What it shows
Sunlight warms the water, some of it evaporates into vapour, the vapour cools on the plastic and condenses into droplets, and the droplets run down as "rain". The same four stages drive the real water cycle.
The science
Heat from the Sun gives water molecules enough energy to escape from the surface as invisible vapour. Higher up the bag the plastic is cooler, so vapour loses energy, gathers into tiny droplets and becomes visible as mist and then drops. When the drops grow heavy enough they slide down, just as rain falls from clouds back to rivers and seas.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A clear zip-lock bag
- A quarter cup of water
- A drop of blue food colour
- A permanent marker
- Tape
- A sunny window
Safety
This project uses safe, everyday materials. Still, read the rules below with a parent before you start.
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.
Draw a sea along the bottom of the bag, a sun in one top corner and a cloud in the other with the marker.
Pour the water in, add the food colour and seal the bag with as little air as possible.
Tape the bag flat on a window that gets morning sun, with the sea at the bottom.
Look at the bag after one hour and draw what you see on a sheet of paper.
Look again in the afternoon and in the evening and draw each time.
Watch for droplets that join together and slide down, and count how many you can see.
Label your drawings with the words evaporation, condensation and precipitation.
Note whether the blue colour travelled up with the vapour or stayed in the sea.
What you should see
Mist and droplets form on the upper part of the bag within an hour of sunshine and run down later. The droplets are clear, not blue, because only the water evaporates and the colour stays behind.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What makes the water turn into vapour?
Heat from the sunlight.
2Why do droplets form at the top of the bag?
The plastic there is cooler, so the vapour condenses.
3Why are the droplets not blue?
Only pure water evaporates; the colour stays in the liquid.
4Name the three main steps of the water cycle.
Evaporation, condensation and precipitation.
Take it further
Tape a second bag on a shaded wall and compare how long the droplets take to appear.
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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