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LC Chemistry > Water > Flashcards

Water Flashcards

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1
Q

hard water

A

water that will not form a lather easily with soap. forms a scum instead. caused by the presence of Ca+2 and Mg +2

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2
Q

what is the chemical formula of sodium searate

A

C17H35OONa

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3
Q

what is the most common substance found in soap

A

sodium stearate

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4
Q

what happens when sodium stearate is added to hard water

A

a grey insoluble scum

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5
Q

what is the name of the substance that forms when sodium stearate is added to hard water

A

calcium stearate

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6
Q

when will a lather form with soap

A

when all the calcium ions in the water have formed calcium stearate.

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7
Q

write the balanced equations for what happens when sodium stearate is added to hard water

A

C17 H35COONa –> C17H35COO- + Na+

Ca+2 + 2C17H35COO- —> (C17H35COO)2Ca(down arrow)

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8
Q

why are modern detergents not affected by water hardness

A

they do not contain soap they come from crude oil

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9
Q

test for water hardness

A

add water to a test tube
add soap to water and shake
if water is hard a lather does not form. soap scum forms instead
if water is soft, a lather forms easily

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10
Q

advantaged of hard water

A

provides calcium for healthy teeth and bones
tastes nicer
good for brewing industry

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11
Q

disadvantages of hard water

A

blocks pipes causes limescale on kettles and washing machines
wastes soap
produces scum

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12
Q

temporary hardness

A

hardness that can be removed by boiling
caused by calcium hydrogencarbonate

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13
Q

chemical formula of calcium hydrogencarbonate

A

Ca(HCO3)2

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14
Q

what forms carbonic acid

A

rain water is slightly acidic - CO2 dissolves in rainwater forming a weak acid ie carbonic acid

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15
Q

write the balanced equation for the formation of carbonic acid

A

H2O + CO2 (equilibrium) H2CO3

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16
Q

write the balanced equation of the reaction between carbonic acid and limestone

A

CaCO3 + H2CO3 —> Ca(HCO3)2

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17
Q

how do we remove temporary hardness

A

boiling

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18
Q

how does boiling remove temporary hardness

A

the calcium ions that were in dissolved in water are now bound in the calcium carbonate, thus the Ca+2 ions that cause hardness are removed from the water

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19
Q

how can you remove limescale

A

reacting it with hydrochloric acid

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20
Q

write the balanced equation by which boiling removes temporary hardness

A

Ca(HCO3)2 —-> CaCO3 (down arrow) + CO2 + H2O

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21
Q

write the balanced equation for removing limescale with HCl

A

CaCO3 + 2HCl —-> CaCl2 + H2O + CO2

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22
Q

how do you confirm CO2 production

A

bubble gas through limewater. if gas is CO2 the limewater changes colour from colorless to milky white

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23
Q

permanent hardness

A

hardness that cannot be removed by boiling
caused by calcium or magnesium sulphate

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24
Q

what happens when water containing Calcium or magnesium sulphate is boiled

A

there is no chemical reaction that causes the precipitation of the calcium or magnesium ions

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25
Q

differences between distilled and deionised water

A

distilled:
formed by distillation of tap water
purest form of water

deionised:
tap water is passed through a mixed- bed resin and positive and negative ions are removed
usually still contains dissolved gases

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26
Q

how is distilled water formed

A

dissolved solids and water have different boiling points, thus separation according to boiling point occurs. the mixture is heated, and water evaporates. water vapour is condensed as pure water

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27
Q

what are the 4 methods of removing water hardness

A

add soap
distillation
washing soda
ion exchange

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28
Q

explain how adding soap is a method of removing water hardness

A

add soap to the sample of hard water and shake
sodium stearate in soap combines with the Ca+2/Mg+2 ions in water to form soap sum
keep adding soap until a lather forms. at this stage all the Ca+2/Mg+2 ions in water have been removed so a lather can form

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29
Q

how does distillation remove water hardness

A

separation of based on their different boiling points
the substances dissolved in water that are responsible for water hardness eg MgSO4 have higher boiling points than water
the sample of hard water is boiled and the vapour is condensed
the distilled water is pure water

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30
Q

what is a disadvantage of distillation

A

expensive

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31
Q

how does washing soda remove water hardness

A

washing soda crystals are added to water to soften it. the carbonate ions in the washing soda react with the calcium ions in the water, forming calcium carbonate, removing calcium ions from water.

