Water

Question 1

What does water naturally contain?

Answer 1

Microorganisms and dissolved salts, which should be of sufficiently low levels to be safe for humans to drink.

Question 2

How is water that is good quality and safe to drink produced?

Answer 2

1. The water is passed through a filter bed to remove any solid particles.
2. Chlorine gas is then added to kill any harmful microorganisms.
3. Fluoride is added to drinking water in order to reduce tooth decay.
4. By choosing an appropriate source.

Question 3

What is the problem with consuming too much fluoride?

Answer 3

It can cause discolouration of teeth.

Question 4

What can be done to improve the taste and quality of tap water?

Answer 4

More dissolved substances can be removed by passing the water through a filter containing carbon, silver and ion exchange resins.

Question 5

Why is water distilled?

Answer 5

To produce pure water.

Question 6

What water can be distilled?

Answer 6

Any water.

Question 7

How is water distilled?

Answer 7

1. The water is boiled to produce steam.

2. The steam is condensed by cooling to produce pure liquid water.

Question 8

What are the disadvantages of distilling water?

Answer 8

The process uses a lot of energy, which makes distillation an expensive process.

Question 9

What determines whether water is described as hard or soft?

Answer 9

The amount of compounds present.

Question 10

Why does soft water readily form a lather with soap?

Answer 10

Because it doesn’t contain many dissolved compounds.

Question 11

What happens when permanently hard ward is boiled?

Answer 11

It remains hard.

Question 12

What happens when temporary hard water is boiled?

Answer 12

It is softened.

Question 13

What ions does temporary hard water contain?

Answer 13

Hydrogencarbonate ions (HCO3-)

Question 14

What happens to hydrogencarbonate ions upon heating?

Answer 14

They decompose to produce carbonate (CO3^2-) ions.

Question 15

How do carbonate ions form precipitates?

Answer 15

By reacting with calcium and/or magnesium ions.

Question 16

What do calcium and magnesium compounds dissolved in water react with soap to form?

Answer 16

Scum.

Question 17

When do calcium or magnesium compounds dissolve in natural water?

Answer 17

As it flows over ground or rocks containing compounds of these elements.

Question 18

What does scum do?

Answer 18

It makes it harder to form a lather.

Question 19

Why are soapless detergents used?

Answer 19

As they don’t form scum.

Question 20

What is an advantage of hard water?

Answer 20

The dissolved compounds in water are good for your health. For example, calcium compounds help the development of strong bones and teeth, and also help to reduce the risk of heart disease.

Question 21

What are two disadvantages of hard water?

Answer 21

1. More soap is needed to form a lather, which increases cost.
2. Using hard water often leads to deposits (called scale) forming in heating systems and appliances like kettles, which reduces their efficiency.

Question 22

How can hard water be made soft?

Answer 22

The dissolved calcium and magnesium need to be removed. This can be done by either adding sodium carbonate solution or passing the hard water through an ion-exchange column.

Question 23

How does adding sodium carbonate solution to hard water soften it?

Answer 23

The carbonate ions react with the calcium and magnesium ions to form calcium carbonate and magnesium carbonate (respectively), which precipitate out of solution as they are both insoluble.

Question 24

How does passing the hard water through an ion-exchange column soften it?

Answer 24

The ion-exchange column contains a resin that supplies hydrogen ions or sodium ions. As the hard water passes through the resin, the calcium and magnesium ions contained in it are replaced by hydrogen or sodium ions from the resin.

Question 25

What does hard water contain?

Answer 25

Dissolved compounds, usually of calcium and magnesium.

Question 26

What is produced when hard water reacts with soap?

Answer 26

Scum.