What does desalination remove from water?

salt. It turns sea water into fresh water.

In distillation, what happens to the salt?

it stays behind in the liquid. Only the water boils off.

What forces water through the membrane in reverse osmosis?

high pressure. Pressure pushes water molecules through and leaves the ions.

Which is a limitation of desalination?

it uses a great deal of energy. Boiling water, or pumping it at high pressure, is costly.

Which is a use of a multipurpose dam?

irrigation. Stored water is released to farms when needed.

Why can farmland downstream of a dam become less fertile?

silt is trapped behind the dam and no longer reaches the fields. Floods used to spread fertile silt over the land.

Desalination and multipurpose dams · Cambridge IGCSE Environmental Management

In plain words

Two big answers to a shortage of water: take the salt out of the sea, or catch a river behind a wall. Both work, both cost a fortune, and both have a list of side effects that the exam likes you to weigh up.

8 things to know

Desalination is the removal of salt from sea water to make potable water.
Distillation: the water is heated until it boils. The salt stays behind in the liquid. The steam is pure water. The steam is cooled and condensed to form potable water.
Reverse osmosis: the water is put under high pressure and passed through a membrane. The membrane lets water molecules through and stops most ions and other molecules.
Benefits of desalination: the sea is an almost unlimited supply, and it does not depend on rainfall, so it suits dry countries with a coast.
Limitations of desalination: it uses a great deal of energy, so it is expensive. The plant must be near the coast. The very salty water left over, called brine, harms marine life when it is returned to the sea.
A multipurpose dam is a dam built for several uses at once: flood control, hydro-electric power, irrigation, storage of water, transport, recreation, tourism and fish farming.
Benefits of a dam: a reliable supply of water and electricity, no carbon dioxide while generating, protection from floods downstream, and jobs.
Limitations of a dam: the reservoir floods land, so people are moved and habitats and farmland are lost. Fish cannot migrate past it. Sediment is trapped behind it, so land downstream no longer receives fertile silt. It is very costly, and a failure would be a disaster.

Worked example

A dry country on the coast has plenty of money and cheap energy, but no large rivers. Should it build a dam or a desalination plant?

A dam needs a large river and a valley to flood. The country has neither.
Desalination needs sea water, energy and money, and the country has all three.
Desalination is the better choice. The brine must be disposed of with care.

Tips and tricks

In distillation the salt stays behind and the water leaves as steam. In reverse osmosis the water is pushed through a membrane and the salt is held back. No heating is needed for reverse osmosis.
For a dam, "multipurpose" is the clue: be ready to list at least four uses.

6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Desalination and multipurpose dams: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

What does desalination remove from water?
- salt (the answer)
- mud
- bacteria
- oxygen
It turns sea water into fresh water.
In distillation, what happens to the salt?
- it turns to steam
- it stays behind in the liquid (the answer)
- it passes through a membrane
- it is killed by chlorine
Only the water boils off.
What forces water through the membrane in reverse osmosis?
- heat
- gravity
- chlorine
- high pressure (the answer)
Pressure pushes water molecules through and leaves the ions.
Which is a limitation of desalination?
- it depends on rainfall
- it uses a great deal of energy (the answer)
- it floods valleys
- it can be done only inland
Boiling water, or pumping it at high pressure, is costly.
Which is a use of a multipurpose dam?
- desalination
- mining
- irrigation (the answer)
- chlorination
Stored water is released to farms when needed.
Why can farmland downstream of a dam become less fertile?Stretch
- the water is too cold
- there is more flooding
- the dam adds salt to the river
- silt is trapped behind the dam and no longer reaches the fields (the answer)
Floods used to spread fertile silt over the land.

6 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

3 questions, 10 marks. Write your answers on paper, then check them.

Desalination and multipurpose dams

Cambridge IGCSE Environmental Management 0680 · 10 marks · papermunch.org

Name ______________________________ Date ______________

Describe how distillation produces potable water from sea water.[4]
Show answerHide answer
The sea water is heated until it boils. The salt remains in the liquid. The steam, which is pure water, is collected. It is cooled and condenses to form potable water.
I got this one
Describe how reverse osmosis removes salt from water.[2]
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The water is forced at high pressure through a membrane. Water molecules pass through, but most ions and other molecules do not.
I got this one
State four uses of a multipurpose dam.[4]
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Any four: flood control; hydro-electric power; irrigation; storage of water; transport; recreation; tourism; fish farming.
I got this one

Answers: Desalination and multipurpose dams

1. The sea water is heated until it boils. The salt remains in the liquid. The steam, which is pure water, is collected. It is cooled and condenses to form potable water.
2. The water is forced at high pressure through a membrane. Water molecules pass through, but most ions and other molecules do not.
3. Any four: flood control; hydro-electric power; irrigation; storage of water; transport; recreation; tourism; fish farming.