Potable Water

Question 1

What must drinking water have to be save to drink

Answer 1

Drinking water has to have sufficiently low levels of dissolved salts such as sodium chloride

Drinking water cannot have high levels of microbes such as bacteria

pH must be between 6.5 and 8.5

Question 2

Define potable water

Answer 2

Potable water is water that is safe to drink

Question 3

What is pure water

Answer 3

Pure water is water that only contains H2O molecules

Question 4

Explain why potable water cannot be classified as pure water

Answer 4

Pure water in the chemical sense only contains no dissolved substances at all (pure water contains H2O molecules only). However potable water does contain dissolved substances, although in quite small amounts

Question 5

What provides most of the potable water in the UK and why?

Answer 5

In the UK, rain water provides most of our potable water. This is because rain water contains low levels of dissolved substances

Question 6

Where does rain water get collected

Answer 6

Rainwater collects in the ground in aquifers and in lakes, rivers and in reservoirs

Question 7

State some good sources of fresh water

Answer 7

Aquifers
Lakes
Rivers
Reservoirs

Question 8

Describe the steps used to produce potable water from fresh water

Answer 8

To produce potable water, first choose a good source of fresh water e.g. a river
Then, pass the water through filter beds. That is to remove solids such as leaves and suspended particles
The water is sterilised to kill microbes.
Sterilising agents used for potable water include chlorine, ozone or ultraviolet light.

(Add chlorine to kill microbes)

Question 9

What does fresh water contain in terms of dissolved minerals

Answer 9

Fresh water contains very low levels of dissolved minerals

Question 10

When might the desalination of salty water or sea water be required to produce potable water.

Answer 10

If supplies of fresh water are limited, potable water is produced by the desalination of salty water or sea water.

Question 11

What does sea water contain in terms of dissolved minerals

Answer 11

Sea water has very high levels of dissolved minerals.

Question 12

What does desalination do

Answer 12

Desalination reduces the levels of dissolved minerals down to an acceptable level for potable water.

Question 13

State some of the ways to carry out desalination and produce potable water from sea water

Answer 13

Distillation
Another way is to pass the water through membranes. This is called reverse osmosis

Question 14

Both ____ and _____ reduce the levels of dissolved minerals

Answer 14

Both distillation and reverse osmosis reduce the levels of dissolved minerals

Question 15

State a disadvantage to using distillation and reverse osmosis to produce potable water

Answer 15

Both processes require very large amounts of energy which makes them very expensive

Question 16

Describe how distillation is used to produce potable water

Answer 16

Sea water is boiled
Water molecules boil (impurities stay behind)
Steam is cooled and condensed to pure water
This process requires a lot of energy for heat. This makes the process very expensive

Question 17

Describe how reverse osmosis is used to produce potable water

Answer 17

Sea water is pushed through a semi-permeable membrane under pressure
Dissolved substances cannot pass through
This process requires a lot of energy for pressure. This makes the process very expensive

Question 18

Describe how to analyse a sample of water for purity

Answer 18

Check the pH of the water by placing a small amount of the water onto a piece of universal indicator. Pure water has a pH of 7. Universal indicator is green if the pH is 7.
If the pH is not 7 then the water sample contains dissolved acid or alkali and is therefore not pure.

If the pH is 7 then that does not mean that the water is pure - the water could still contain dissolved solids, so now we need to test for those.

To do this, use a balance to weigh an empty evaporating basin. Record the mass of the evaporating basin

Fill the evaporating basin with our water sample and place this on a tripod and gauze

Use a Bunsen Burner to gently heat the water until it has all evaporated.

At this point, all the evaporating basin to cool and then weigh it again.

If the water sample contained any dissolved solids then the mass of the empty evaporating basin will have increased. That is because the water has evaporated but the dissolved solids will have formed crystals on the surface of the evaporating basin.

If our water sample contained any dissolved solids, then the water is not pure.
However if the mass of the evaporating basin did not increase, then the water did not contain any dissolved solids and could be pure.

Question 19

Describe how to purify a sample of water by distillation

Answer 19

Gently heat the water using a Bunsen Burner
At this point, the water will evaporate and form water vapour.
The water vapour travels up the conical flask and into the condenser The condenser cools the vapour and condenses it. The vapour turns back into a liquid , and collects in the receiving flask. This is distilled water
The salt has been left behind in the round bottomed flask

Question 20

What does distilled water contain

Answer 20

Distilled water contains no dissolved solids and has a pH of 7
Distilled water is pure water

Question 21

State the equipment used to purify a sample of water by distillation

Answer 21

Conical flask on a tripod and a gauze
Delivery tube
Test tube
Beaker containing ice and water

Question 22

state the pH of pure water

Answer 22

7

Question 23

state the boiling point for pure water

Answer 23

100°C

Question 24

state boiling point of impure water/potable water

Answer 24

Impure/potable water boils slightly higher than 100°C

Question 25

Name the substance removed from seawater by desalination

Answer 25

Sodium Chloride

Question 26

Explain why it is more difficult to produce drinking water from waste water than from water in lakes

Answer 26

Water needs more processes because it contains more microbes and more toxic chemicals or detergents

Question 27

State an advantage of adding fluoride to drinking water

Answer 27

Water needs more processes because it contains more microbes and toxic chemicals or detergents

Question 28

How could the student check that all the water had evaporating after investigating how much solid was dissolved in sea water

Answer 28

Reheat the evaporating basin
Weigh the evaporating basin again. If all the water had evaporating there would be no change in mass

Question 29

Types of water

Answer 29

Pure water
Sea water
Potable water
Waste water (from homes, industry, agriculture)
Ground water (underground streams & aquifiers)
Fresh water (lakes rivers, ice caps, glaciers, underground streams & aquifiers

Question 30

Which types of water contain microbes - and how much

Answer 30

Sea water - contains microbes
Ground water - contains microbes
Fresh water - contains microbes (very low from some sources)
Waste water - contains microbes
Potable water - does not contain microbes (or very low amounts)
Pure water - does not contain microbes

Question 31

Which types of water contains dissolved substances

Answer 31

Sea water - high
Waste water - high
Fresh water - low
Potable water - low
Pure water does not contain any dissolved substances