Unique properties of water arising from hydrogen bonding, causes and removal of temporary and permanent hardness, methods of softening water, and stages of water treatment for domestic use.
Water is the most important substance in chemistry and in life. Its unusual physical properties all arise from a single structural feature: the ability to form hydrogen bonds. Understanding what makes water hard, how hardness is removed, and how water is made safe to drink are core examination topics.
Water's unusual properties arise from hydrogen bonds between molecules — each O–H group in one molecule attracts the lone pair on the oxygen of a neighbouring molecule. These bonds are weak individually but collectively make water unlike almost any other molecular compound:
| Property | Significance |
|---|---|
| High specific heat capacity | Absorbs a lot of heat for a small temperature rise — moderates climate and body temperature |
| High boiling point (100 °C) | Remains liquid over a wide temperature range suitable for life |
| Excellent solvent | Dissolves ionic and polar substances, allowing nutrients and waste to be transported in living systems |
| Density maximum at 4 °C | Ice is less dense than liquid water — ice floats, insulating aquatic organisms beneath |
| High surface tension | Allows surface-dwelling insects; important in capillary action in plants |
All these properties would be very different if water molecules could not form hydrogen bonds. A molecule of similar size without H-bonding (e.g. H₂S) is a gas at room temperature and boils at −60 °C.
Not all water behaves the same way. Water that flows through limestone or chalk picks up dissolved calcium and magnesium ions, and these affect how it reacts with soap and what it deposits in pipes and kettles.
Hard water does not lather readily with soap. It contains dissolved Ca²⁺ and Mg²⁺ ions, usually picked up as rainwater passes through limestone or dolomite rock. When soap (a sodium salt of a fatty acid) is added, these ions react with it to form an insoluble scum instead of a lather.
Cause: dissolved calcium hydrogen carbonate, Ca(HCO₃)₂. This forms when rainwater containing dissolved CO₂ passes through limestone:
Temporary hardness is removed by:
Cause: dissolved calcium or magnesium sulfates (CaSO₄, MgSO₄). These do not decompose on boiling, so permanent hardness cannot be removed by boiling.
Permanent (and temporary) hardness is removed by:
Ion exchange resins — the hard water passes through a resin that exchanges Ca²⁺ and Mg²⁺ for Na⁺ or H⁺ ions, which do not cause hardness
Distillation — removes all dissolved solids, giving pure water
Boiling only removes temporary hardness — it has no effect on permanent hardness. Washing soda and ion exchange remove both. This distinction appears frequently on Paper 01.
Hard water is not entirely undesirable. Ca²⁺ ions contribute to strong bones and teeth, and hard water reduces the leaching of lead from old pipes (a thin calcium carbonate layer forms on the inside). The main disadvantages are: soap is wasted forming scum, limescale builds up in heating systems reducing efficiency, and hot water systems corrode more quickly.
Softening hard water solves a domestic inconvenience. Making water safe to drink is a public health necessity, and the two problems are handled differently.
Natural water sources contain suspended solids, microorganisms, and dissolved impurities. Treatment makes it safe:
| Stage | Process | Purpose |
|---|---|---|
| Sedimentation | Allow particles to settle (sometimes with aluminium sulfate as a flocculant to clump fine particles) | Removes coarse suspended matter |
| Filtration | Pass through sand or gravel beds | Removes remaining suspended particles and some microorganisms |
| Chlorination | Add chlorine gas or sodium hypochlorite | Kills bacteria and other pathogens |
| pH adjustment | Add lime (Ca(OH)₂) if too acidic | Prevents corrosion of pipes |
Boiling water kills pathogens and is the most accessible treatment in an emergency. It does not remove dissolved salts, heavy metals, or chemical pollutants.
Be precise about the order of treatment stages: sedimentation before filtration (you remove the large particles first, then the fine ones). Chlorination comes after filtration.