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LC Chemistry > Water > Flashcards

Water Flashcards

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1
Q

Hard Water

A

A water that will not easily form a lather with soap

due to the presence of Ca²⁺ or Mg²⁺ ions in solution

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2
Q

Temporary Hardness

A

Can be removed by boiling the water

due to Ca(HCO₃)₂ , becomes CaCO₃ on heating, leads to blocked pipes

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3
Q

Permanent Hardness

A

not removed by boiling the water, caused by CaSO₄ or MgSO₄

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4
Q

Methods of removing water hardness

A
  • distillation
  • washing soda
  • ion exchange
  • boiling (only works for temp. hardness)
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5
Q

Water treatment

A
  1. screening
  2. flocculation
  3. sedimentation
  4. filtration
  5. chlorination
  6. fluoridation
  7. pH adjustment
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6
Q

Biochemical Oxygen Demand (B.O.D)

A

the amount of dissolved oygen consumed by biological action when a sample of water is kept at 20°C in the dark for 5 days

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7
Q

Hardness in water

A

Calcium + Magnesium ions mix with soap and form a scum. Modern detergents do not contain soap so will form a lather even with hard water

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8
Q

Advantages of hard water

A
  • provides calcium for teeth and bones
  • nicer taste
  • good for brewing, and for tanning
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9
Q

Disadvantages of hard water

A
  • blocks pipes, leaves scale on kettles
  • wastes soap
  • produces scum
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10
Q

Removing water hardness

A
  • distillation
  • using washing soda
  • ion exchange resin
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11
Q

Distillation

A

evaporating off the water (leaves the dissolves salts behind). The condensed water will no longer contain the salts and be ‘soft.’

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12
Q

Using washing soda

A

Hydrated sodium carbonate Na₂ CO₃. IOH₂O (washing soda) removes hardness from water and allows it to lather more easily with soap

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13
Q

Using washing soda equation

A

Ca + Na₂ CO₃ -> CaCO₃ + 2Na+

CaCO₃ is soluble in water

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14
Q

Ion exchange resin

A

-Pass water through ion exchange resin
-Cation exchange swaps ions that cause hardness
(Ca²⁺ + 2RNa -> R₂Ca + 2Na⁺) (R represents the resin)
-mixed-bed ion-exchange resin removes all ions from water (Water in deionised and cannot be hard)
-Contains 1) Cation exchange (to remove + ions) and
RH⁺ Na⁺ -> RNa + H⁺ 2)Anion exchange (to remove -ions)
ROH + Cl⁻ -> RCl + OH⁻

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15
Q

Calculating Water Hardness

A

in hardback

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16
Q

Water Pollution - dissolved oxygen

A
  1. Dissolved oxygen in rivers, lakes, sea -> vital for survival of fish + other forms of life
  2. Solubility of oxygen in water -> low, depends on temp of water
  3. When organic waste discharged into water, bacteria + organisms present in water provided w/ nutrients -> cause organisms to multiply
  4. Organisms break down waste -> carbon dioxide + water -> use up oxygen dissolved in water

organic matter + oxygen -> carbon dioxide + water

  1. Dissolved oxygen used up by bacteria may not be replaced fast enough by more oxygen from air dissolving in water
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17
Q

Pollution

A

release of substances into environment that damage the environment

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18
Q
  1. screening
A
  • wire mesh

- removes large solids + floating debris (twigs, plastics, etc)

