What it shows
Metals expand when heated, and a thin metal lid heats faster than the glass jar below it. A tight cap that will not turn becomes loose after a short dip in warm water, which shows thermal expansion at work.
The science
Most solids expand a little when heated, because their particles vibrate more and push apart. A thin metal lid warms up faster than thick glass, and metal expands more than glass for the same rise in temperature. The lid grows slightly wider than the neck of the jar and the grip loosens.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 3 glass jars with tight metal lids, such as jam or pickle jars
- A bowl of warm tap water at about 45 C (adult checks)
- A bowl of cold water and ice
- A thermometer
- A cloth for gripping
- A stopwatch
- A notebook
Safety
For this project: An adult must prepare the warm water and keep it below 50 C. Do not use boiling water on glass because it can crack. Grip jars with a cloth and keep them over a table, not a hard floor.
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
7 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.
Screw the lids on three jars as tightly as you can and ask a helper to check that they are hard to open.
Try to open jar 1 by hand and note how hard it is on a scale of 1 to 5.
Hold the lid of jar 2 in the warm water bowl for thirty seconds, with the jar kept upright and the water not touching the lid threads inside.
Dry the lid with the cloth and try opening it again; note the new effort score.
Cool the lid of jar 3 in the ice water for thirty seconds and try to open it.
Repeat the warm test with different times, 15, 30 and 60 seconds, for jar 2.
Draw a table of effort scores against treatment and explain the pattern.
What you should see
The warmed lid opens much more easily, while the cooled lid is harder to open than the untreated one. This shows that the lid expanded when warm and shrank when cold.
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 lid loosen when warm?
The metal expands and grips the glass less.
2Why does the lid expand more than the glass?
Metals expand more than glass for the same heating, and the thin lid heats faster.
3Why not use boiling water?
Sudden strong heating can crack glass.
4Where is expansion allowed for in buildings?
Gaps are left in railway tracks and bridges.
Take it further
Measure the length change of a thin metal wire when heated, using a pointer made from a straw.
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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