You will need:
- a large bowl
- a beaker or short glass
- cling film
- some tape or an elastic band
- a pebble
- jug of water
Make some salty water by dissolving a few teaspoons of salt into the jug of water. Pour this water into the large bowl so that the water is a few centimetres deep.
Stand the beaker in the middle of the salt water so that its top is higher than the water but lower than the rim of the bowl.
Stretch some cling film over the top of the bowl and make sure the bowl is sealed. It may be necessary to use tape or an elastic band to ensure the bowl is airtight.
Place the pebble or any small weight in the centre of the cling film over the middle of the beaker. This will force the cling film to slope slightly into the centre and will help collect the water.
Place the bowl on a windowsill, preferably in the sunshine and leave for several hours. If it is not a sunny day and you want to speed the experiment up then place the bowl under a bright lamp as this will have a similar effect to the sun.
The heat from the sun or the lamp has caused the water in the bowl to evaporate. Importantly, the salt has been left behind. In fact if the experiment was left long enough, until all the water had evaporated, salt crystals would be left in the bowl.
The water vapour rises from the surface as it would from the surface of the sea, a pond or a puddle in the playground. In those cases it would rise high into the sky and only condense when it started to cool, forming clouds.
In the experiment, the cling film stops the water vapour from escaping and instead it cools on the underside of the cling film and turns back into water droplets. The weight on the cling film helps these droplets to make their way to the centre and fall into the beaker.
This water is fresh water, without salt and is known as distilled water.
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