CC2.3 — Drinking water (Edexcel 1SC0)

Water treatment

Raw water from rivers or reservoirs is treated to make it safe to drink:

1. Sedimentation: large particles settle to the bottom.
2. Filtration: water passes through sand/gravel beds → removes smaller particles and some microorganisms.
3. Chlorination: chlorine gas or sodium hypochlorite added → kills remaining bacteria and pathogens.
4. pH adjustment: lime added to correct pH if needed.

Desalination

Used to obtain fresh water from seawater in areas with limited fresh water supplies.

• Distillation: seawater boiled → water vapour condensed → pure water collected. Energy-intensive.
• Reverse osmosis: high pressure forces water through a semipermeable membrane; salt and other dissolved substances remain behind. More energy-efficient than distillation.

Water testing

• Flame test: identify metal cations by characteristic flame colours.
• Precipitate test: add NaOH → different coloured precipitates for different metal cations.
• Sulfate test: add dilute HCl then Ba²⁺ → white precipitate of BaSO₄.
• Halide test: add dilute HNO₃ then Ag⁺ → AgCl (white), AgBr (cream), AgI (yellow).

Potable vs pure water

Pure water (chemistry definition): only H₂O — no dissolved substances. Potable water: water that is safe to drink — may contain dissolved minerals; does not need to be chemically pure.