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32
Q

what is ion exchange

A

swap ions that cause hardness for ions that do not cause hardness

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33
Q

explain household ion exchange

A

cation exchange resin
-water that contains Ca+2 ions is passed through the cation exchange resin
-Ca+2 ions are removed from water and replaced with Na+ ions, which do not cause hardness
- one Ca+2 is replaced by 2 Na+ ions

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34
Q

give the balanced equation of household ion exchange

A

Ca+2 + 2RNa–> R2Ca + 2Na+

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35
Q

explain deionised water ion exchange

A

mixed bed resin
-all ions are removed from deionised water but it may still contain dissolved gases and suspended solids
-water that contains a mixture of positive and negative ions is passed through a deioniser that has a mixture of cation and anion exchange resins - mixed bed resin
-the cation exchange resin removes the cations and replaces them with H+ ions.
- the anion exchange resin removes anions and replaces them with OH- ions

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36
Q

write the balanced equations for a mixed bed resin

A

cation exchange: RH + Na+ —> RNa + H+
anion exchange: ROH + Cl- —> RCl + OH-
H+ + OH- —> H2O

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37
Q

what does EDTA stand for

A

ethylenediaminetetraacetic acid

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38
Q

criteria for drinking water

A

colourless, safe to drink( chlorinated), fluoridated

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39
Q

what are the 7 steps of water treatment

A

screening
flocculation
sedimentation
filtration
chlorination
fluoridation
pH adjustment

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40
Q

explain screening

A

water from a river or lake is passed through a wire mesh.
acts like a large sieve and removes large pieces of floating debris

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41
Q

explain flocculation

A

is the coagulation of small suspended particles in water to form larger particles

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42
Q

what chemicals are used in flocculation

A

aluminium sulphate

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43
Q

explain sedimentation

A

water passes into sedimentation/ settlement tanks
- water flows in at the bottom of the tank and slowly rises to the surface to allow maximum settlement of particles
-clear water overflows into channels
- removed 90% of suspended particles

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44
Q

explain filtration

A

water from the sedimentation tanks is passed through beds of sand to filter any remaining suspended solids

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45
Q

explain chorination

A

water from filtration is now colourless and clear but may contain harmful bacteria
- chlorine is added to water to make it safe to drink by killing harmful microorganisms
- a small amount of chlorine is used as too much causes water to taste and smell bad

46
Q

explain fluoridation

A

1ppm of fluoride components are added to water.

47
Q

what are examples of fluorine compounds added to water during treatment

A

NaF, H2SiF6

48
Q

why is only a small amount of fluorine added to water during treatment

A

too much can stain teeth

49
Q

why is fluorine added to water

A

it strengthens enamel of teeth

50
Q

explain pH adjustment

A

optimum pH 7.2. acidic water may corrode pipes. calcium hydroxide is added to water to increase the pH . if pH is too high sulphuric acid or CO2 is added to lower the pH.

51
Q

what is Al2(SO4)3 or aluminium oxide used for in water treatment

A

floculating agent

52
Q

what happens if you add too much Al2(SO4)3 or aluminium oxide in water treatement

A

affects taste and corrodes pipes

53
Q

what is chlorine gas used for in water treatment

A

chlorination

54
Q

what happens if to much chlorine is added in water treatment

A

affects taste and smell

55
Q

what is sodium fluoride used for in water treatment

A

fluoridation

56
Q

what happens if to much sodium fluoride is added in water treatment

A

stains teeth

57
Q

what is calcium hydroxide used for in water treatment

A

pH adjustment increase

58
Q

what happens if to much calcium hydroxide is added in water treatment

A

causes hard water

59
Q

what does sulphuric acid do in water treatment

A

pH adjustment decrease

60
Q

what happens if to much sulphuric acid is added in water treatment

A

corrosion of pipes

61
Q

what does sodium carbonate do in water treatment

A

softens the water

62
Q

what happens if to much sodium carbonate is added in water treatment

A

affects taste

63
Q

biochemical oxygen demand

A

the amount of dissolved oxygen in ppm consumed by biological action
when a sample of water is stored at 20 C in the dark for 5 days

64
Q

what is the bod test used for

A

used to measure the amount of dissolved oxygen in water

65
Q

why is dissolved oxygen vital

A

to maintain life in oceans and lakes

66
Q

what does the solubility of oxygen depend on

A

temperature - as temp increases solubility decreases

67
Q

what affects dissolved oxygen in lakes or oceans

A

organic waste ie slurry or sewage
-bacteria in organic waste are provided oxygen and nutrients in water
-the bacteria multiply and break down the organic waste but use up the oxygen in the water to form co2 and h2o
- if there is enough organic material in water the dissolved oxygen levels falls below the level that is needed to sustain life

68
Q

what else can affect dissolved oxygen levels

A

photosynthesis- o2 levels will increase
respiration- o2 levels will decrease

69
Q

why do we dilute samples for bod tests with pure water

A

ensures dissolved oxygen remains present throughout the 5 day incubation period an ensures we can measure a value for O2 concentration

70
Q

procedure for bod test

A

-2 water bottles completely filled to brim with water being tested
-wrinkler titration method is used to measure dissolved oxygen, one bottle is measure immediately, the other is incubated in the dark, for 5 days, at 20 C and then O2 is measured again
- the BOD is the difference between the initial and final measurement in ppm

71
Q

what are the conditions at which the water being BOD tested is stored

A

dark, for 5 days, At 20 C

72
Q

what are the 3 precautions when carrying a bod test

A

1.when filling the bottles, hold the bottle under the surface of the water to prevent air bubbles containing oxygen being trapped in the bottle