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19
Q
  1. flocculation
A
  • flocculating agent (flocculant), usually, aluminium sulphate, added
  • makes smaller suspended solids coagulate/stick together in large clumps
  • makes them sink faster in the sedimentation tank
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20
Q
  1. settlement
A
  • large tanks
  • water gases is at bottom, rises slowly to top at <2m/hr
  • particles settle to bottom
  • 90%+ of suspended solids removed at this stage
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21
Q
  1. filtration
A
  • large beds of sand
  • removes remaining suspended solids
  • sand supported on layer of gravel
  • sand cleaned regularly
  • water now clear, but may contain harmful bacteria
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22
Q
  1. chlorination
A
  • Cl₂ gas added
  • sterilises water
  • v. small quantities (0.2-0.5 ppm)
  • controlled carefully
  • monitored by bacteriological exam of water
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23
Q
  1. fluoridation
A
  • NaF pr H₂SiF added (hexafluorosilicic acid)
  • Added by law -> helps reduce dental decay by strengthening enamel
  • small quantities -> ~1ppm
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24
Q
  1. pH adjustment
A
  • optimum level between 7-9
  • too acidic -> damage to pipes
  • corrected by addition of Ca(OH)₂ (lime) to raise pH
  • If v. hard water, softened by addition of Na₂CO₃ which is a base
  • Too basic corrected by addition of dilute H₂SO₄ to lower pH
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25
Q

Excess - flocculation

A

excess aluminium sulfate affects taste + causes corrosion of pipes

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26
Q

Excess - chlorination

A

too much chlorine, unpleasant taste + smell

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27
Q

Excess - fluroidation

A

excess fluoride, gives rise to condition which causes staining of teeth (fluorosis)

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28
Q

Excess - pH adjustment

A
  • calcium hydroxide added must be controlled, too much affects taste
  • excess sulfuric acid, corrosion in pipes
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29
Q

chemicals

A
  • Aluminium sulfate
  • Chlorine
  • Fluorine compound eg. sodium fluoride
  • Calcium hydroxide
  • Sodium carbonate
  • Sulfuric acid
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30
Q

Purpose - Aluminium sulfate

A

Coagulation of small suspended particles

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31
Q

Purpose - Chlorine

A

Sterilise water (kill microorganisms)

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32
Q

Purpose - Fluorine compound eg. sodium fluroide

A

Prevents teeth decay

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33
Q

Purpose - Calcium hydroxide

A

Raise pH

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34
Q

Purpose - Sodium carbonate

A

Soften water

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35
Q

Purpose - Sulfuric acid

A

Lower pH

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36
Q

Excess - Aluminium sulfate

A

taste affected

corrosion of pipes

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37
Q

Excess - Chlorine

A

taste + smell affected

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38
Q

Excess - Fluorine compound

A

staining of teeth

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39
Q

Excess - Calcium hydroxide

A

hardness of water

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40
Q

Excess - Sodium carbonate

A

taste affected

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41
Q

Excess - Sulfuric acid

A

corrosion of pipes

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42
Q

B.O.D test

A
  • Carried out by completely filling 2 bottles w/ water to be tested
  • Measurement of dissolved oxygen carried out immediately in one bottle so oxygen content does not increase (photosynthesis of plant material)
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43
Q

oxygen in water

A
  • Dissolved from air into water
  • Low solubility in water
  • Temp inversely affects solubility
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44
Q

3 main types of water pollution

A

eutrophication
organic waste
heavy metals

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45
Q

eutrophication

A

1- Enrichment with nutrients (fertilisers/nitrates/phosphates)

2- Due to runoff from land/pollution/dispersion in water/absorption by plants

3- Rapid growth of plants (algae) on surface/algal blooms formed

4-Light blocked from plants below surface

5- Decay by bacteria of algae, algal blooms/inc in bacteria

6- DIssolved oxygen depleted (lowered, used up), oxygen level falls

7– Fish levels reduced or killed off

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46
Q

discharge of organic waste

A

1- Domestic sewage, slurry, silage effluent, effluent from food processing factories, milk, industrial waste, etc

2- Micro-organisms feed

3- Waste is broken down, O₂ used up (organic matter + O₂ ->CO₂ + H₂O)

4- O₂ used up, reducing levels of fish + possibly killing off fish life

5- O₂ gone, anaerobic bacteria take over, river will smell due to presence of by-product, Hydrogen Sulphide H₂S

