LC Chem 9.2 Water Hardness

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Hard water

Water that will not easily form a lather with soap.

Ca2+ or Mg2+ ions

Causes of Hardness

ppm CaCO3

Units and substance in which water hardness is usually expressed


Hardness that can be removed by boiling

Calcium and Magnesium Hydrogencarbonates

Causes of Temporary hardness

Temporary hardness

Caused by the action of water and carbon dioxide on limestone


Causes a chemical reaction which removes the Ca2+ ions from the water to soften the water


Soap scum caused by hard water that clogs pipes and boilers


Hardness that cannot be removed by boiling

Calcium and Magnesium Chlorides and Sulfates

Causes of Permanent hardness

Advantages of Hard Water

1. Provides calcium for teeth and bones.
2. Nicer taste.
3. Good for brewing and tanning.

Disadvantages of Hard Water

1. Blocks pipes, leaves scale on kettles and boilers.
2. Wastes soap.
3. Produces scum.

Removes Hardness from Water

1. Distillation.
2. Using washing soda.
3. Ion exchange resin


Boiling the water and then cooling the vapour to remove all dissolved solids and dissolved liquids

Washing soda

Addition of hydrated sodium carbonate, Na2CO3.10H2O. The carbonate ions react with the calcium ions in the water and removes them as insoluble calcium carbonate. Ion exchange resin Substance which swaps the ions in water which cause hardness for ions which do not cause hardness

Cation exchange

Resins which swap the positive ions in the water (Ca2+ and Mg2+ ions) for Na+ ions: Ca2++ Na2R --> CaR + 2Na+

Anion exchange

Resins which swap the negative ions in water for OH- ions: Cl- + ROH --> RCl + OH-

Mixed resin

Replaces all the dissolved salts with water molecules so the water is deionised


Used to estimate the total hardness of a water sample as it forms metal complexes with alkaline earth metal ions Ca2+ and Mg2+ but not with alkali metal ions

Ethylenediaminetetraacetic acid

Full name of edta


pH needed for the estimation of the total hardness of a water sample using edta

Eriochrome Black T

Indicator used in the estimation of the total hardness of a water sample using edta

Wine red to Blue

Colour change observed in the estimation of the total hardness of a water sample using edta

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