Water Treatment

Clean water is essential for health. Water from rivers, lakes, or reservoirs can carry soil, plastics, dissolved metal compounds, sewage, nitrates and phosphates, and microbes. Some things help life (for example, dissolved oxygen), but others are harmful. Water treatment makes water safe to drink.

How domestic water is treated

• Sedimentation: Water is left in large tanks so heavy solids settle to the bottom. Think of muddy water in a jar—the sand settles first.
• Filtration: The clearer water passes through layers of sand and gravel. The tiny spaces trap fine particles, like a very fine sieve.
• Activated carbon: Water flows through carbon granules with huge surface area. Smelly and bad-tasting molecules stick to the carbon (this is called adsorption), improving taste and odour.
• Chlorination: A small, controlled amount of chlorine is added to kill harmful microbes. In water, chlorine forms chemicals that disinfect. A simple equation is:

[ \mathrm{Cl_2 + H_2O \rightarrow H^+ + Cl^- + HOCl} ]

HOCl (hypochlorous acid) kills microbes. A tiny “residual” amount stays in pipes to keep the water safe as it travels to homes.

Why laboratories use distilled water

Distilled water has far fewer dissolved ions than tap water. Tap water can contain calcium and magnesium that cause limescale and may interfere with chemical reactions. Distilled water helps experiments give reliable results.

Checking purity quickly

• Pure water boils at 100 °C at standard pressure. Impurities usually raise or lower the boiling point.
• Pure water freezes at 0 °C. Impurities lower the freezing point.

Common misconceptions

• Filtration removes particles, not dissolved substances.
• Boiling kills microbes but does not remove dissolved solids.
• Carbon filters improve taste and odour but do not kill microbes; chlorination is needed for disinfection.