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47
Q

heavy metals

A

1-Some heavy metal ions known to be cumulative poisons

2- metal ions

3- The concentration of heavy metal ions can be measured using Atomic Absorption Spectrometry (AAS)

4-Heavy metals removed from water by precipitation reactions, eg. lead reacts w/ HCl to form an insoluble precipitate that fall out of solution

Pb²⁺ + 2HCl -> PbCl₂ + 2H⁺

5- The Eu sets limits on the conc of certain species in water

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48
Q

Metal Ions

A

Lead (Pb²⁺)
Mercury (Hg²⁺)
Cadmium (Cd²⁺)

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49
Q

Lead (Pb²⁺) - source, harmful effects

A

Old plumbing + paints

Damage to nervous system

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50
Q

Mercury (Hg²⁺) - source, harmful effects

A

Industrial effluent

Birth defects

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51
Q

Cadmium (Cd²⁺) - source, harmful effects

A

Discarded batteries

Carcinogenic

52
Q

Sewage treatment stages

A
  • Primary treatment (physical)
  • Secondary tretment (biological)
  • Tertiary treatment (chemical)
53
Q

Primary treatment (pysical stage)

A

1- Incoming sewage is screened to remove debris and non-biodegradable material

2- Passes into large deep sedimentation (settlement) tanks where about 50% of suspended solids + about 33% of BOD removed

3- Liquid on top passed onto to secondary treatment

54
Q

Secondary treatment (biological stage)

A
  • Incoming sewage from 1st stage of treatment passed into aeration tanks where mechanical stirrers aerate it
  • Allows aerobic bacteria + other micro-organisms to decompose solid matter into harmless sludge called Activated Sludge
  • Sewage flows into settling tanks, sludge can be reused, or recycled to make methane
  • ~95% of BOD removed
55
Q

Tertiary treatment (chemical stage)

A
  • When liquid passes out of second round of settling, it is clean + inoffensive, may contain compounds of nitrate + phosphate
  • Phosphates from washing powders + washing up liquids, and nitrates from organic material in sewage
  • Must be removed before water discharged into raver as could cause eutrophication
56
Q

Phosphate removal

A
  • alluminium sulphate added, aluminium phosphate precipirates out
  • Iron (III) chloride added, iron (III) phosphate precipitates out
  • Lime may also be used
  • Insoluble phosphate compounds allowed to settle out before discharge into waterway
57
Q

Nitrate removal

A

Difficult + expensive as nitrogen may be present in forms NH₃, NO₂₋, NO₃₋, organic compounds containing N

Denitrifying bacteria

58
Q

Instrumental methods of water analysis

A
  • AAS (heavy metals)
  • pH sensor
  • Colorimetry
59
Q

Principle of colorimetry

A

Amount of light absorbed by a coloured solution is proportional to conc of solution

60
Q

Colorimeter

A

Device used to test concentration of solution by measuring its absorbance of a specific wavelength of light

61
Q

Using a colorimeter

A
  • Diff soltions must be made + a control (usually mix of distilled water and another solution) is first filled into a cuvette
  • Placed inside colorimeter to calibrate machine
  • After calibrated, use to find densities and/or conc of other solutions
62
Q

Colorimetry

A

in hardback

63
Q

Principle

A
  1. If solution is coloured, then intensity of colour is proportional to conc
  2. % of light absorbed by coloured solution in colorimeter is proportional to conc
64
Q

Processes

A
  • Light of particular wavelength is passed thru number of samples of known concentration
  • Graph of absorbance against conc is plotted
  • Absorbance of unknown is noted + using graph, conc of unknown can be found
65
Q

Uses

A

Analyses: lead in water, fertilisers in water (Eg. nitrates + phosphates)

66
Q

Atomic Absorption Spectrometry

A
  1. White light passed thru gaseous sample of element, will have specific frequencies removed from it
  2. Emission spectrum is coloured lines on black bg, while absorption spectrum is black lines on a coloured bg
  3. Technique used to determine conc of elements
67
Q

Atomic Absorption Spectra

A

Identifying elements
Concentration of lements
Concentration of heavy metals in water

68
Q

What happens in an AAS?

