What it shows
Liquids expand when heated and shrink when cooled. Coloured water in a sealed bottle rises up a narrow straw when the bottle is warmed and falls when it is cooled, which is exactly how a liquid thermometer works.
The science
The particles in a liquid move faster when they are heated and need a little more room, so the liquid takes up slightly more space. In a wide bottle the change is too small to see, but if the only way out is a narrow straw, even a tiny increase in volume pushes the water a long way up the straw. Cooling does the opposite and the level drops. Clinical and room thermometers use a thin tube for the same reason, with a liquid such as alcohol or mercury.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A small glass or plastic bottle with a tight lid (free)
- A clear drinking straw
- Water and a few drops of food colour or ink
- Modelling clay, hot glue or candle wax to seal the straw into the lid
- A card strip and a pen for the scale
- Two bowls, one with warm water and one with cold water
Safety
For this project: An adult must make the hole in the lid and use hot glue; use warm tap water only, never boiling.
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
9 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 the bottle to the very top with water coloured with a few drops of ink so the level is easy to see.
Ask an adult to make a hole in the lid just big enough for the straw and push the straw through so it dips into the water.
Seal around the straw with clay or hot glue so no air can get in or out; the seal is the most common point of failure.
Screw the lid on tightly; the coloured water should rise a few centimetres up the straw, and if it does not, top up the water.
Tape the card strip behind the straw and mark the water level at room temperature.
Stand the bottle in the bowl of warm water and watch the level climb; mark the highest point and write "warm".
Move it to the cold water and mark the lowest point with "cold".
Hold the bottle in your hands and see whether your body heat is enough to move the level.
Leave it on a shelf for a day and note how the level changes between morning and afternoon.
What you should see
The coloured water climbs several centimetres up the straw in warm water and drops in cold water, moving between the marks you made. It even shows the difference between morning and afternoon room temperature.
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 the water rise in the straw when warmed?
The water expands as it warms, and the narrow straw makes the small extra volume show as a big rise.
2Why must the lid be sealed?
If air can leak in or out, the water level will not show the expansion properly.
3Why is the straw narrow instead of wide?
A narrow tube makes a tiny change in volume into a large, easy-to-read change in height.
4What liquids are used in real thermometers?
Coloured alcohol in most school thermometers, and mercury in some older ones.
Take it further
Calibrate the scale by comparing with a real thermometer at two known temperatures and marking degrees in between.
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.
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