Hard Water

Depending on which part of the country you live in, you will have hard or soft water
Hard water contains dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions
These dissolve in the water when it has come into contact with rocks such as limestone that contain calcium compounds
- E.g when rainwater flows over rocks on its way to a resevoir
Soft water has only low concentrations of these ions
The difference between these two types of water can be observed via their reaction with soap:
- Soft water forms a lather much more easily than hard water
- The minerals in hard water react with soap to form scum making it harder to form a lather

person washing hands on sink

Soft water forms a lather more easily than hard water

The relative amount of hardness in different water samples can be determined by:
- Measuring the volume of soap solution required to produce a given lather
  OR
  By measuring the lather produced by a given volume of soap solution
Details of this investigation can be located in 'Determining hardness in water'

Exam questions will ask you to compare water samples for hardness levels.

Remember: The more soap needed to form a lather, the harder the water.

Temporary & Permanent Hardness

Hard water can be softened by removing the calcium and magnesium ions
Sometimes this can be achieved by boiling the water:
- Water that can have its hardness removed by boiling it is called temporary hard water
- This method of removing hardness leads to the formation of scale which causes 'furring' of the heating element in a kettle making it less efficient
- Scale also forms in washing machines, pipes and immersion heaters and can cause pipes to eventually become clogged
Temporary hardness is caused by dissolved calcium hydrogencarbonate, Ca(HCO₃)₂ which decomposes when the water is heated
Some types of hard water cannot have their hardness removed by boiling
- This type of water is called permanent hard water
- Permanent hardness is caused by dissolved calcium sulfate, CaSO₄which does not decompose when the water is heated

Image taken from Wikimedia

Water that can have its hardness removed by boiling is called temporary hard water

Higher Tier students must be able to explain how boiling removes temporary hardness
Temporary hardness is caused by dissolved calcium hydrogencarbonate, Ca(HCO₃)₂
- This is formed when rainwater containing carbon dioxide flows over rocks containing calcium carbonate

calcium carbonate + water + carbon dioxide → calcium hydrogencarbonate

CaCO₃(s) + H₂O (l) + CO₂(g) → Ca(HCO₃)₂(aq)

When the water is boiled, the calcium hydrogenencarbonate decomposes to form calcium carbonate, water and carbon dioxide

calcium hydrogencarbonate → calcium carbonate + water + carbon dioxide

Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O (l) + CO₂(g)

The calcium carbonate is insoluble which leads to the 'furring' of the kettle elements

Water samples A, B, C and D were tested for hardness by adding soap solution 1 cm³ at a time to a sample.

The total volume of soap solution needed to produce a permanent lather was recorded.

Each sample was tested before and after boiling.

Water sample	Volume of soap solution required (cm³)
Water sample	Before boiling	After boiling
A	1	1
B	9	9
C	14	7
D	14	1

Explain which water sample contains only temporary hardness.

Answer:

D contains only temporary hardness because all hardness is removed by boiling
- The volume of soap required after boiling to form a lather is the same as water sample A
Sample A is soft water as it requires little soap to form a lather with the same volume required before and after boiling
Sample B is permanent hard water as a large volume of soap is required for a lather to form and this does not change after boiling
Sample C has some temporary hardness as the volume of soap required to form a lather reduces, but not to the level of A so not all hardness has been removed