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Flashcards in Water Deck (37):
1

Distinguish between the terms temporary and permanent hardness in water referring to a compound which causes each type of hardness and a method of removal

Temporary - calcium hydrogen carbonate, boiling
Permanent - calcium sulfate, ion exchange resin

2

Describe what takes place in the primary treatment of sewerage

It is a mechanical process involving screening and settling
Screening - floating solids screened by passing through steel bars, pebbles and grit removed by allowing them to settle into grit channels
Settling - sewage then flows into settling tanks allowing solids to settle to the bottom

3

Describe what takes place in secondary treatment of sewerage

Biological process using micro-organisms
Sewage is pumped into organic waste to break it down. It is then pumped into aeration tank and the sludge provides nutrients for organisms. The tank is aerated to prevent levels of dissolved oxygen getting too low. the sewerage is moved to a settling tank and sludge is removed

4

What are the substances removed in the tertiary treatment process

Nitrates and phosphates

5

Why is tertiary treatment considered necessary

To prevent eutrophication

6

What term describes the process of removing suspended solids in the treatment of water for drinking

Flocculation

7

Name the substance that is added to water during flocculation

Aluminium sulfate

8

Why is it necessary to adjust the PH of water if it is too acidic / What problems would arise if the PH were outside range 6-8

Prevent corrosion of pipes

9

What is used to adjust the PH of drinking water

Calcium hydroxide [slaked lime]

10

How does boiling remove any temporary hardness caused by the presence of calcium hydrogencarbonate in a water sample

Calcium hydrogencarbonate decomposes and is converted to calcium carbonate and a precipitate forms which is insoluble

11

[To determine the BOD of the water in a lake, two samples were taken and one was tested immediately]
Under what conditions was the second sample stored?

20C in dark

12

[To determine the BOD of the water in a lake, two samples were taken and one was tested immediately]
At what time interval was it tested?

5 days

13

The result of the BOD test suggested that eutrophication had occurred in the lake. State one of its harmful effects

Organisms in water are deprived of oxygen and die

14

Suggest a possible cause of eutrophication

Slurry run off

15

Why is there concern about lead in drinking water?

Can be harmful to human health [toxic]

16

Name an instrumental method used to analyse water for heavy metals

AAS

17

How can lead be removed from a contaminated water supply

Precipitation reaction

18

How does temporary hardness arise in rainwater flowing over limestone

Rain water is slightly acidic as it has reacted with CO2 in air so therefore it reacts with carbonate of limestone and forms hydrogen carbonate which is soluble

19

Write a balanced equation to describe the formation of limescale when this water is boiled

Ca(HCO3)2 -> CaCO3 + H2O + CO2

20

Explain in words how water with temporary hardness is deionised using a cation exchange resin and an anion exchange resin

two hydrogen ions from the cation-exchange resin replace a Ca2 (positive ions) in the water
a hydroxide ion from the anion-exchange resin replaces a HCO3– (negative ions) in the water
hydrogen ion and hydroxide ion combine to form water

21

Explain how hard water is caused

Dissolution of calcium ions

22

How does hard water waste soap

Reacts with calcium and magnesium ions to form scum

23

How can hard water be softened by ion exchange so that it is suitable for use as deionised water in the lab?

Passes through and positive ions are replaced by H+ ions and negative ions are replaced with OH- ions

24

In water treatment, what is the purpose of adding a flocculating agent and state a problem that would arise if it is added in excessive quantity

Clumping together of fine particles
Corrosion and tooth decay

25

In water treatment, what is the purpose of adding chlorine and and state a problem that would arise if it is added in excessive quantity

Kills pathogens (micro-organisms)
Toxic, strong odour is repulsive

26

In water treatment, what is the purpose of adding a fluorine-containing compound and state a problem that would arise if it is added in excessive quantity

Prevents tooth decay
Toxic, stains teeth

27

In water treatment, what is the purpose of adding calcium hydroxide and state a problem that would arise if it is added in excessive quantity

Raises PH
Too high and cause hardness

28

In water treatment, what is the purpose of adding sulfuric acid and state a problem that would arise if it is added in excessive quantity

Lowers PH
Corrosion and tooth decay

29

What is meant by biochemical oxygen demand (BOD) of a water sample

p.p.m of oxygen consumed when a sample of water is kept in th dark for 5 days at 20 degrees celcius.

30

Describe how BOD was reduced by about 30% in primary sewage treatment

Removal of solids by screening and settlement

31

Explain the processes by which BOD was further reduced in secondary sewerage treatment

Digestion and Aerobically (Aeration) involving micro organisms

32

What treatment is carried out to ensure low levels of micro-organisms in drinking water

Chlorination

33

A sample of hard water is treated for domestic use by ion exchange, assuming that the hardness is due to Ca(HCO3)2, explain in words how a cation exchange resin softens this water supply

Each calcium ion is replaced by two Na+ ions

34

What is meant by the term flocculation

Joining together of suspended particles

35

A whitish deposit is often found on the insides of kettels in hard water districts. If some of this deposit is scraped into a test tube and dilute hydrochloric acid is added a reaction is observed. Write a balanced equation for this reaction

CaCO3 + 2HCL -> CaCl2 + H20 + CO2

36

Give one reason why heavy metal ions may be found in rivers and lakes

From industrial effluent
Improper disposal of batteries

37

In measuring B.O.D of a sample of water, it is normal practice to keep a sample in the dark at 20 degrees for five days, why is this necessary?

Darkness - prevent photosynthesis as this would increase the level of oxygen in water
Temperature - Valid comparison of different water samples since the amount of oxygen dissolved in water depends on temperature. Also, the rate at which oxygen is consumed depends on temperature
Five days - Ensure a standard period of time to allow all the dissolved oxygen to be used up