1. Water Hardness

2. Limescale deposits in hot water pipes and on filament in kettles CaCO3

3. 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

4. Where do the ions that cause water Hardness come from?
Mg+ ions come from dolomite rock
MgCO3
Ca+ ions come from Limestone rock

5. 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 precipitate out thus forming a scum.
2C17H35COONa + Ca (C17H35COO)2Ca↓ + 2Na+
Soap actually softens hard water by removing the Ca2+ ions from the water Na
Sodium Stearate

6. Modern detergents Made from crude oil Not affected by hard water ions
Too severe on skin

7. Soft water
Is water that lathers easily
No Ca 2+ or Mg 2+ ions

8. Types of Hardness
Temporary Hardness
Permanent Hardness

9. Temporary Hardness
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

10. Causes of Temporary Hardness
Temporary Hardness can be removed by Boiling
Rainwater slightly acidic due to CO2 (carbonic acid)
Acidic rainfall will dissolve limestone (CaCO3)
Forming soluble Calcium Hydrogen Carbonate Ca(HCO3)2 and Mg (HCO3)2
If heated a reaction occurs in which the Ca ions form insoluble CaCO3
Pg 264 book for equations (must know)

11. Water Cycle

12. Causes of Temporary Hardness
Limestone + Carbonic Acid
Carbonic Acid
Calcium Hydrogen Carbonate (aq)

13. Temporary Hardness= Rain to Acidic Rain to Hard water to Lime scale
carbonic acid
H CO2 = H2CO3
CaCO H2CO3 = Ca(HCO3 )2 (soluble)
Ca(HCO3)2 + heating = CaCO3 + CO2 + H2O
insoluble
CaCO3 destroys heating elements and blocks pipes
Limestone carbonic acid = calcium Hydrogencarbonate
Insoluble soluble

14. Ca(HCO3)2 heat CaCO3 + CO2 + H2O
Temporary Hardness
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.

15. Why Be Concerned About Hard Water?
Hard water does cause soap scum, clogs pipes and clogs boilers as limescale

16. Permanent hardness
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

17. Hard water disadvantages
Limescale is deposited on heating elements which reduces their efficiency e.g. washing machines and kettles
Limescale blocks pipes
Soap not as efficient
Produces scum

18. Hard water Advantages
Source of calcium which is good for teeth and bones
Some people believe it tastes nicer
Makes better tasting beer

19. To soften the hard water it is necessary to remove the calcium and magnesium ions
Addition of washing soda
Distillation
Ion exchanges
Deionisers

20. Addition of washing soda
Washing soda is hydrated sodium carbonate (Na2CO3.10H2O)
Calcium ions+ carbonate ions=calcium carbonate (insoluble)
Washing soda is usually included in clothes detergent
Ca2+ + CO3- = CaCO3 (precipitate)

21. Distillation
Distilling water removes all insoluble and soluble particle
Including calcium and magnesium ions
Hence water is softened
Too expensive
Pure water formed

22. Ion exchangers
Swaps the hardness ions with ions that don’t cause hardness
Sodium ions used as swap for mg and Ca ions
Cation Exchange
Uses filters
Not as pure as distillation

24. Removal of Hardness by Ion Exchange
Ca Na2R → CaR Na+
The Hardness causing calcium and magnesium ions are removed from the water and replaced by sodium ions, which do not cause hardness

25. Deionised water
To make deionised water it must be passed over a mixed bed resin (cation and anion resin)
RH (cation) + Na+ = RNa + H+
ROH (anion) + CL- = RCL + OH-
H+ + OH- = H2O
Only ions removed so product not as pure as distillation

26. Deionisers Removes all ions from water
As water passes through all the positive ions are replaced with H+ and negative ions replaced with OH-

28. Deionised Water
Ion exchange resins with a mixture of a cation exchanger and anion exchanger remove all the ions from water.

29. Deionised Water M+ + HR → MR + H+ A- + ROH → RA + OH- H+ + OH- → H2O
All the metal ions in the water are replaced by hydrogen ions
A- + ROH → RA + OH-
All 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

30. Distilled vs Deionised
Purer
Dissolved and suspended solids removed
Not as pure
Dissolved gases present
Non-ionic molecules present

31. Deionised Water and Distilled Water
Deionised water is not quite as pure as distilled water Deionised water has all the dissolved ions in the water removed Distilled water has all the dissolved solids and covalent compounds e.g. dissolved gases removed