What it shows
The solar disinfection (SODIS) method leaves clear water in sunlight for hours so heat and ultraviolet light weaken germs. Measuring temperature in different bottle set-ups shows how to make it work better.
The science
Sunlight heats water and also has ultraviolet radiation that damages microbes. Higher temperature and UV together work faster than either alone. A black back absorbs light and warms the water, while foil reflects more light through the bottle.
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.
- Four clear PET bottles with lids
- Black paper or paint
- Foil
- Four thermometers
- Tap water
- A sunny roof or open space
- Tape and a watch
Safety
For this project: This project only measures temperature; do not drink water from the experiment. An adult must supervise roof work. Do not claim that treated water is safe without proper testing.
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.
Fill four clear bottles with equal tap water, leaving space for air.
Tape black paper behind bottle 2 and foil behind bottle 3.
Leave bottle 1 with no backing and place bottle 4 in shade as a control.
Put a thermometer in each bottle through a hole in the cap, sealed with tape.
Lay the bottles on their sides in full sun, side by side.
Record the temperature in each every 30 minutes for six hours.
Note the time of day and the cloudiness.
Plot a graph of temperature against time for the four bottles.
What you should see
Bottles with black or foil backing warm more than the plain bottle and far more than the shaded one. The graph shows how a simple backing improves heat.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does SODIS stand for?
Solar disinfection of water.
2Why is clear water needed?
Muddy water blocks sunlight.
3Why use PET bottles?
They let UV light through and are cheap.
4Does this prove water is safe?
No, only lab tests can show that.
Take it further
Compare the temperature on a clear, a hazy and a cloudy day.
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.
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