Drinking water from the sea
Sea water is a mixture of water, salt and other minerals, and sometimes small bits of seaweed or shell. We cannot drink it, because the salt makes the body lose water faster. How can we make drinking water from a mixture? First, filter the sea water to catch the large bits. Salt and water still pass through the filter. Next, heat the water so that it evaporates. The vapour rises, and the salt stays behind. Then cool the vapour so that it condenses into drops of fresh water. Desalination plants on the coasts of Egypt make fresh water on a very large scale. Some work by evaporating and condensing like this; many others push sea water hard through special, very fine filters that hold back the salt. There are problems: heating or pushing so much water needs a lot of energy and money, and the very salty water left over must be returned to the sea carefully so that sea life is not harmed. Many people work together on this, such as engineers, plant operators and water scientists. In school we only make a small model, and the water is never for drinking.
| Step | What it does | What is left? |
|---|---|---|
| filter | catches big bits | water and salt pass |
| evaporate | water becomes vapour | salt stays behind |
| condense | vapour cools to drops | fresh water |
| Good | Problem |
|---|---|
| gives fresh water by the sea | needs a lot of energy and money |
| the sea never runs out of salt water | very salty leftover can harm sea life |
Mini still (a grown-up helps, sunny day, hats on, nobody looks at the Sun or at shiny surfaces). Stir two spoonfuls of salt into a bowl of water. Put a small empty cup in the middle, cover the bowl with cling film and put a coin on the film above the cup. Leave it in the sunshine for a few hours. Look for drops on the film and water in the cup. Never taste it. Wash your hands.