Earth science

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Video

Materials

• 1 large bowl
• 1 small bowl - This should be higher than 3 cm (1 inch) but lower than the large bowl. It should be able to stand in 3 cm (1 inch) deep water in the large bowl.
• Salt
• 1 coin (or a pebble)
• Plastic wrap
• Tape
• Water

Short explanation

When salt water evaporates, the salt doesn't follow. When the water vapor then condenses into raindrops, the water has turned into fresh water.

Long explanation

When ordinary salt (table salt, sodium chloride) comes into contact with water, the water molecules separate the salt crystals. They are separated into ions (atoms that have an excess or deficit of electrons), more specifically into sodium and chloride ions. The reason for this is that water molecules have charged ends, which attract the charged ions. Salt water, both in the sea and in your bowl, thus consists of water molecules mixed with sodium and chloride ions.

Your covered bowl acts as a small greenhouse. Light comes in, is converted into thermal radiation (infrared radiation) which bounces around a bit inside, before it leaves the greenhouse. Therefore, it gets warmer inside than outside. When water is heated, it evaporates, i.e. changes from a liquid to a gaseous state. This is because a heated water molecule is a water molecule in rapid motion, so fast that it can escape the electric attraction to other water molecules. However, the ions do not evaporate. Because they are charged ions, they are more attracted to the water molecules and, when the water level becomes low, to each other. A much higher temperature is required for the ions to move so fast that they can evaporate.

When the water vapor hits the plastic wrap, it condenses to liquid water again. This is because the plastic wrap is cold, as it has direct contact with the surrounding cold air. Some of this water rains down into the bowl in the center, and is now fresh water!

In addition to being a tool for a scientific demonstration, this type of device is actually available to buy as emergency equipment for shipwrecked.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you use a wider large bowl?
• What happens if the water in the large bowl is deeper?
• What happens if you use a wider small bowl?
• What happens if you do the demonstration indoors?
• What happens if you color the salt water with food coloring?

Variation

You can also put soil and grass in the large bowl. However, don't drink the water in the small bowl, as bacteria and viruses can accompany the water when it evaporates.

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