Potable water, waste water and the water practical

In chemistry, pure means one substance only. Potable water is water that is safe to drink: it has low levels of dissolved salts and microbes, but it is not pure.

Which treatment is used depends on where the water starts:

• Fresh water falls as rain and collects in the ground (in porous rock called aquifers) and in lakes, rivers and reservoirs. The UK has enough rain to rely on this.
• Sea water has far too much dissolved salt to drink. Dry countries with a coast may desalinate it.
• Waste water (sewage from homes, farms and factories) must be cleaned before it goes back into rivers.

Water for chemical analysis is distilled or deionised, because the ions in tap water could react or be mistaken for ions in the sample.

• Filter beds of sand and gravel trap insoluble solids. Dissolved substances pass straight through.
• Sterilising with chlorine, ozone or ultraviolet light kills microbes. A trace of chlorine is left in so microbes cannot grow in the pipes.
• Desalination removes the dissolved salts from sea water, by distillation (boil, then condense the steam) or by reverse osmosis (pushing the water through membranes at high pressure). Both need large amounts of energy.

1. A plant needs 5000 m³ of water a day. Distillation needs 20 kWh per m³; reverse osmosis needs 4.0 kWh per m³
2. energy saved per m³ = 20 − 4.0 = 16 kWh
3. energy saved per day = 16 × 5000 = 80000 kWh
4. percentage saved = 16 ÷ 20 × 100 = 80%

1. Screening removes large objects and grit.
2. Sedimentation: the sewage stands in tanks and separates into sludge (bottom) and liquid effluent (top).
3. The sludge is broken down by anaerobic bacteria (no oxygen). This makes biogas, mostly methane, and a solid that can be spread as fertiliser.
4. The effluent has air bubbled through it so aerobic bacteria break down the organic matter left. Then it can go back into a river.

Sewage needs more stages than ground water because it carries much more organic matter and many harmful microbes. Some factory waste water also contains harmful chemicals, so it needs extra treatment.

Anaerobic for the sludge, aerobic for the effluent. Swapping them is the most common slip.

The aim: measure the pH and the mass of dissolved solids in some water samples, then distil one to make pure water.

• pH: calibrate a pH meter in a buffer of known pH, rinse the probe with distilled water, then read each sample. (Universal indicator paper and a colour chart is the rougher option.)
• Dissolved solids: weigh a dry evaporating basin, add a measured volume of sample, evaporate gently over a water bath, cool and reweigh. Heat and reweigh again until the mass stops changing.
• Purify: heat the sample in a flask with anti-bumping granules; the thermometer bulb sits level with the side arm; cold water enters the condenser at the bottom. Pure water boils at 100 °C and leaves no residue.

mass of dissolved solids per dm³ = mass of solid (g) ÷ volume evaporated (dm³)
1 dm³ = 1000 cm³

1. River water: 250 cm³ evaporated; basin 41.36 g empty, 41.47 g after
2. mass of solid = 41.47 − 41.36 = 0.11 g
3. 250 cm³ = 0.250 dm³, so 0.11 ÷ 0.250 = 0.440 g/dm³
4. Compare samples per dm³: a bigger volume leaves more solid even if the water is no saltier