What it shows
A hot liquid cools by evaporation, convection and radiation, and a lid or a narrow shape cuts the losses. You record temperature every five minutes and plot cooling curves for four setups.
The science
A hot liquid loses heat faster the larger its exposed surface and the bigger its temperature difference from the room. Evaporation takes away a lot of energy because the fastest molecules leave. A lid stops most evaporation and convection, so the water cools more slowly, and the cooling curve flattens as the temperature approaches room temperature.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two steel or glass vessels of the same volume: a wide bowl and a tall glass
- A lid or plate for each
- A thermometer to 100 C
- Hot water from a kettle (adult pours)
- A stopwatch
- Graph paper
- A notebook
Safety
For this project: An adult must heat and pour the water, limited to around 70 C. Keep the vessels away from the edge of the table and do not hold hot vessels without a cloth.
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.
Pour 200 ml of hot water at about 70 C into the wide bowl and note the exact starting temperature.
Pour the same amount at the same temperature into the tall glass.
Start the stopwatch and read the temperature every five minutes for an hour.
Repeat both setups with a lid on, leaving a small hole for the thermometer.
Take care to place all four setups in the same part of the room.
Plot four cooling curves, temperature against time, on the same graph.
Compare the temperature drop in the first twenty minutes for each setup.
What you should see
The wide open bowl cools fastest, and the covered tall glass keeps heat longest. All curves fall steeply and then flatten as they approach the 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 curve flatten?
The difference to room temperature gets smaller, so the heat loss slows.
2Why does a lid help?
It stops evaporation and convection.
3Why is the wide bowl faster?
It has a larger surface area.
4What law describes cooling?
Newton’s law of cooling.
Take it further
Plot the logarithm of the temperature difference against time and check whether it forms a straight line.
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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