Hard Water

Section 1 of 8Hard and soft water

Where hardness comes from

• Where hardness comes from Rainwater flowing over rocks such as limestone dissolves calcium compounds,
  • so the water picks up calcium (Ca²⁺) and magnesium (Mg²⁺) ions.
• Whether your tap water is hard or soft depends on the rocks in your region.

Hard water

• Hard water Hard water contains relatively high concentrations of dissolved calcium and magnesium ions.

Soft water

• Soft water Soft water has only low concentrations of these ions.

Section 2 of 8Telling them apart with soap

1Add soap

• Add soap Soap solution is added to a sample of water and shaken.

2Ions react

• Ions react In hard water, calcium and magnesium ions react with the soap and form an insoluble scum.

3Less lather

• Less lather Because soap is used up making scum, more soap is needed before a lather forms.

4Compare

• Compare The more soap needed to give a lasting lather, the harder the water. Soft water lathers easily.

Section 3 of 8Temporary and permanent hardness

Feature	Temporary hardness	Permanent hardness
Dissolved compound responsible	Calcium hydrogencarbonate, Ca(HCO3)2	Calcium sulfate, CaSO4
Effect of boiling	Hardness is removed	Hardness is not removed
Scale in kettles	Forms when heated	Does not form on heating
Reason	The compound decomposes on heating	The compound does not decompose on heating

Section 4 of 8Problems caused by scale

Furring up

• Furring up When temporary hard water is heated, solid scale forms on kettle elements and makes them less efficient.

Other places

• Other places Scale also builds up in washing machines, immersion heaters and pipes, and can eventually block pipes.

Section 5 of 8Why boiling removes temporary hardness (HT only)

1

Rainwater containing dissolved carbon dioxide flows over rock containing calcium carbonate and slowly dissolves it: CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq).

2

This dissolved calcium hydrogencarbonate causes the temporary hardness.

3

On heating, it decomposes: Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g).

4

The calcium carbonate formed is insoluble, so calcium ions leave the solution and the insoluble solid is deposited as scale.

Section 6 of 8Worked example: interpreting soap volumes

Sample result	Interpretation
Small volume of soap needed before and after boiling	Soft water
Large volume before boiling, same large volume after	Permanent hardness only
Large volume before, falls to the soft-water level after	Temporary hardness only
Large volume before, falls but stays above soft-water level	Both temporary and permanent hardness

Section 7 of 8Key terms

Term	Meaning
Hard water	Water containing dissolved calcium and magnesium ions that reduce lathering with soap.
Soft water	Water with few dissolved calcium and magnesium ions that lathers easily.
Temporary hardness	Hardness caused by calcium hydrogencarbonate that is removed by boiling.
Permanent hardness	Hardness, for example from calcium sulfate, that is not removed by boiling.
Scale	Insoluble calcium carbonate deposit left when hard water is heated.
Scum	The insoluble solid formed when soap reacts with calcium or magnesium ions.

Section 8 of 8Exam technique

1

Saying hard water contains 'hard particles' - the cause is dissolved calcium and magnesium ions.

2

Stating that boiling removes all types of hardness; it only removes temporary hardness.

3

Mixing up the compoundstemporary is hydrogencarbonate, permanent is sulfate.

4

Writing that harder water needs less soap; it needs more soap to form a lather.