Water

Question 1

What is soft water?

Answer 1

Water containing no dissolves calium and/or magnesium salts

It easily forms a lather with soap

Question 2

What is hard water?

Answer 2

Water containing dissolved calcium and/or magnesium ions

This makes it difficult to form a lather with soap as these ions react to form a scum

Question 3

Which products do not react with hard water to form a scum?

Answer 3

Soapless detergents

Question 4

What causes hard water to become hard in the first place?

Answer 4

• When water comes in contact with rocks some compounds dissolve
• If the rocks contain calcium or magnesium ions, these will dissolve in the water making it hard
• We know the water is hard because these ions will react with soap to form a scum

Question 5

What is scale and how is it produced?

Answer 5

The insoluble substance produced when temporarily hard water boils

Question 6

Why is scale bad?

Answer 6

It can be deposited in kettles, boilers, and pipes

This reduces the efficiency of the heating system and causes blockages

Question 7

Why is it better to drink hard water than soft water?

Answer 7

Calcium compounds are good for our health, helping to develop strong bones and teeth

Calcium may also resduce the risk of heart disease

Question 8

Does soft water still contain dissolved substances?

Answer 8

Yes!

But it does not contain the dissolved salts that react with soap to form scum

Also, it does not produce scale when heated

Question 9

What happens when you heat/boil permanent hard water?

Answer 9

It is not softened and does not produce scale

Question 10

Why is temporary hard water softened by heating?

Answer 10

• Temporary hard water contains hydrocarbon ions, HCO₃^-_(aq)
• These ions decompose when heated to produce carbonate ions, water, and carbon dioxide
• The carbonate ions then react with the calcium and magnesium ions in the water to produce precipitates of calcium carbonate and magnesium carbonate
• These are deposited as scale

Question 11

Write a balanced symbol equation showing how temporary hard water is softened by heating

Answer 11

2HCO₃^-_(aq) → H₂O_(l) + CO_2(g) + CO₃^2-_(aq)

Question 12

What are the two ways of softening both types of hard water?

Answer 12

By using washing soda or an ion-exchange column

Question 13

How can washing soda be used to soften water?

Answer 13

• Washing soda is sodium carbonate
• This reacts with the calcium and magnesium ions in the water to form solid calcium and magnesium carbonate precipitates
• These precipitates cannot react with soap

Question 14

How can an ion-exchange column be used to soften water?

Answer 14

• It is packed with a resin containing sodium or hydrogen ions
• When hard water is passed through, calcium and magnesium ions become attached to the resin
• The sodium or hydrogen ions then take their place in the water
• Sodium and hydrogen ions do not react with soap

Question 15

What other types of methods can be used to remove water hardness?

Answer 15

Any method that removes dissolved calcium and magnesium ions from hard water will soften it

Question 16

What should drinking water not contain?

Answer 16

• Harmful substances

It should also have low levels of:

• Dissolved salts
• Microbes

Question 17

How is drinking water treated to make it safe to drink?

Answer 17

• The water must come from a suitable source
• It is treated by sedimentation and filtration to remove solids
• This is followed by disinfection to kill microbes
• Chlorine is often used to kill microbes in drinking water

Question 18

What can water filters be used to do and how?

Answer 18

• To improve the taste
• They often contain carbon and an ion-exchange resin that removes some soluble substances
• They may also contain silver or another substance to prevent the growth of bacteria

Question 19

How is pure water made and why is this not used to treat mains tap water?

Answer 19

Can be made by distillation

This requires amount of energy to boil the water and so it would be vert expensive to do on a large scale

Question 20

What are the advantages and disadvantages of softening water?

Answer 20

Advantages:

• Requires less soap
• Does not produce scum
• Does not produce scale so does not damage heating systems etc.

Disadvantages:

• Reduces health benefits
• Costs money
• (Tases manky)

Question 21

Why is it important to control the amount of chlorine added to water supplies?

Answer 21

It is particularly effective at killing the microbes in water so that it is safe to use

However, it is poisonous and can produce other toxic compounds

Therefore it must be carefully controlled to minimise risks

Question 22

What are the advantages and disadavtages of adding fluoride to drinking water?

Answer 22

Advantages:

• Can help prevent tooth decay
• Might save dentists/people time and money

Disadvantages:

• People should be able to choose to take extra fluoride or not
• Costs money