  1. put the lid on the bottle below the surface of the water to ensure there is no air pocket at the top of the bottle- some oxygen in the air could dissolve in the water sample thus making the experiment less accurate
  2. immediately place the second bottle in the dark to prevent photosynthesis
73
Q

what does a high bod value mean

A

lots of organic material is present so more oxygen is removed from water

74
Q

bod value of clean drinking water

A

1-2

75
Q

bod for polluted water ie the level that kills fish

A

100

76
Q

bod for raw sewage

A

300

77
Q

bod for silage effluent

A

54000

78
Q

how to calculate bod

A

amount of o2 consumed = initial - final O2 conc
calculate dilution factor
multiply amount of O2 consumed by dilution factor

79
Q

eutrophication

A

the enrichment of water with nutrients (nitrogen and phosphorus) which can lead to excessive growth of algae and other plants

80
Q

how can bod be reduced

A

if there is excess of plant nutrients in the water eg nitrate and phosphate ions

81
Q

what is natural eutrophication

A

occurs in lakes an is cause by nitrogen and phosphorus gradually increasing over time as sediment builds up in the lake

82
Q

artificial eutrophication

A

caused suddenly, e.g. fertiliser run off from a farm or domestic waste

83
Q

what causes algal blooms

A

nitrates and phosphates are absorbed by plants, this causes plants and floating algae to undergo rapid growth which leads to algal blooms

84
Q

what are algae

A

tiny plants that live in wet environment

85
Q

how do algae affect the dissolved oxygen content in lakes

A

initially it increases as algae undergo photosynthesis and produce oxygen, however, algae do not live long and when they die, they are broken down by microorganisms which use up more O2 from the water

86
Q

what are high levels of nitrates in water associated with?

A

stomach cancer

87
Q

how are heavy metals removed from water

A

precipitation reactions

88
Q

examples of heavy metals

A

lead cadmium and mercury

89
Q

why are heavy metals heavy

A

the have high relative atomic masses

90
Q

how is heavy metal concentration in water measured

A

atomic absorption specectroscopy

91
Q

what type of poison are heavy metals

A

commulative poisons- the build up in the body over time

92
Q

where do heavy metals come from

A

discharge from factories
batteries not disposed of correctly
old houses had lead pipes

93
Q

is mercury dangerous?

A

v dangerous if ingested or vapours are inhaled

94
Q

what are the effects of lead poisoning

A

birth defects and death

95
Q

how are lead ions removed

A

reacted with dilute HCl and lead chloride is precipitated

Pb+2 + 2HCl —> PbCl2(down arrow) + 2H+

or

reacted with calcium hydroxide to form lead hydroxide
Pb+2 + 2OH- –> Pb(OH)2n(down arrow)

96
Q

how is mercury removed from water

A

reacted with sodium sulphide to form mercury sulphide

Hg+2 + S-2 –> HgS (down arrow)

97
Q

primary sewage treatment

A

a mechanical process in which large solids are removed by screening and some suspended solids are removed by sedimentation

98
Q

explain primary sewage treatment

A

floating solids are passed through steel bars and removed
sewage is passed through grit channels- grit and sand settles at bottom of tank
remaining suspended particles settle in the sedimentation tank. solids settle at bottom and from a sludge
liquid is passed onto secondary treatment

99
Q

secondary sewage treatment

A

biological oxidation process in which the levels of suspended and dissolved organic materials are reduced
sewage is decomposed by means of bacteria which use the nutrients from sewage together with oxygen from air to break down the sewage

100
Q

explain secondary sewage treatment

A

swage is decomposed by bacteria
activated sludge process- liquid passes from sedimentation tank to an aeration tank
the microorganisms respire
liquid is aerated by a mechanical rotor
liquid flows into settlement tank some sludge removed, some recycled back as activated sludge.
95% of bod is removed

101
Q

tertiary treatment

A

a process that involves the removal of phosphorus compounds by precipitation and the removal of nitrogen compounds by biological and ion exchange methods

102
Q

describe how phosphates are removed by precipitation

A

aluminum sulphate forming aluminium phosphate

Al+3 + PO4-3 —> AlPO4 (down arrow)

103
Q

how is nitrogen present in tertiary sewage treatment

A

present as ammonia or nitrate compounds

104
Q

why is a biological process used to remove nitrogen in tertiary treatment

A

removal is expensive

105
Q

how is nitrogen removed in tertiary treatment

A

denitrifying bacteria converts nitrate ions to nitrogen gas or ion exchange is used

106
Q

how is water analysed

A

pH measurement
Atomic absorption spectroscopy
colorimetry

107
Q

explain the principle of colourimetry

A

the amount of UV light absorbed by a coloured solution is proportional to the concentration of the solution

108
Q

what does colorimetry measure

A

conc of coloured substances in solution

109
Q

explain the use of aas with water

A

detects presence of heavy metals

110
Q

explain the use of pH measurement and water

A

place a pH probe in the water and read the pH from the digital display

LC Chemistry (22 decks)

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