☰ خطة العام · العلوم (بالإنجليزية)

Fresh water from the sea: designing a solar still

الأسبوع 23 · المحور الثالث «حماية كوكبنا»، الوحدة الثالثة «الموارد الموجودة على سطح الأرض»: المشروع البيني «تحلية مياه البحر» (ب): تصميم مقطر شمسي · 3 دروس
1

Taking salt out of sea water

Discovery Education Science P5: Term 2, Unit 3, pp. 89–91
GoalExplain how the water cycle separates salt from water, and how a still copies it.
New words
condense · to change from a gas into a liquid when cooleddistillation · heating a liquid until it evaporates, then cooling the vapour to catch it as a liquid againsolar still · a device that uses the Sun’s heat to make fresh water from salt water
Explanation

Nature already takes salt out of sea water. The Sun warms the sea, and water evaporates into the air as vapour, but the salt does not go with it. High up, the air is cooler, so the vapour condenses into tiny drops that make clouds. When the drops grow heavy they fall as rain, and the rain is fresh. This is the water cycle. People copy it in a process called distillation: heat the salt water so it evaporates, cool the vapour so it condenses, and catch the drops. The device is a still, and one that uses sunshine is a solar still. Making fresh water from sea water is called desalination. Big desalination plants use a lot of energy, so many people are looking for cleaner and cheaper ways.

evaporates, then coolsfallsflows backseacloudrain
Worked examples
Look at a pot lid after a grown-up cooks rice. Drops cling to the cool underside. Steam met a cooler surface and condensed. A still uses the same idea on purpose.
ProcessSaltFresh water
salt flatkeptlost to the air
stillleft behindcaught
Salt flats in Egypt show the other half of the story. Sea water was left to evaporate, the water went into the air, and a white crust of salt stayed. Salt flats give salt, but nobody caught the water.
free heatsalt waterfresh water
Where does the heat come from in a solar still? From the Sun. That is good news in a sunny country like Egypt, because the energy is free and does not pollute the air.
Class activity

Lid watcher (a grown-up helps, from a distance). While a grown-up boils water in a pot with a lid, watch from a safe distance and then ask the grown-up to show you the underside of the lid. Draw what you see and label evaporation and condensation. Do not touch the pot, the steam or the lid.

Practice
1
Why does the rain from the water cycle not taste salty?
2
Put the three words in order for the water cycle: A condensing, B evaporating, C raining.
3
What is distillation? Use the words “heat”, “vapour” and “cool”.
4
Give one good thing about a solar still and one limit.