Cleaning used water, a model watershed and thirst by the sea
Week 22 · Theme 3 Protecting Our Planet, Unit 3 Our Earth’s Resources, Concept 3.3 (C), Unit Project We All Live Downstream, Interdisciplinary Project Water by the Sea (A) · 3 lessons
3
Thirsty at the seaside
Discovery Education Science P5: Term 2, Unit 3, pp. 86–88
GoalExplain why sea water cannot be drunk, show that evaporation leaves salt behind, and ask how salt could be taken out of sea water.
New words
salt water · water with a lot of salt dissolved in it, like sea waterevaporate · to change from liquid water into a gas called water vapourdesalination · taking the salt out of sea water
Explanation
A family on a beach at Alexandria runs out of drinking water. The sea is right there, but it is salty, and a person must not drink it. Too much salt makes the body lose water and makes people more thirsty, not less. About 97 out of every 100 parts of the Earth’s water are salty, so most of the water on our planet is not ready to drink. Salt does not simply disappear. When salt water is left in the sun, the water evaporates and rises into the air as vapour, while the salt stays behind. This is how salt flats form. Salt flats give us salt but no water. A challenge for engineers is to take the salt out and also keep the water. Taking salt out of sea water is called desalination, and next week we will design a way to do it.
Worked examples
Mina leaves a saucer of salty water on a sunny windowsill (a grown-up helps). After a few days the water has gone and white crystals are left in the saucer. The water evaporated, but the salt stayed. Nobody tastes the crystals.
Sea water
Salt in it (about)
1 L
35 g
2 L
70 g
4 L
140 g
Sea water has about 35 g of salt in every litre. So 2 L of sea water holds about 2 × 35 = 70 g of salt, and 4 L holds about 4 × 35 = 140 g. If all the water evaporated, that salt would be left.
sea water→heat makes vapour→catch and cool the vapour→fresh water
Two ideas from the team: Idea 1, boil the sea water and catch the steam. Idea 2, wait for the Sun to evaporate it and catch the vapour. Both give fresh water because the vapour has no salt. The trick is catching it. But a person must never boil water without a grown-up.
Class activity
Salt saucer (a grown-up helps). Put 2 spoons of salt in a cup of warm tap water and stir until it is gone. Pour a thin layer into a saucer and put it in a sunny, safe place indoors. Look each day and write what you see. What is left when the water has gone? Do not taste it. Wash your hands. Then write an idea for catching the vapour as well.
Practice
1
Why must a person not drink sea water?
It has too much salt. It makes the body lose water and makes you more thirsty.
2
What is left in the saucer after the water has evaporated? Say the letter.
B: the salt. Evaporation takes only the water.
3
Sea water has about 35 g of salt in every litre. How much salt is in 3 L?
3 × 35 = 105 g.
4
Look at the squares. About how many parts out of 100 of Earth’s water are salty?
