Physical And Enviro Chem - Water

Question 1

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What is meant by Hard Water

Answer 1

Water which doesn’t easily form a lather with soap

Question 2

How is Hard water caused

Answer 2

presence of calcium(Ca^2+) or magnesium ions (Mg^2+)

Question 3

What is soap

Answer 3

Soap is a salt of a fatty acid (a carboxylic acid)

Question 4

What is an example of a common soap?

Answer 4

Sodium stearate ion

Question 5

Explain with the aid of a balanced chemical equation why does hard water not form a lather with soap/ How does hard water waste soap/how does hard water form on insoluble grey scum with soap.

Answer 5

Balanced Chemical equation
2C^17H^35COO - + Ca ^ 2+ → (C^17H^35COO)^2 Ca

Soap is a salt of a fatty acid.

The soap we will use is Sodium stearate

The stearate ion present in this soap example reacts with CA^2+ or Mg^2+ ions in hard water.

They then form calcium/magnesium stearate, an insoluble grey ‘scum’

The soap does not form a lather with the water until all of the Ca2+ or Mg2+ ions in the water have been used up

Soap is also wasted in this way

Question 6

Name the 2 types of hardness in water

Answer 6

Temporary Hardness
Permanent Hardness

Question 7

What is Temporary Hardness

Answer 7

Hardness that can be removed by boiling the water

Question 8

What is Temporary hardness caused by

Answer 8

Temporary Hardness is caused by the presence of

calcium hydrogen-carbonate (Ca(HCO^3)^2)
OR
magnesium hydrogen-carbonate(Mg(HCO^3)^2)

Question 9

How do we precipitate the mentioned hydrogen carbonate ions out of water

Answer 9

by boiling/heating the substances it causes the solution to precipitate out in the form of calcium carbonate or magnesium carbonate(Limescale)

Question 10

What is Permanent Hardness

Answer 10

Hardness that cannot be removed by boiling the water

Question 11

What is Permanent Hardness caused by

Answer 11

The presence of Calcium sulfate (CaSO^4)
or Magnesium sulfate

Question 12

Is it possible to heat/boil these to remove the magnesium/calcium ions.

Answer 12

No you cannot i.e. it doesn’t soften water

Question 13

Describe with the aid of balanced chemical equations how temporary hardness in water arises

Answer 13

Rainwater is slightly acidic due to carbon dioxide from the air dissolving in it.

This then upon contact forms a weak carbonic acid

H2O + CO2 ⇄ H^2CO^3

• As this acidic rainwater (carbonic acid) flows over limestone rocks (calcium carbonate), it reacts
  with it, forming calcium hydrogencarbonateH^2CO^3 + CaCO^3 ⇄ Ca(HCO^3)^2

i.e. temporary hard water is a dilute solution of calcium hydrogencarbonate

Question 14

Explain with the aid of a balanced chemical equation how boiling removes temporary
hardness?

Answer 14

The soluble calcium hydrogencarbonate breaks down into calcium carbonate, carbon dioxide
and water

Calcium carbonate is insoluble and precipitates out of solution
i.e. the calcium ions are no
longer dissolved in the water – the water has been ‘softened’

                     boil Ca(HCO^3)^2 ---→ CaCO^3 + H^"O +CO^2

Question 15

By what common name is this insoluble calcium carbonate precipitate known?

Question 16

Give two disadvantages of hard water

Answer 16

Causes limescale which build up on (machines that use hot water) washing machines, dishwashers, kettles

Hard water wastes soap i.e. the soap will not form a lather and begin cleaning until the Ca^2+/Mg^2+
ions causing the hardness have been used up

Question 17

What is Total Hardness

Answer 17

The sum of the permanent hardness and temporary hardness

Question 18

How can total hardness in a water sample be removed?

Answer 18

Distillation

Washing soda tablets/Bath salts; use sodium carbonate (Na2CO3)

Using an ion-exchange resin