What it shows
Sunlight evaporates water from salty or muddy water, and the vapour condenses on a cool plastic sheet as water with the salt left behind. It models how free heat can separate fresh water from sea or borewell water.
The science
Heat from the sun turns the surface water into vapour, leaving the salt behind. The vapour rises, meets the cooler plastic and turns back into liquid droplets. Because the plastic slopes towards the middle, droplets slide down and drip into the cup.
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 wide steel or plastic basin
- A small heavy glass or steel cup
- A clear plastic sheet or cling film
- A rubber band or string
- A small pebble
- Salt, tap water and a measuring spoon
- A black plastic sheet or black cloth
Safety
For this project: This is a learning model: do not drink or taste the collected water, because a home still does not make water safe to drink. Keep the still out of reach of small children and handle the warm basin with 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.
Step 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.
Line the bottom of the basin with the black plastic or cloth to absorb more sunlight.
Dissolve two spoons of salt in 500 ml water and pour it into the basin.
Place the empty cup in the centre, making sure no salt water can splash into it.
Stretch the plastic sheet over the basin and fix it with the rubber band.
Put the small pebble on the sheet just above the cup so it dips in a gentle cone.
Place the still in full sun for four to six hours.
Measure the water in the cup with a measuring spoon.
Without tasting either, compare the collected water with the original by drying a teaspoon of each on a clean steel spoon in the sun and looking at how much salt crust is left.
What you should see
A few tablespoons of water collect in the cup, and it leaves much less salt crust than what was in the basin. The amount depends on how bright and warm the day is.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Where does the salt go?
It stays behind in the basin because salt does not evaporate.
02Why is the plastic sloped?
So the droplets run to the lowest point above the cup.
03Which two changes of state are used?
Evaporation and condensation.
04Why is a still slow?
Only the sunlight on a small area supplies the heat.
Take it further
Compare yield with and without the black lining, and with a steeper plastic sheet.
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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