Using resources 2

Question 1

What is potable water?

Answer 1

Water that is safe to drink.

Question 2

How is potable water obtained?

Answer 2

Either it’s naturally safe or it has to be treated.

Question 3

What is pure water?

Answer 3

Water only containing H₂O molecules.

Question 4

Is potable water pure?

Answer 4

No because it can contain lots of other dissolved substances.

Question 5

What should drinking water have for it to be considered ‘potable’.

Answer 5

Sufficiently low levels of dissolved salts and microbes (including bacteria).

A pH between 6.5 and 8.5.

Question 6

What do the methods used to produce potable water depend on?

Answer 6

Available supplies of water
Local conditions

Question 7

What does rain provide in the UK?

Answer 7

Water with low levels of dissolved substances (freshwater).

This collects in the ground, in
lakes, and in rivers.

Question 8

Briefly describe how most potable water is produced.

Answer 8

An appropriate source of fresh water is chosen.

The water is passed through filter beds.

The water is sterilised.

Question 9

List the sterilising agents used for potable water.

Answer 9

Chlorine
Ozone
Ultraviolet light

Question 10

If supplies of fresh water are limited, where can we get water from?

Answer 10

Salty or sea water.

Question 11

What is desalination?

Answer 11

The removal of salt from seawater using distillation or by processes that use membranes such as reverse osmosis.

Question 12

Why is using freshwater rather than salt water preferable?

Answer 12

Desalination requires large amounts of energy.

Question 13

How can rainwater collect?

Answer 13

As surface water (in lakes, rivers, and reservoirs).

As groundwater (in rocks called aquifers that trap water underground).

Question 14

Why does most of the domestic water supply in warm areas e.g southeast of
England come from groundwater?

Answer 14

Surface water dries up too quickly

Question 15

Filtration

Answer 15

A wire mesh screens out large items such as twigs.

Then, a gravel and sand bed is used to filter out any remaining insoluble solids.

Question 16

Sterilisation

Answer 16

The water is sterilised to kill any harmful bacteria or microbes. This can be done by bubbling chlorine gas through it or by using ozone or ultraviolet light.

Question 17

Why may seawater have to be used rather than freshwater to provide potable water?

Answer 17

In very dry countries there’s not enough surface or groundwater so potable water must be obtained from seawater.

Question 18

Steps to desalinating saltwater using distillation:

Answer 18

-Test the pH of the water using a pH probe and meter

-Test the water for the presence of sodium chloride (the main salt in seawater)

-Distil the water by pouring it into a distillation apparatus (diagram?) and heating the flask. The water will boil forming steam, leaving any (dissolved) salts in the flask.

-The steam will condense back to liquid water in the condenser and can be collected as it runs out.

-Retest the distilled water for sodium chloride to ensure it has been removed.

-Retest the pH of the water to check that it’s neutral.

Question 19

What must we do if the pH of the water is too high or low?

Answer 19

Neutralise it e.g. by titration.

Question 20

Why do we use a pH meter rather than an indicator to test the pH of water?

Answer 20

It won’t contaminate the water.

Question 21

How do we test for sodium ions?

Answer 21

Do a flame test on a small sample.

If sodium ions are present the flame will turn yellow.

Question 22

How do we test for chloride ions?

Answer 22

Take a small sample of your water and add a few drops of dilute nitric acid, followed by a few drops of silver nitrate solution.

If chloride ions are present, a white precipitate will form.

Question 23

Reverse osmosis

Answer 23

The salty water is passed through a membrane that only allows water molecules to pass through.

Ions and larger molecules are trapped by the membrane so separated from the water.

Question 24

Why are desalination or reverse osmosis not practical for producing large quantities of fresh water?

Answer 24

The processes require loads of energy so are really expensive and hence not practical.

Question 25

Industrial waste water may require what?

Answer 25

The removal of organic matter and harmful chemicals.

Question 26

Sewage and agricultural waste water require what?

Answer 26

The removal of organic matter and harmful microbes.

Question 27

Urban lifestyles and industrial processes produce large amounts of waste water that require what?

Answer 27

Treatment to ensure that we don’t pollute the natural environment.

Question 28

Where does water from flushed toilets, sinks etc go?

Answer 28

Into the sewers and towards sewage treatment plans.

Question 29

Why is the adding of additional chemicals e.g. fluoride to water controversial?

Answer 29

People aren’t given any choice as to whether they consume them or not.

Question 30

How do agricultural systems produce waste water?

Answer 30

Nutrient run-off from fields

Slurry from animal farms

Question 31

Why does sewage from domestic or agricultural sources need to be treated before it can be put back into freshwater sources (like rivers and lakes)?

Answer 31

To remove any organic matter and harmful microbes - which would pollute the freshwater sources and pose health risks.

Question 32

Give an example of an industrial process resulting in waste water.

Answer 32

The Haber process.

Question 33

What might industrial waste contain?

Answer 33

Organic matter

Harmful chemicals - this means it has to undergo additional stages of treatment before it is safe to release to the environment.

Question 34

What does sewage treatment include?

Answer 34

• screening and grit removal
• sedimentation to produce sewage sludge and effluent
• anaerobic digestion of sewage sludge
• aerobic biological digestion of effluent.

Question 35

1- Screening (before treatment of waste water)

Answer 35

Large insoluble solids like twigs, alongside grit, are removed from the sewage.

Question 36

2- Sedimentation

Answer 36

The sewage is set aside to stand in a settlement tank and undergoes sedimentation, whereby heavier suspended solids sink to the bottom to produce sludge, and the lighter effluent floats to the top.

Question 37

4- aneorobic digestion of sewage sludge

Answer 37

The sludge from the bottom of the settlement tank is removed and transferred into large tanks. It gets broken down here by bacteria in a process called anaerobic digestion.

Question 38

3 - aerobic biological digestion

Answer 38

The effluent in the settlement tank is removed and treated by aerobic biological digestion, whereby air is pumped through the water to encourage aerobic bacteria to break down any organic matter in the water. This includes other microbes.

Question 39

step 5 in treatment of sewage

Answer 39

When anaerobic respiration breaks down the organic matter in the sludge, this releases methane gas. The methane gas can be used as an energy source and the remaining digested waste can be used as a fertiliser.

Question 40

step 6 in treatment of sewage

Answer 40

For waste water containing toxic substances, additional stages of treatment may involve adding chemicals (e.g. to precipitate metals), UV radiation, or using membranes.

Question 41

Sewage treatment positive

Answer 41

Uses less energy than the desalination of salt water so could be used as an alternative in areas where there’s not much fresh water.

For example, Singapore is treating waste water and recycling it back into drinking supplies.

Question 42

Sewage treatment negatives

Answer 42

Requires more processes than treating freshwater.

People don’t like the idea of drinking water that used to be sewage.