8.2Chapter 8: Pure Substances and Solutions

Drinking seawater — Alhussan Chemistry - Stage 8 Revision Notes & Topical Questions

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8.2 Drinking seawater — Desalination and Simple Distillation — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define desalination, describe how simple distillation produces pure water from seawater, and evaluate the benefits and problems of desalination.Dry countr…

Dry countries
Apparatus details
Reduced-pressure distillation
Reverse osmosis
Other users
Electricity

Section 1 of 6Why seawater is a water source

Dry countries

Saudi Arabia has very little rain, no permanent rivers and only one lake.
About half of its drinking water comes from the sea.

Seawater

Seawater it contains about 3.5% dissolved salts, which the body cannot remove in large amounts.
Drinking seawater dehydrates you because the kidneys need extra water to remove the salt.

Desalination

Desalination a process that removes salt, and other dissolved substances, from seawater to make pure water that is safe to drink.

Section 2 of 6Simple distillation in the laboratory

Heat the seawater in a flask until it boils and makes steam.

The steam passes into a condenser, a tube surrounded by cold water flowing in at the bottom and out at the top.

The steam cools and condenses to liquid water.

Pure liquid water drips into the collecting beaker, and the salt remains in the flask.

Principle

Principle it separates a solvent from a solution because the solvent has a much lower boiling point than the solute.
Water boils at about 100 °C while the salt does not boil.

Apparatus details

Apparatus details the thermometer bulb is at the side arm and shows the boiling point of the pure liquid, 100 °C.
Anti-bumping granules stop violent boiling.
The apparatus must not be sealed, so pressure cannot build up.

Section 3 of 6Large-scale desalination

Large-scale desalination

Reduced-pressure distillation

Reduced-pressure distillation it is similar to laboratory distillation,
- but the pressure is reduced so water boils at a lower temperature,
- which makes it cheaper.
Large plants use multi-stage flash distillation.

Reverse osmosis

Reverse osmosis it works a bit like filtering, forcing seawater at high pressure through a membrane that lets water through but holds back salt particles.
It uses less energy than distillation but needs electricity for pumps, and the membranes must be cleaned and replaced.

Other users

Other users ships and submarines make pure water by desalination too.

Method	Advantage	Disadvantage
Distillation	Very pure water	Uses a lot of energy
Reverse osmosis	Less energy	Expensive membranes and pumps

Section 4 of 6Benefits and problems of desalination

Benefit

Benefit it supplies drinking water for millions of people independent of rainfall.

Taste

Taste the water is completely pure, and some people do not like its taste.

Health

Health pure water has no dissolved calcium compounds, which other sources contain and which benefit health.

Electricity

Electricity some methods need huge amounts of electricity.
Fossil fuel energy contributes to global warming, so some countries use solar energy instead.

Environment

Environment sea animals and plants may die where very salty brine returns to the sea.

Cost

Cost plants are expensive to build and run.

Alternatives

Alternatives rainwater collection, recycling waste water and reducing leakage.

Section 5 of 6Key terms

Term	Meaning
Desalination	removing salt from seawater.
Distillation	separating by evaporating and condensing.
Condenser	a tube that cools vapour into liquid.
Distilled water	pure water made by distillation.
Reverse osmosis	forcing water through a membrane to remove salts.
Brine	very salty water left after desalination.