C3- Water

Question 1

What does hard water contain?

Answer 1

Calcium and magnesium ions.

Question 2

How does hard water react to soap?

Answer 2

The calcium and/or magnesium ions in hard water react with soap to form an insoluble solid called scum.

Question 3

Why can temporary hard water be softened by boiling?

Answer 3

Because temporary hard water contains dissolved hydrogencarbonate ions, HCO₃^-, as well as calcium and/or magnesium ions.

When heated, the HCO₃^- decomposes to form carbonate ions, CO₃^2-, which in boiled water react with the calcium and/or magnesium ions to form scale.

Question 4

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

Answer 4

Hard water can contain calcium, which is good for our health; helping to develop strong bones and teeth, and calcium may also reduce the risk of heart disease.

Question 5

What problems may be presented by hard water?

Answer 5

• Unusual taste.
• Scale can be deposited in kettles, boilers and pipes. This reduces the efficiency of heating systems and causes blockages.

Question 6

What is the advantage to using soft water instead of hard water?

Answer 6

Soft water does not contain calcium and/or magnesium ions, so doesn’t produce scum when it reacts with soap and doesn’t produce scale when it’s heated.

Question 7

How can hard water be made soft?

Answer 7

By removing the calcium and/or magnesium ions from the water.

There are two methods:

• Adding sodium carbonate to the water.
• Using ion exchange columns.

(Temporary hard water can be made soft by boiling it).

Question 8

Why can’t permanent hard water be softened by boiling?

Answer 8

In contrast, permanent hard water contains dissolved sulfate ions, SO₄^2-, which do not decompose when heated so do not react with the calcium and/or magnesium ions therefore the water stays hard, even when boiled.

Question 9

Why can (both types of) hard water be made soft by adding sodium carbonate into the water?

Answer 9

Sodium carbonate, Na₂CO₃, (aka washing soda) reacts with the calcium and/or magnesium ions to form solid calcium carbonate and/or magnesium carbonate.

Question 10

Why can (both types of) hard water be softened using an ion-exchange resin?

Answer 10

1. Ion-exchange column is packed with a resin containing sodium or hydrogen (or silver) ions.
2. When hard water passes through the resin the calcium and/or magnesium ions swap places with the sodium or hydrogen ions.

Question 11

How is hard water formed?

Answer 11

When natural waters flow over ground or rocks containing calcium or magnesium compounds.

Question 12

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

Answer 12

Water is made fit to drink by filtering it to remove solids and adding chlorine to kill microbes.

Question 13

Pure water can be made by distillation.

Why is distillation not used to treat mains tap water?

Answer 13

Because it requires a large amount of energy to boil the water and so would be very expensive to do on a large scale.

Question 14

What is meant by the term, ‘drinking water’?

Answer 14

Drinking water should not contain any harmful substances and shows contain only low levels of dissolved substances and microbes.

Drinking water does not have to be pure, but it should not contain anything that will cause us harm.

Question 15

An ion exchange column is used for a few weeks.

Sodium chloride solution now needs to be passed through the ion exchange column.

Suggest why.

Answer 15

To replenish sodium ions.

Question 16

What are the arguements for and against water companies adding chlorine to sterilise water?

Answer 16

✅Prevents disease.

✅Kills microorganisms.

❌Toxic/poisonous.

❌No consumer choice.

Question 17

What are the advantages and disadvantages of using an ion exchange column to soften hard water?

Answer 17

✅Quick method to use.

✅Continuous process.

❌Increases sodium content of water (if sodium ions are used).

❌Resin needs periodic replenishment with sodium/hydrogen/silver ions.

Question 18

What are the advantages and disadvantages of adding sodium carbonate to soften hard water?

Answer 18

✅Easy method to use.

✅Relatively cheap.

❌Increases sodium content of water.

❌Leaves a residue of precipitated carbonate in the water.

❌Batch process.

Question 19

Why might filters used in the home contain particles of silver?

Answer 19

To prevent growth of microbes/kill microbes (sterilise).