What it shows
Water evaporating from wet sand between two clay pots cools the inner pot. This is how farmers can store vegetables cooler without a fridge.
The science
Evaporation of water takes heat from what is around it, which is why wet things feel cool. Water in the sand moves outward through the clay and evaporates from the outer pot surface, taking latent heat with it. Dry air and a breeze give the most cooling.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two clay pots, one that fits inside the other with a 2 to 3 cm gap
- Clean sand
- Water
- A wet cloth for the top
- Two thermometers
- Tomatoes or leafy greens for the test
Safety
For this project: Wear gloves while handling wet sand and wash hands after handling vegetables. Do not eat produce kept in an experiment longer than a day or two.
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 the gap between the two pots with sand and pour water slowly until the sand is damp.
Place the vegetables in the inner pot and cover it with a wet cloth.
Put the pot in a shaded, breezy spot outside, not in the sun.
Put one thermometer in the inner pot and another in a similar pot in the same place with no wet sand.
Add water to the sand each morning and evening to keep it damp.
Record both temperatures twice a day for five days.
Look at the vegetables each day and note any wilting or spoiling.
Plot the temperature curves and compare how long the vegetables stay fresh.
What you should see
The pot-in-pot cooler stays several degrees cooler than the dry pot, with greater cooling on dry days. The vegetables inside stay fresher for longer.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does evaporation cool?
The fastest water molecules leave and take heat energy with them.
2Why does it work best in dry air?
Water evaporates faster when the air is dry.
3Why keep it in the shade?
Direct sun would add heat.
4Who could use this?
Farmers and vendors without electricity.
Take it further
Compare the cooling in dry summer weather and in the humid monsoon.
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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