What it shows
When salt water is left in the sun, only the water escapes as vapour and the salt stays behind as white crystals. This is how salt pans on the coast of Gujarat and Tamil Nadu produce the salt we eat.
The science
Evaporation happens when water molecules at the surface gain enough energy from sunlight to escape into the air. Salt particles are held tightly by the water and by each other and cannot evaporate at room temperature, so they are left behind. As the water shrinks the solution becomes saturated and salt crystals start to grow at the edges, where the layer is thinnest.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 3 tablespoons salt
- 1 cup water
- A shallow steel or plastic tray, or a wide plate
- A dark cloth or black paper to put under the tray
- A sunny windowsill or balcony
- A kitchen balance, optional
Safety
For this project: Place the tray where it cannot be knocked over; keep it away from pets. No heat is needed beyond sunlight.
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.
Stir the salt into the water until it is fully dissolved and the water looks clear again.
Weigh the empty tray if a balance is available, then pour in the salt water.
Put the tray on the dark cloth in the sun; the dark colour absorbs heat and speeds up evaporation.
Mark the water level on the side of the tray with a pen or a strip of tape.
Check every morning and evening, marking the new level each time and noting where the first crystals appear.
When the tray is dry, scrape the salt gently into a bowl with a spoon.
Weigh the salt and compare it with the three tablespoons you started with; it should be close.
Look at the crystals with a magnifier and record their cube shape.
What you should see
A white crust of salt crystals in the tray, with the heaviest crust at the edges, and a mass of salt close to what was dissolved, showing that nothing was lost except the water.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Where did the water go?
It evaporated into the air as water vapour.
2Why did the salt not evaporate?
Salt needs far more heat to turn to vapour, so at room temperature it stays solid.
3Why put the tray on a dark cloth?
Dark surfaces absorb more sunlight and get warmer, so the water evaporates faster.
4Is this how sea salt is made?
Yes, sea water is let into shallow pans and the sun dries it to salt.
Take it further
Set up a second tray in the shade and compare how many days each takes to dry.
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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