C12.2 Water Safe to Drink

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: types of water, how fresh water is made potable 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, why it is not pure in the chemical sense, and compare it with other types of water
• 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.

• Pure; Where it is found made in a lab, e.g. by distillation; Dissolved substances none; Microbes none
• Potable; Where it is found from the tap, after treatment; Dissolved substances low, safe levels; Microbes none or very few
• Fresh; Where it is found rain, lakes, rivers, ice caps; Dissolved substances low levels; Microbes some
• Ground; Where it is found underground in porous rocks (aquifers); Dissolved substances low levels; Microbes some
• Sea (salt); Where it is found seas and oceans; Dissolved substances high levels (mainly salt); Microbes yes
• Waste; Where it is found used water from homes, farms and industry; Dissolved substances high levels; Microbes many

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 0.60 mg per dm³ of water. It treats 8.0 × 10⁶ dm³ a day.
mass of chlorine = 0.60 × 8.0 × 10⁶ = 4.8 × 10⁶ mg
4.8 × 10⁶ mg ÷ 1000 = 4800 g ÷ 1000 = 4.8 kg

Answer: 4.8 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 3000 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: 3000 × 3.5 = 10 500 kWh. Plant Y: 3000 × 12 = 36 000 kWh.
Difference = 36 000 − 10 500 = 25 500 kWh per day

Answer: reverse osmosis uses 25 500 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 taken straight from the sea, Water with no dissolved substances at all.)

Answer: Water that is safe to drink

Working: Potable water is safe to drink. It still contains low, safe levels of dissolved substances, so it is not pure.

Question 2. Name the method of desalination in which sea water is pushed through a membrane at high pressure.

Answer: reverse osmosis

Working: Reverse osmosis: high pressure forces water molecules through a membrane that holds back the salt ions.

Question 3. True or false? In the UK most potable water is made by desalinating sea water.

Answer: False

Working: False. The UK has plenty of rain, so most potable water is made from fresh water by filtering and sterilising. Desalination needs much more energy.

Related lessons

• Previous lesson: C12.1 Finite and Renewable Resources
• Next lesson: C12.3 Treating Waste Water
• In this chapter: C12.4 Extracting Metals from Ores
• In this chapter: C12.5–6 Life Cycle Assessments and Reduce, Reuse, Recycle
• In this chapter: C12 RP Required Practical: Water Purification

Continue in the full course

This lesson is section C12.2 (AGCSC2902) of our free GCSE Combined Science 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 Combined Science course.