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23 terms

LC Chem 9.2 Water Hardness

chemistry
STUDY
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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
Temporary
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
Boiling
Causes a chemical reaction which removes the Ca2+ ions from the water to soften the water
Limescale
Soap scum caused by hard water that clogs pipes and boilers
Permanent
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
Distillation
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
edta
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
9-10
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