What it shows
Water evaporates in the sun and condenses on cool plastic, leaving salt behind. It is a small model of how the water cycle and distillation work.
The science
Sunlight heats the salty water and it evaporates, leaving the salt behind. The water vapour rises, touches the cooler cling film and condenses into drops. These run down to the lowest point, which is above the cup, and fall in. This is distillation using solar energy. It is also how rain forms in nature.
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.
- A large steel or plastic bowl
- A small glass or cup
- Cling film or a clear plastic sheet
- A small stone
- Salty water made with 2 spoons of salt in a bowl
- Tape
- A sunny spot
Safety
For this project: Do not drink the collected water unless the equipment is clean and an adult has checked it. Keep hands away from hot water.
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.
Pour salty water to a depth of 3 cm in the bowl.
Put the empty cup in the middle, with its rim lower than the bowl rim.
Cover the bowl tightly with cling film and tape it.
Place a small stone in the centre of the film so it dips down above the cup.
Put the still in direct sunlight for several hours.
Observe droplets forming under the film and falling into the cup.
Measure the water collected after 4 hours.
Test it with a battery and bulb conductivity tester, which should stay dark for pure water.
What you should see
A few millilitres of salt-free water collects in the cup, while the salt remains in the bowl.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What happens to the salt?
It stays behind in the bowl.
2What is it called when vapour turns to water?
Condensation.
3Why is the film sloped?
So drops run to the centre and into the cup.
4Which stage of the water cycle does this show?
Evaporation and condensation.
Take it further
Add a black cloth under the bowl and see if the yield increases.
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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