What it shows
Equal drops of water, alcohol-based hand sanitiser and cooking oil evaporate at very different speeds: the sanitiser disappears in minutes, water in about an hour, and oil hardly at all. The difference shows how strongly each liquid’s molecules hold on to each other.
The science
In a liquid the molecules attract each other. Evaporation happens when surface molecules gain enough energy to break free into the air. Alcohol molecules attract each other weakly, so they escape easily and the drop vanishes fast. Water molecules are held by stronger hydrogen bonds, so water evaporates more slowly. Oil molecules are long and heavy and cling strongly, so oil barely evaporates at room temperature. Moving air carries vapour away and speeds up every evaporation.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A steel or glass plate
- A dropper or syringe (no needle)
- Water, alcohol-based hand sanitiser and cooking oil
- A marker that writes on the plate or three sticky labels
- A timer
- A small fan or a hand fan, optional
Safety
For this project: An adult must supervise the sanitiser, which is flammable: keep it well away from the kitchen stove and any flame, and do not touch eyes. Use only a few drops.
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.
Mark three spots on the plate and label them water, sanitiser and oil.
Put three drops of each liquid on its spot at the same moment and start the timer.
Watch the edges of the sanitiser drop shrink and write the time when it is gone.
Check the water drop every five minutes and note when it disappears.
Observe the oil drop at the end of an hour; it should look unchanged.
Repeat the test on a fresh plate with the fan blowing gently across it and compare the times.
Repeat once more with the plate in a sunny spot and compare again.
Put all the times in a table with columns for still air, fan and sun, and explain the order.
What you should see
Sanitiser evaporates in one to three minutes, water in thirty to ninety minutes depending on the weather, and oil not at all within the test; fan and sunshine shorten both the water and sanitiser times.
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 sanitiser disappear fastest?
It contains alcohol, whose molecules escape into the air easily because they attract each other weakly.
2Why does the fan speed things up?
It blows away the vapour above the drop so more molecules can leave the surface.
3Why does the sanitiser drop feel cold?
Evaporation takes heat from the skin and the plate.
4Why does oil not evaporate?
Its molecules are heavy and hold each other strongly, so almost none escape at room temperature.
Take it further
Measure the mass of a cup of water each day for a week to work out how much evaporates per day in your room.
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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