What it shows
Water leaves wet cloth by evaporation, which speeds up with heat, wind and open surface area. You hang identical wet cloths in different conditions and weigh them each hour to find which dries fastest.
The science
Evaporation is when the faster water molecules at the surface escape into the air. Heat gives them energy, wind removes the damp air next to the cloth so more can escape, and a spread out cloth has more surface for them to leave. Damp humid air slows the process.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 6 identical cotton handkerchiefs
- A bucket of water
- A kitchen scale
- A clothesline and clips
- A fan or a windy spot
- A shaded place and a sunny place
- A notebook
Safety
For this project: Make sure the floor stays dry. An adult must set up the fan so cables are off the ground and fingers stay away from the blades.
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
7 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.
Soak all handkerchiefs in water and wring each one the same number of times.
Weigh each wet cloth and record the weight.
Hang one in the sun, one in the shade, one in front of the fan, one folded in the sun, one in a closed room and one in a plastic bag as a control.
Weigh each cloth every hour using the same scale.
Calculate the percentage of water lost since the start.
Plot a graph of water lost against time for each condition.
Rank the six conditions from fastest to slowest drying.
What you should see
The cloth in sun with wind dries fastest, shade is slower, folded cloth dries much slower and the cloth in the bag hardly dries at all. The graph shows the effect of heat, wind and area.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is evaporation?
Water turning to vapour at the surface below the boiling point.
2Why does wind help?
It carries away the damp air so more water can leave.
3Why is folded cloth slow?
Less surface is exposed to air.
4Why does it take longer in monsoon?
The air is already full of water vapour.
Take it further
Measure the humidity with a wet and dry bulb thermometer and relate it to the drying rate.
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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