot from the tap or a flask, about 60 °C, and cold from the fridge
- Food colouring or ink, a few drops
- A dropper or a straw
- A small bottle
- A thermometer
- Tray
Safety
For this project: An adult must handle hot water, which should not be boiling. Keep the glasses on a tray and away from the table edge. Colouring can stain clothes.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fill one glass with cold water and the other with warm water and let them stand until completely still.
Using a straw as a dropper, carefully place two drops of food colouring at the bottom of the cold-water glass.
Watch the colour: it stays near the bottom and spreads slowly.
Place two drops at the bottom of the hot water glass in the same way, and watch the coloured water rise in a plume and then spread out.
Now put a small bottle of hot coloured water into a tray of cold clear water and watch the colour rise out of the neck in a stream.
Put a bottle of cold coloured water into a tub of warm water and watch it sink.
Draw arrows to show the loops of the current and label hot and cold regions.
Write one sentence explaining the sea breeze using what you saw.
What you should see
The colour rises as a plume in hot water and the hot coloured water floats upward through cold water, while the cold coloured water sinks, showing a loop of motion.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does hot water rise?
It expands and becomes less dense than the cold water around it.
2What is the difference between convection and conduction?
In convection the hot fluid itself moves, in conduction heat passes without the material moving.
3Where does convection occur in nature?
Sea breezes, ocean currents and the monsoon winds.
4Why is the heater of a room placed low?
Warm air rises and circulates through the room, so the whole room warms.
Take it further
Place a block of coloured ice at one end of a tray of warm water and photograph the currents every 30 seconds.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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