14.2 Water Safe to Drink

A free GCSE Chemistry lesson for AQA (8462) from OBA School: what potable water is, how fresh water is treated in the UK, and how sea water is desalinated. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain what potable water is and why it is not pure in the chemical sense
• Describe how fresh water from a suitable source is made potable by filtering and sterilising
• Describe desalination by distillation and by reverse osmosis, and explain why it is costly
• Carry out calculations about water treatment, including concentrations and energy use

Key words

• Potable water: Water that is safe to drink. It has low levels of dissolved salts and microbes.
• Pure water: In chemistry, water with nothing else in it (only H₂O molecules).
• Fresh water: Water with low levels of dissolved salts, e.g. rain, rivers, lakes and groundwater.
• Sterilise: Kill microbes (bacteria and viruses), e.g. with chlorine, ozone or ultraviolet light.
• Desalination: Removing dissolved salts from salty water, e.g. sea water.
• Reverse osmosis: Pushing salty water through a membrane at high pressure. The membrane lets water through but not the salt ions.

Potable water is not pure water

Water is essential for life. Drinking water must be potable: safe to drink, with low levels of dissolved salts and microbes. It is not pure in the chemical sense, because it still contains small amounts of dissolved substances such as calcium, magnesium and chloride ions.

Key fact: Pure water has a pH of exactly 7, boils at 100 °C and leaves no residue when it is evaporated. Potable water may have a pH a little above or below 7 and leaves a small solid residue.

Testing for purity is not the same as testing for water. White anhydrous copper(II) sulfate turning blue (or blue cobalt chloride paper turning pink) shows that water is present, but impure water gives the same result. To show water is pure, check that it boils at exactly 100 °C (at normal pressure).

Treating fresh water in the UK

We use water for drinking and washing, in farming, and in industry as a raw material, a solvent and a coolant. In the UK, rain provides plenty of fresh water. It collects in lakes, rivers and reservoirs, and it also soaks down through porous rock into underground stores called aquifers (ground water), which are reached by drilling wells.

Water from natural sources is never pure. Rain dissolves some gases from the air (such as carbon dioxide) as it falls, then dissolves minerals from soil and rock. It also picks up microbes from soil and rotting material. The treatment works must lower both to safe levels. To make it potable:

• Choose a source of fresh water that is as clean as possible.
• Filter it: first through a wire mesh to remove large objects, then through filter beds of sand and gravel to remove small insoluble particles. (Some works first add chemicals that make tiny particles clump together and settle out.)
• Sterilise it to kill harmful microbes, using chlorine, ozone or ultraviolet light.

Worked example: chlorine dose

A treatment works adds chlorine at 0.80 mg per dm³ of water. It treats 5.0 × 10⁶ dm³ a day.
mass of chlorine = 0.80 × 5.0 × 10⁶ = 4.0 × 10⁶ mg
4.0 × 10⁶ mg ÷ 1000 = 4000 g ÷ 1000 = 4.0 kg

Answer: 4.0 kg of chlorine per day

Exam tip: Chlorine stays in the water as it travels through the pipes, so it keeps killing microbes on the way to homes. Ozone and UV leave no lasting protection.

Desalination of sea water

Where fresh water is scarce (for example in some hot, dry countries), sea water can be desalinated. There are two main methods:

• How it works; Distillation boil the water, then condense the steam; Reverse osmosis force water through a membrane at high pressure
• Energy; Distillation very large amount to boil the water; Reverse osmosis large amount to pump at high pressure
• Other costs; Distillation large plant; heat losses; Reverse osmosis membranes must be replaced; water pre-treated

Lowering the energy cost of distillation. Large plants boil sea water at reduced pressure. At lower pressure water boils at a lower temperature, so less energy is needed to heat it. The steam is then condensed as usual.

Reverse osmosis involves no heating, so it usually uses less energy than distillation. But energy is still needed to pump the water at high pressure, the membranes must be cleaned or replaced, and salty water corrodes pumps and pipes.

Both methods need large amounts of energy, so desalination is expensive. In the UK it is cheaper to treat fresh water, so desalination is only used on a small scale.

Worked example: energy comparison

A town needs 2000 m³ of potable water a day. Plant X (reverse osmosis) uses 3.5 kWh per m³; Plant Y (distillation) uses 12 kWh per m³.
Plant X: 2000 × 3.5 = 7000 kWh. Plant Y: 2000 × 12 = 24 000 kWh.
Difference = 24 000 − 7000 = 17 000 kWh per day

Answer: reverse osmosis uses 17 000 kWh less per day

Common mistakes

• Saying potable water is pure. It is safe to drink but contains small amounts of dissolved substances.
• Saying filtration removes microbes or dissolved salts. Filter beds remove insoluble solids; sterilising kills microbes.
• Saying chlorine “cleans” the water. It kills microbes.
• Forgetting the main drawback of desalination: it needs a lot of energy, so it is expensive.

Revision questions

Try these, then check your answers below each one.

Question 1. What is potable water? (Choose from: Water that is safe to drink, Pure water containing only H₂O molecules, Water with no dissolved substances at all, Water taken straight from a river.)

Answer: Water that is safe to drink

Working: Potable water is safe to drink. It has low levels of dissolved salts and microbes, but it is not pure.

Question 2. Name one method of desalinating sea water.

Answer: distillation

Working: Sea water can be desalinated by distillation or by reverse osmosis using membranes.

Question 3. True or false? Desalination is widely used in the UK because it needs very little energy.

Answer: False

Working: False. Desalination needs large amounts of energy, so it is expensive. The UK has plenty of fresh water, which is cheaper to treat.

Related lessons

• Previous lesson: 14.1 Finite and Renewable Resources
• Next lesson: 14.3 Treating Waste Water
• In this chapter: 14.4 Extracting Metals from Ores
• In this chapter: 14.5–14.6 Life Cycle Assessments and Reduce, Reuse, Recycle
• In this chapter: 14.7 Required Practicals: The Earth's Resources

Continue in the full course

This lesson is section 14.2 (AGCCH1402) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.