The document discusses hard water and its causes. Hard water is water that does not easily form soap lathers, due to the presence of calcium and magnesium ions that react with soap. There are two types of hardness: temporary hardness caused by calcium and magnesium bicarbonates that can be removed by boiling, and permanent hardness caused by calcium and magnesium chlorides and sulfates that cannot be removed by boiling. Methods for softening hard water include using ion exchange resins that replace calcium and magnesium ions with sodium ions.
Report
Share
Report
Share
1 of 14
More Related Content
Section 9.2 hardness in water
2.
Hard water is water that will not easily form a
lather with soap.
Hardness is caused by the presence of Ca2+ or
Mg2+ ions dissolved in the water. These ions react
with the soap to form a scum rather than a lather.
The concentration of the Ca2+ ions is greater than
the concentration of any other metal ion in our
water.
Water hardness is usually expressed in ppm CaCO3
3.
Soaps (e.g. Sodium Stearate)
Soap is a Sodium salt of Stearic Acid, C17H35COONa) which is a long
chain fatty acid
O
NaHO
palmitic acid
Sodium Stearate
Soap scum is formed when the Ca 2+ ion (or Mg2+ ion) reacts with the
soap. This causes an insoluble salt (calcium stearate) to precipitates
to form the scum.
2C17H35COONa + Ca2+
(C17H35COO)2Ca↓ + 2Na+
Soap actually softens hard water by removing the Ca 2+ ions from the
water
6. Temporary Hardness is hardness that can be
removed by boiling.
Temporary hardness is caused by:
◦ Calcium Hydrogencarbonate Ca(HCO3)2
◦ Magnesium Hydrogencarbonate Mg(HCO3)2
8. When boiled
Ca(HCO3)2
heat
CaCO3 + CO2 + H2O
Ca2+ (and Mg2+) ions are removed from the
water and now form the precipitate CaCO3 at
the bottom of the kettle etc.
This is referred to as limescale. This can
damage kettles and hot pipes.
9.
Hard water does cause soap scum, clogs
pipes and clogs boilers as limescale
10.
Permanent Hardness is hardness that cannot
be removed by boiling.
Permanent hardness is caused by
◦ Calcium Chloride CaCl2
◦ Magnesium Chloride MgCl2
◦ Calcium Sulfate CaSO4
◦ Magnesium Chloride Mg SO4
11. Ca2+ +
Na2R → CaR + 2Na+
The Hardness
causing calcium
and magnesium
ions are removed
from the water and
replaced by sodium
ions, which do not
cause hardness
12. Ion exchange resins with a mixture of a cation exchanger and
anion exchanger remove all the ions from water.
13. M+ + HR → MR + H+
All the metal ions in the water are replaced by hydrogen ions
A- + ROH → RA + OHAll the anions in the water are replaced by hydroxide ions
The hydrogen ions and hydroxide ions to give water
H+ + OH- → H2O
This mixed resin replaces all the dissolved salts with water molecules
The water is said to be DEIONISED
14. Deionised water is not quite as pure as distilled
water
Deionised water had all the dissolved ions in
the water removed
Distilled water has all the dissolved solids and
dissolved gases removed