Water Purification Methods

Water purification appears in the Cambridge 0620 syllabus under experimental techniques and environmental chemistry. Examiners test the distinction between pure water and drinkable water, the steps in water treatment, and the role of distillation.

Pure water vs drinkable water

This distinction catches many candidates. In chemistry, “pure” has a specific meaning:

Feature	Pure water	Drinkable (potable) water
Composition	Only H₂O molecules	H₂O + dissolved minerals + chlorine
Boiling point	Exactly 100 °C	Slightly above 100 °C
Freezing point	Exactly 0 °C	Slightly below 0 °C
Taste	Flat/tasteless	Has a taste (from dissolved minerals)
How to make	Distillation	Water treatment plant
Safe to drink?	Yes (but lacks minerals)	Yes

Tap water is drinkable but not pure. The dissolved minerals (calcium, magnesium, sodium ions) are not harmful — they contribute to taste and provide essential minerals.

Water treatment process

Water treatment plants take raw water from rivers, lakes, or reservoirs and make it safe to drink through several stages:

Stage 1: Screening

Large objects (leaves, twigs, debris) are removed by passing water through metal screens/grilles. This is a physical process.

Stage 2: Sedimentation

Water is held in large tanks where heavy insoluble particles (sand, clay, silt) settle to the bottom under gravity. Chemicals called flocculants (e.g. aluminium sulfate) may be added to clump small particles together, making them heavy enough to settle faster.

Stage 3: Filtration

Water passes through beds of fine sand and gravel. This removes smaller suspended particles that did not settle during sedimentation. The sand bed also traps some bacteria.

Stage 4: Chlorination

Chlorine gas or sodium hypochlorite solution is added in carefully controlled amounts. Chlorine kills bacteria, viruses, and other harmful microorganisms. The water is now safe to drink.

Some treatment plants also add fluoride (to reduce tooth decay) — this is common in certain states of Malaysia.

Obtaining pure water by distillation

If chemically pure water is needed (for laboratory use or some industrial processes), tap water or seawater must be distilled:

1. Heat the water in a flask until it boils
2. Steam rises and passes into a condenser (a tube surrounded by cold water)
3. The steam condenses back to liquid water
4. The pure water (distillate) is collected in a clean container
5. Dissolved salts and minerals remain behind in the flask

This is simple distillation. The distillate is pure water — it contains only H₂O.

Testing purity

To confirm water is pure:

• Measure its boiling point — pure water boils at exactly 100 °C at standard pressure
• Measure its freezing point — pure water freezes at exactly 0 °C
• Impure water has a higher boiling point and lower freezing point than pure water

Desalination

In water-scarce regions, seawater can be converted to drinkable water by desalination:

Method	Process	Energy requirement
Distillation	Boil seawater, condense the steam	High
Reverse osmosis	Force seawater through a membrane that blocks salt	Moderate

Both methods remove dissolved sodium chloride and other salts. Reverse osmosis is increasingly used because it requires less energy than distillation.

Water purification in Malaysia

Malaysia’s water treatment follows the standard stages described above. The country’s tropical climate and heavy rainfall provide abundant raw water, but the humid conditions also promote bacterial growth, making chlorination especially important. Several states (including Johor and Selangor) have implemented advanced treatment processes to handle increasing urban water demand.

Worked exam question

Describe how river water is made safe to drink. Your answer should include at least three steps. [3]

The water is first passed through screens/filters to remove large debris [1]. It is then allowed to settle in tanks so that heavy particles sink to the bottom (sedimentation) [1]. The water is filtered through sand beds to remove smaller particles [1]. Finally, chlorine is added to kill bacteria/microorganisms (chlorination) [1]. (Any 3 for 3 marks.)

Explain why tap water is described as drinkable but not pure. [2]

Tap water contains dissolved minerals/salts and chlorine [1]. Pure water in chemistry means it contains only water molecules and boils at exactly 100 °C — tap water does not meet this definition [1].

Common exam mistakes

1. Saying tap water is “pure” — in chemistry, pure means only one substance. Tap water is drinkable/potable but not chemically pure.
2. Confusing filtration with distillation — filtration removes insoluble solids; distillation separates dissolved solutes by boiling and condensing.
3. Forgetting chlorination — many candidates describe the physical steps but omit the chemical treatment that kills bacteria.
4. Stating that boiling water “purifies” it — boiling kills bacteria but does not remove dissolved salts. Only distillation produces pure water.