Hardness of water

Question 1

Calcium hydrogencarbonate

Answer 1

Ca(HCO₃)₂

Question 2

Sodium Carbonate

Answer 2

Na₂CO₃

Question 3

Calcium Sulfate

Answer 3

CaSO₄

Question 4

Sodium Sulfate

Answer 4

Na₂SO₄

Question 5

When is hard water formed?

Answer 5

Hard water is formed when rainwater dissolves some of the rock that it flows over.

Question 6

What may rainwater contain?

Answer 6

Rainwater may contain dissolved carbon dioxide. This makes it slightly acidic.

Question 7

Rocks such as:

Answer 7

• 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.

Question 8

The word equation for the formation of temporary hard water is:

Answer 8

calcium carbonate + carbon dioxide + water ⇒ calcium hydrogencarbonate

Question 9

When is permanent hardness formed?

Answer 9

Permanent hardness is produced when calcium sulfate rock dissolves.

Question 10

Water hardness can be measured by how much water soap is needed to produce a lather.

Answer 10

• 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.

Question 11

How is temporary hardness removed?

Answer 11

• 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.

Question 12

Use of ion-exchange resins.

Answer 12

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.

Question 13

Thermal decomposition symbol Eq. for temporary hardness:

Answer 13

Ca(HCO₃)₂ ⇒ CaCO₃ + CO₂ + H₂O

Question 14

Washing Soda

Answer 14

• 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.

Question 15

Reaction between hard water and washing powder.

Answer 15

CaSO₄ + Na₂CO₃ ⇒ Na₂SO₄ + CaCO₃