Water Hardness

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

Water Hardness

Soaps (e.g. Sodium Stearate)

Soap is a Sodium salt of Stearic Acid, C17H35COONa) which is a long

chain fatty acid

Soap scum is formed when the Ca2+ 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 Ca2+ ions from

the water

O

HOpalmitic acidSodium

Stearate

Na

Temporary Hardness

Permanent Hardness

Types of Hardness

Water Cycle

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

Temporary Hardness

Causes of Temporary Hardness

Carbonic AcidLimestone + Carbonic Acid Calcium Hydrogen Carbonate

(aq)

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.

Temporary Hardness

Why Be Concerned About Hard Water?

Hard water does cause soap scum, clogs pipes and clogs boilers as limescale

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

Permanent hardness

Ca2+ + Na2R → CaR + 2Na+

Removal of Hardness by Ion Exchange

The Hardness

causing calcium

and magnesium

ions are removed

from the water

and replaced by

sodium ions,

which do not

cause hardness

Deionised Water

Ion exchange resins with a mixture of a cation

exchanger and anion exchanger remove all the ions

from water.

Deionised Water

M+ + HR → MR + H+

A- + ROH → RA + OH-

All the anions in the water are replaced by hydroxide ions

All the metal ions in the water are replaced by hydrogen 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

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

Deionised Water and Distilled Water