Testing a Water Sample from a Borehole
A village borehole supplies water that lathers poorly with soap and leaves white scale in kettles. A sample gives a white precipitate with sodium carbonate solution and a white precipitate with dilute ammonia solution that dissolves only partly in excess ammonia. A second sample, taken after the water has been boiled and filtered, gives much less precipitate with sodium carbonate. The village has a limited budget and can afford either a water softener or a reverse-osmosis unit. A student claims the water is 'pure because it is clear'. The committee asks you to interpret the tests and advise on the best treatment.
1. Interpret the test results and identify the likely ions present, explaining the effect of boiling.
Read the worked answer
Poor lathering and scale indicate hard water containing dissolved calcium and/or magnesium ions. The white precipitate with sodium carbonate shows Ca2+ (and possibly Mg2+) forming insoluble carbonates. The precipitate with ammonia that partly dissolves in excess suggests a mixture: calcium ions give a precipitate with ammonia only in the presence of carbonate, while magnesium hydroxide is precipitated and is not very soluble in excess ammonia. On boiling, hydrogencarbonates decompose: Ca(HCO3)2 → CaCO3 + H2O + CO2, removing some calcium as scale and softening the water. The much smaller precipitate after boiling shows the hardness was largely temporary (hydrogencarbonate) hardness, though some permanent hardness remains.
2. Explain why the student's claim that the water is 'pure because it is clear' is wrong.
Read the worked answer
Clear water can still contain dissolved ions that have no colour. The tests show calcium and magnesium ions, and the scale shows dissolved hydrogencarbonates. Pure water in the chemical sense contains only H2O molecules and would give no precipitate with sodium carbonate and would lather freely with soap. The sample is a solution, not a pure substance. Purity would need to be checked by tests such as measuring boiling point (pure water boils at exactly 100 °C at 1 atm) or by evaporation to see whether a residue remains.
3. Compare a water softener with reverse osmosis for this village and recommend one, giving reasons.
Read the worked answer
An ion-exchange water softener replaces calcium and magnesium ions with sodium ions, removing hardness and preventing scale, but it does not remove all dissolved solids, and the sodium output may be undesirable for drinking. It needs regular salt regeneration and a reliable supply of salt. Reverse osmosis forces water through a membrane that removes most dissolved ions, giving low-mineral water, but it is expensive, wastes water as reject, needs power and membrane replacement, and may need remineralisation. For a village with a limited budget whose main problem is hardness and scale, a softener is the more practical choice if salt is available; reverse osmosis would be justified only if other contaminants (for example nitrate or salts) also needed removal.