Which one among the following determines the Hardness Index of fresh water ?

Correct — A, (a) Total concentration of the Ca2+ and Mg2+ ions in the fresh water. Hardness is a measure of dissolved divalent metal ions, and in natural fresh water those are overwhelmingly calcium and magnesium.

Where they come from. Rain falling through the air picks up carbon dioxide and becomes weakly acidic; percolating through limestone, chalk, dolomite or gypsum, it dissolves calcium and magnesium out of the rock. Water from such catchments is hard; water from granite country, which yields little of either ion, is soft. Hardness is therefore a property of geology as much as of water.

How it is expressed. Total hardness is conventionally reported as an EQUIVALENT concentration of calcium carbonate, in milligrams per litre or parts per million, so that calcium and magnesium can be added on one scale; classifications commonly in use run from soft, below about 60 milligrams per litre, through moderately hard and hard, to very hard above roughly 180. In the laboratory it is measured by titration with EDTA, which binds both ions.

Why anyone measures it. Hard water wastes soap, because calcium and magnesium react with it to form an insoluble scum instead of a lather, and it deposits scale in kettles, geysers, pipes and above all boilers, where a layer of scale insulates the metal, wastes fuel and can be dangerous. TEMPORARY hardness, caused by the bicarbonates of calcium and magnesium, is removed by boiling, which drives off carbon dioxide and precipitates the carbonate; PERMANENT hardness, caused by chlorides and sulphates, survives boiling and is removed chemically, by washing soda, or by ion exchange and zeolite softeners that swap the calcium and magnesium for sodium.

Note what hardness is NOT: it is not acidity. That is what the other options in this set are built on.

Why the others are wrong

• (b)Difference of the pH value of the fresh water with respect to sulphuric acid — Two ideas welded together, neither of which is hardness. A pH value is a measure of hydrogen ion activity, that is of acidity or alkalinity, and there is no measurement defined as the difference between a sample's pH and that of an acid. Hard water is not acidic; indeed water made hard by bicarbonates is mildly alkaline and has a buffering capacity, which is a different property again, called alkalinity.
• (c)The pH value of fresh water measured at room temperature and at one atmospheric pressure — The most plausible of the wrong options, because it names a real measurement made under properly stated conditions, and candidates associate any single number describing water quality with pH. But pH and hardness are independent: soft water can be acidic or alkaline, and so can hard water. A soap test or an EDTA titration measures hardness; a pH meter does not.
• (d)The molar ratio of D2O to H2O in fresh water — This is about heavy water — water in which the hydrogen is the isotope deuterium — which occurs naturally in minute proportion and is separated industrially for use as a moderator in certain nuclear reactors, including India's pressurised heavy water reactors. It is a genuine quantity, and it has nothing to do with hardness. It is offered because it looks technical, and because 'heavy' water is easy to associate loosely with 'hard' water.

Hardness is one of a small set of routine water-quality measurements, each answering a different question. HARDNESS asks how much dissolved calcium and magnesium there is, and it governs soap consumption and scale. pH asks how acidic or alkaline the water is. ALKALINITY asks how much acid the water can neutralise before its pH falls, and comes mainly from bicarbonate. TURBIDITY asks how much suspended matter it carries. DISSOLVED OXYGEN and biochemical oxygen demand ask about the health of a water body and its organic pollution load. Hardness is not a health hazard at ordinary levels — hard water supplies calcium and magnesium in the diet — but it is an economic nuisance, which is why industrial and boiler feed water is softened or demineralised before use.

Key facts

• Hardness is the total concentration of dissolved calcium and magnesium ions in water.
• It is conventionally expressed as an equivalent concentration of calcium carbonate in milligrams per litre or parts per million.
• Calcium and magnesium enter water from limestone, chalk, dolomite and gypsum in the catchment.
• Temporary hardness comes from bicarbonates and is removed by boiling; permanent hardness comes from chlorides and sulphates and is not.
• Hard water forms an insoluble scum with soap instead of a lather, and deposits scale in kettles, pipes and boilers.
• Softening methods include boiling, addition of washing soda, lime treatment, and ion exchange or zeolite softeners.
• Total hardness is measured in the laboratory by titration with EDTA.
• pH measures acidity or alkalinity and is independent of hardness; heavy water refers to the deuterium isotope and is unrelated.

Common traps

• Reaching for pH whenever a question asks about a numerical property of water.
• Confusing hardness with alkalinity; they are related in origin but measure different things.
• Reading 'heavy water' as a synonym for 'hard water'.
• Assuming hard water is unsafe to drink; the objection to it is economic, not toxicological.

• practice — not a real PYQ

  Temporary hardness of water can be removed by which one among the following ?

  • (a)Boiling the water
  • (b)Adding common salt
  • (c)Filtering through sand
  • (d)Exposing the water to sunlight

  Answer(a) Boiling the water

• practice — not a real PYQ

  Hard water forms a scum with soap because calcium and magnesium ions do which one among the following ?

  • (a)Raise the pH of the solution sharply
  • (b)Form insoluble salts with the soap
  • (c)Decompose the soap into glycerol
  • (d)Prevent the soap from dissolving at all

  Answer(b) Form insoluble salts with the soap