Solar Purifier
What you need
- Water — plain water for the first round, and later a cup of water with a spoon of salt stirred into it until it dissolves. Also a few drops of black ink.
- A clear cover: a pane of window glass, or a piece of transparent polythene, a little wider than the mouth of the dish. Tape or a few small stones to hold polythene down.
- A wide, shallow dish of glass or metal — a steel plate with a raised rim, or a glass bowl. Wide matters more than deep.
The manual states these. What it does not state is not here.
How it runs
The teaching sequence as the manual gives it.
-
01
Pour water into the dish to about the depth of a finger. Shallow and wide is what you want, because evaporation happens at the surface and a wide dish has more of it. Stand the dish outside in full, direct sunlight — on a step, a stool or a wall, out of the wind. A clear day, late morning onwards, works best. -
02
Add a few drops of black ink and stir it in. The water should be properly dark, dark enough that nobody can argue afterwards about what was in the dish and what was not. -
03
Cover the dish with the glass pane or the polythene sheet. It has to sit right across the mouth with no gap at the edge, or the vapour escapes and nothing collects. If you are using polythene, tape the edges down or weigh them with small stones, and let the middle sag a little so that drops run to one low point instead of falling straight back into the dish. Tilt a glass pane slightly for the same reason. -
04
Leave it in the sun for fifteen to twenty minutes and do not lift the cover to check. Then look at the underside of the cover without disturbing it. It should be beaded with small, clear drops. If nothing has formed, either the sun is too weak or the cover is not sealed at the edge — move the dish to a brighter spot, press the cover down properly and give it another fifteen minutes. -
05
Now have the students look hard and say what they see. The water in the dish is black. The drops on the cover are not. Ask them where the ink went. It is still in the dish, because ink does not evaporate and water does — only the water left. They may taste a drop, provided the water you started with was clean drinking water. -
06
Do the whole thing again with the salt water, and this time have them taste both: the water left in the dish and a drop off the cover. The dish is salty. The drops are not. Salt cannot evaporate either. Whatever is dissolved in the water stays behind, and only the water travels.
Talking about it
Discussion points, worked examples and the handouts the manual prints with this session.
Points for discussion
What happened here, step by step? Sunlight warmed the water. The fastest-moving water molecules escaped from the surface as invisible vapour, leaving the ink and the salt behind because neither of them can do that. The vapour rose, met the cooler underside of the cover, gave up its heat and turned back into liquid. Evaporation, then condensation. Now ask where else this happens. The sun does it to the Arabian Sea; the vapour rises, cools as it climbs, condenses into cloud and comes back down on this country as the monsoon. The dish is a monsoon under a sheet of glass — and it is the reason rain is fresh even though the sea it came from is salt.
Ending it
How the manual closes this session, and where it can go next.
If it goes well
Ask how they would get drinking water out of sea water — or out of the brackish water that comes up from tube wells in parts of Rajasthan and Gujarat, where a village pumps water it cannot drink. Then ask what would have to get bigger: the surface catching the sun, the area of the cover, the channel that collects the drops. And ask what this device could honestly supply. A family's drinking water, perhaps. A field of wheat, no.
Where this comes from
Bridge the Gap is a compilation. This is what the manual records for this session.
The manual prints no source for this session. It is published here as Swechha has used it, adapted over years of classroom work; if you recognise its origin, tell us and we will credit it.
Before you start sets out where the compendium as a whole comes from.
Next
Where to go from here.