A
  • White light is passed through (river water)
  • Elements in water (eg. Pb, Hg will absorb certain characteristic wavelengths of light)
  • Amount of light absorbed is directly proportional to conc of element in sample
69
Q

ON what principle does an AAS work?

A

-Every element:

absorbs a particular wavelength of light, amount of light absorbed is proportional to concentration of the element

70
Q

Experiment: water hardness

indicator used + 
function of buffer solution
A

-erichrome black t indicator

71
Q

Experiment: water hardness end point colour change

A

goes from wine red to dark blue

72
Q

calculating water hardness

A

Use Va x Ma …. formula

Get Ma, then turn mol/L intro mg/L (ppm)

73
Q

what is ppm?

A

mg/L

74
Q

homologous series edta is a member of

A

carboxylic acids

75
Q

Experiment: water hardness

solution added before titration proceeds + purpose

A
  • pH 10 buffer solution
  • added before titration proceeds
  • keeps the pH at about 10, ensuring necessary conditions for effective operation of indicator are maintained
76
Q

Experiment: water hardness - rectifying splash of solution up side of conical flask + why

A

side of flask washed down with d.i. water

-doesn’t affect concentration of reactants in flask as it contains no ions

77
Q

how to use ion exchange resin

A
  • pass water through resin to replace positive ions (Ca²⁺ / Mg²⁺) with hydrogen ions (H⁺)
  • negative ions replaced with hydroxyl ions (OH⁻)
  • H⁺ and OH⁻ ions combine to form water
78
Q

how suspended solids are removed from a water sample + weighed

A
  • filter known volume through weighted filter paper
  • dry filter paper
  • reweigh and find increase (mass of suspended solids)
79
Q

how to determine if water contained temp hardness

A
  • boil the water

- scale as a result of boiling indicates temp hardness

80
Q

how boiling removes temp hardness caused by presence of calcium hydrogencarbonate

A
  • calcium hydrogen carbonate Ca(HCO₃)₂ decomposes /converted to calcium carbonate CaCO₃
  • a precipitate is formed
81
Q

Experiment: water hardness - effect of boiling the water

A

the titre would be smaller as boiling removes temp hardness

82
Q

purpose of tertiary sewage treatment

A
  • remove nitrates (nitrogen compounds)
  • remove phosphates (phosphorous compounds)

(-prevent algal blooms/eutrophication )

83
Q

processes in primary treatment of sewage

A
  • screening
  • passing over grit channels (traps)
  • settlement (sedimentation)
84
Q

explain how high nitrate levels can result in a reduction in the dissolved oxygen content of lakes and rivers

A
  • causes rapid growth of water plants (algae)
  • decay of these plants by aerobic microorganisms uses up oxygen
  • surface plants block light preventing photosynthesis
85
Q

water of crystallisation experiment - primary standard reagent which could have been used to standardise the HCL solution

A

anhydrous sodium carbonate

86
Q

water of crystallisation experiment - suitable indicator, colour change,

A

methyl orange

yellow to red

87
Q

water of crystallisation experiment - why not more than 1-2 drops of indicator should be used

A

methyl orange is a weak acid

88
Q

describe the correct procedure for rinsing the burette before filling it with the solution to deliver + describe filling it

A

rinse with deionised water
rinse with solution

  • use funnel, pour in at top, take meniscus into account
  • ensure area below tap is filled
89
Q

why is it important to fill the part below the tap of the burette?

