Potable Water

Access to clean drinking water is essential for human health. In this lesson you will learn what makes water potable, how potable water is produced in the UK, and how processes differ between regions with plentiful rain and those where fresh water is scarce. This is a key topic in AQA GCSE Chemistry: Using Resources.

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What Is Potable Water?

Potable water is water that is safe to drink. It is not the same as pure water in the chemical sense.

Term	Definition
Pure water	Water that contains only H2O molecules and nothing else; boils at exactly 100 degrees C and freezes at exactly 0 degrees C
Potable water	Water that has sufficiently low levels of dissolved salts and microbes to be safe for drinking

Potable water still contains dissolved substances — small amounts of dissolved minerals are actually beneficial and give water its taste. The key requirement is that the levels of dissolved salts and harmful microorganisms are low enough not to cause illness.

Exam Tip: A very common exam question asks you to explain the difference between pure water and potable water. Remember: pure water contains only water molecules, but potable water can contain dissolved substances — it just has to be safe to drink.

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Producing Potable Water in the UK

In the UK and other countries with adequate rainfall, potable water is produced from fresh water sources such as rivers, lakes and reservoirs. The process involves several stages:

Stage	What Happens	Purpose
1. Collection	Water is collected from rivers, lakes or reservoirs	Gathering raw water from the environment
2. Sedimentation	Water is held in large tanks where solid particles settle to the bottom	Removes large insoluble particles (mud, grit, debris)
3. Filtration	Water passes through filter beds of sand and gravel	Removes finer insoluble particles and some bacteria
4. Sterilisation	Water is treated with chlorine, ozone or UV light	Kills harmful bacteria and other microorganisms

graph LR
    A["Fresh Water Source<br>River / Lake / Reservoir"] --> B["Sedimentation<br>Solids settle out"]
    B --> C["Filtration<br>Sand and gravel beds"]
    C --> D["Sterilisation<br>Chlorine / Ozone / UV"]
    D --> E["Potable Water<br>Safe to drink"]
    style A fill:#bbdefb,stroke:#1565c0
    style B fill:#fff9c4,stroke:#f57f17
    style C fill:#ffe0b2,stroke:#e65100
    style D fill:#c8e6c9,stroke:#2e7d32
    style E fill:#b3e5fc,stroke:#0277bd

Sterilisation Methods

Method	How It Works	Advantages	Disadvantages
Chlorine	A measured amount of chlorine gas or solution is added to the water	Cheap, effective, provides residual protection in pipes	Can produce harmful by-products; some people dislike the taste
Ozone	Ozone gas (O3) is bubbled through water	Very effective at killing microorganisms; no lasting taste	Expensive; does not provide residual protection
UV light	Water is exposed to ultraviolet radiation	No chemicals added; effective against most pathogens	Expensive equipment; no residual protection; water must be clear

Exam Tip: You may be asked to compare sterilisation methods. Chlorine is most commonly used in the UK because it is cheap and continues to protect water as it travels through pipes to homes (residual protection). This is a key advantage over ozone and UV.

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Producing Potable Water in Arid Countries

In countries where fresh water is scarce (such as parts of the Middle East and North Africa), potable water is often produced by desalination — removing salt from seawater or brackish water.

There are two main desalination methods:

1. Distillation

• Seawater is heated until it boils and becomes steam
• The steam is then cooled and condensed to produce pure liquid water
• Dissolved salts are left behind in the boiling vessel

2. Reverse Osmosis

• Seawater is forced through a semi-permeable membrane at high pressure
• The membrane has pores small enough to allow water molecules through but blocks dissolved salt ions and larger molecules
• Clean water passes through; concentrated brine is left behind

Method	Energy Requirement	Cost	Quality of Water
Distillation	Very high (requires large amounts of heat energy)	Expensive	Very pure
Reverse osmosis	High (requires high-pressure pumps)	Less expensive than distillation but still costly	Very pure

Exam Tip: Both desalination methods require large amounts of energy, which makes them expensive. This is a key reason why desalination is mainly used in countries where there is no adequate fresh water supply. AQA may ask you to explain why the UK does not routinely use desalination.

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Why Desalination Is Not Used Routinely in the UK

The UK has sufficient rainfall and fresh water sources. Since the processes of sedimentation, filtration and sterilisation require far less energy than desalination, it is much cheaper and more practical to treat fresh water. Desalination would only be considered in the UK during extreme drought conditions.

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Water Quality Testing

Before water is distributed to homes, it is tested to ensure it meets safety standards:

Test	What It Checks
pH testing	Ensures water is close to neutral (pH 6.5-9.5 in the UK)
Dissolved solids	Checks that levels of dissolved salts are within safe limits
Microbiological testing	Confirms absence of harmful bacteria such as E. coli
Chemical analysis	Tests for specific pollutants like lead, nitrates and pesticides

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Summary

• Potable water is water safe to drink; it is not chemically pure.
• In the UK, potable water is produced by sedimentation, filtration and sterilisation of fresh water.
• Sterilisation can be achieved using chlorine, ozone or UV light.
• In arid countries, desalination (distillation or reverse osmosis) is used to make seawater potable.
• Desalination requires large amounts of energy, making it expensive.
• The UK does not routinely use desalination because it has adequate fresh water supplies.
• Water is tested for pH, dissolved solids, microbes and chemical pollutants before distribution.

Exam Tip: When comparing water treatment in the UK with desalination, always mention energy costs. The key difference is that desalination requires far more energy, making it impractical where fresh water is readily available.

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