Why Does Water Need to Be Purified?
Water found in nature — in rivers, lakes, and underground aquifers — is not pure. It contains dissolved minerals, suspended particles of mud and sand, bacteria, viruses, algae, and sometimes harmful chemicals from agriculture or industry. Drinking this water without treatment can cause serious diseases such as cholera, typhoid fever, and dysentery.
Even if water looks clear, it may still contain invisible contaminants. That is why water must go through several stages of purification before it reaches your tap.
Never drink water directly from rivers, streams, or lakes without purifying it first — even in places that look clean. Invisible microorganisms can cause serious illness within hours or days of drinking contaminated water.
Stages of Water Treatment
Modern water treatment plants clean water through a series of carefully designed steps:
1. Screening
Large objects — leaves, branches, fish, plastic rubbish — are removed by passing the water through coarse screens or grilles. This protects the equipment downstream and removes the biggest particles.
2. Sedimentation
The water is pumped into large, calm settling tanks. Gravity causes heavier particles (sand, clay, silt) to slowly sink to the bottom, forming a layer called sludge. The cleaner water above is then moved to the next stage.
To speed up sedimentation, chemicals called coagulants (such as aluminium sulfate, Al₂(SO₄)₃) are often added. These cause tiny particles to clump together into larger flocs, which settle faster.
3. Filtration
The water passes through layers of different materials that trap remaining particles:
| Layer | Material | Particle size removed |
|---|---|---|
| Top | Sand (coarse) | Large particles |
| Middle | Sand (fine) | Medium particles |
| Bottom | Gravel | Supports the layers |
| Extra | Activated charcoal | Chemicals, odours, colours |
Activated charcoal (also called activated carbon) is especially useful because its highly porous surface traps chemical contaminants, removes unpleasant tastes and odours, and can even remove some pesticides.
4. Chlorination
Even after filtration, microscopic bacteria and viruses can remain in the water. A small amount of chlorine (Cl₂) or a chlorine compound is added to kill these microorganisms. This process is called chlorination and it is the reason tap water sometimes has a faint smell.
The concentration of chlorine used is carefully controlled — enough to kill pathogens, but small enough to be completely safe for drinking.
5. UV Treatment (Optional)
Some water treatment plants use ultraviolet (UV) light to disinfect water. UV light damages the DNA of bacteria and viruses, preventing them from reproducing. It is a chemical-free alternative to chlorination and leaves no taste or smell.
Real world: Your tap water
The next time you fill a glass from the tap, think about the journey that water has made. It was collected from a reservoir or river, screened, settled, filtered through sand and charcoal, treated with chlorine, tested by scientists, pumped through underground pipes, and delivered to your home. Clean tap water is the result of advanced chemistry and engineering — and in many countries, it is taken completely for granted.
Drinking Water vs Distilled Water
Not all purified water is the same:
| Type | How it is made | Contains minerals? | Typical use |
|---|---|---|---|
| Tap water | Treated by water plant | Yes (some) | Drinking, cooking |
| Filtered water | Passed through filter jug | Reduced | Drinking |
| Boiled water | Heated to 100 °C | Yes | Emergency purification |
| Distilled water | Evaporated then condensed | No | Lab use, car batteries, irons |
Distilled water is made by boiling water into steam and then condensing the steam. Dissolved minerals and most contaminants are left behind. However, because it contains no minerals, distilled water is not recommended for regular drinking — our bodies need minerals like calcium and magnesium.
Water Hardness
You may have noticed that in some areas, soap is harder to lather, or that white deposits form inside kettles and on shower heads. This is caused by hard water — water that contains high concentrations of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions.
These ions come from rocks like limestone (calcium carbonate, CaCO₃) that water flows through underground. Hard water is not harmful to drink — in fact, some calcium is good for your bones — but it can damage pipes and appliances over time.
The white crusty deposits you see inside a kettle are called limescale. They form when hard water is heated and the dissolved calcium carbonate comes out of solution and sticks to surfaces. This is a physical/chemical change, not a sign that the water is dangerous.
Boiling Water to Purify It
In emergencies — for example, during a flood when the water supply may be contaminated — simply boiling water for at least one minute kills most harmful bacteria and viruses. Boiling does not remove dissolved chemicals or minerals, but it is a reliable way to make biologically unsafe water drinkable.
Clean Water as a Global Challenge
Access to clean drinking water is not equally distributed around the world. According to the World Health Organization, hundreds of millions of people still lack access to safe drinking water, relying on rivers, unprotected wells, or contaminated sources.
Real world: Water in developing regions
In many rural areas of sub-Saharan Africa and South Asia, families must walk several kilometres to collect water from rivers or wells that may be contaminated. Simple purification technologies — such as solar disinfection (leaving clear plastic bottles in sunlight for 6 hours), boiling, or basic sand filters — can dramatically reduce illness and save lives. Chemistry knowledge directly improves human health and quality of life in these communities.
Practice Problems
List the four main stages of water treatment in the correct order.
Show answer
What is hard water, and what causes it?