Hardness of water
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Calcium hydrogencarbonate
Ca(HCO3)2
Sodium Carbonate
Na2CO3
Calcium Sulfate
CaSO4
Sodium Sulfate
Na2SO4
When is hard water formed?
Hard water is formed when rainwater dissolves some of the rock that it flows over.
What may rainwater contain?
Rainwater may contain dissolved carbon dioxide. This makes it slightly acidic.
Rocks such as:
• Chalk, limestone and marble are forms of calcium carbonate. These react with water and carbon dioxide to form calcium hydrogencarbonate. • Calcium hydrogencarbonate dissolves to form temporary hard water.
The word equation for the formation of temporary hard water is:
calcium carbonate + carbon dioxide + water ⇒ calcium hydrogencarbonate
When is permanent hardness formed?
Permanent hardness is produced when calcium sulfate rock dissolves.
Water hardness can be measured by how much water soap is needed to produce a lather.
• When a soap flake is shaken in a water sample, calcium ions in the water react with the soap and turn it into scum. • Eventually, as more flakes are added and the mixture shaken, the soap reacts with all of the calcium ions. • After that, any more soap produces a stable lather for the first time. The number of soap flakes needed is a measure of water hardness.
How is temporary hardness removed?
• Temporary hardness is removed by boiling. The calcium hydrogencarbonate decomposes easily in hot water to form calcium carbonate (limescale), water and carbon dioxide. • Heating removes the soluble calcium ions from the water, by changing them into insoluble calcium carbonate. • Permanent hardness is not affected by heating - calcium sulfate is too stable.
Use of ion-exchange resins.
Ion-exchange resins are a way of removing permanent hardness. The water flows over beads of solid resin, which trap the calcium ions on its surface, exchanging them for sodium ions. Ion- exchange resin removes temporary and permanent hardness.
Thermal decomposition symbol Eq. for temporary hardness:
Ca(HCO3)2 ⇒ CaCO3 + CO2 + H2O
Washing Soda
• Washing soda can softened both temporary and permanent water. It is sodium carbonate. • When washing soda dissolves, it reacts with the calcium sulfate in the water to form insoluble calcium carbonate, so ‘locking up’ the calcium ions.
Reaction between hard water and washing powder.
CaSO4 + Na2CO3 ⇒ Na2SO4 + CaCO3