A

air will be displaced by solution

90
Q

describe procedure for weighing + making up solution from hydrated sodium carbonate crystals

A
  • weigh on clock glass using electronic balance
  • transfer solution to beaker + rinse clock glass with d.i water using wash bottle to ensure all is transferred
  • dissolve in d.i water
  • transfer to volumetric flask using funnel + rinse with d.i water to ensure all is transferred
  • add d.i water to up to near mark on flask
  • add final amount using dropper, taking meniscus into account
  • stopper, invert several times
91
Q

describe procedure for washing the pipette and using it to measure the solution

A

-rinse with d.i water, followed by solution

  • fill up to mark with pipette filler taking meniscus into account
  • transfer to flask with tip against side of flask to avoid splashing
92
Q

three precautions with burette that should be taken in order to ensure an accurate measurement

A
  • vertical/read at eye level
  • add dropwise using dropper near end-point
  • rough titration first
93
Q

sodium carbonate crystals, is not a primary standard but anhydrous sodium carbonate (Na₂CO₃) may be used as a primary standard. Why is this the case?

A
  • Na₂CO₃ is pure, stable, anhydrous (not hydrated), no water loss
  • solution of exact concentration can be made, no need to standardise by titration, any opposite point for sodium carbonate crystals
94
Q

why a water sample could have high pH

A

-very hard due to running over limestone or subject to intense photosynthetic activity due to presence of algae

95
Q

why a water sample could have low pH

A
  • water is acidic/peaty

- organic material decomposes to form acidic substances

96
Q

suggest a way to determine if a water supply contained temp hardness

A
  • boil water

- scum as a result of boiling indicates temp hardness

97
Q

how does boiling remove any temp hardness, caused by presence of calcium hydrogencarbonate Ca(HCO₃ )₂ , in a water sample?

A

calcium hydrogen carbonate decomposes / converted to calcium carbonate

98
Q

harmful effects of eutrophication

A

excessive plant growth / algal bloom / oxygen depletion / fish kill

99
Q

causes of eutrophication

A

fertilizer run-off / silage run-off / slurry run-off

100
Q

why is there concern about lead (Pb²⁺) in drinking water?

A

lead is toxic

101
Q

how does temporary hardness arise in rainwater flowing over limestone?

A
  • rain water acidic due to dissolved carbon dioxide

- rain water reacts with limestone to form calcium hydrogen carbonate

102
Q

balanced equation of formation of limescale when water is boiled

A

Ca(HCO₃)₂ –> CaCO₃ + H₂O + CO₂

103
Q

explain in words how water with temp hardness is deionised using cation-exchange resin and anion-exchange resin

A
  • two hydrogen ions from cation-exchange resin replace a Ca²⁺/cations in water
  • hydroxide/hydroxyl ion from anion-exchange resin replaces a HCO₃⁻/anions

hydrogen ions and hydroxide ions combine to form water

104
Q

suggest a reason why a sample of water that was boiled and filtered would have a lower ppm CaCO₃

A

temporary hardness removed

105
Q

problem of too low free chlorine level

A

infection / more viruses

106
Q

problem of too high free chlorine level

A

taste impaired / unpleasant smell / skin irritation

107
Q

how hard water is caused + how it wastes soap

A
  • presence of Ca²⁺ or Mg²⁺ ions (calcium and magnesium ions)

- soap react with Ca and Mg ions to give scum

108
Q

why is water pollution by heavy metal ions a cause of concern?

A

danger to health (poisonous/toxic)

109
Q

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

A

-removal of solids by screening and settlement

110
Q

explain the processes by which the BOD was further reduced in secondary sewage treatment

A
  • digestion/decomposition
  • aerobic/aeration
  • activated sludge
111
Q

methods of removing all of the hardness in a water sample

A
  • distillation
  • ion exchange resin
  • washing soda
112
Q

the chemical that is the main component of scale

A

calcium carbonate (CaCO₃)

113
Q

describe how suspended solids are removed in water treatment

A
  • settlement
  • flocculation
  • decanting/allow water to overflow
  • filtration
114
Q

problems if pH of water supply outside range of 6-8

A

damage to pipes
tooth decay
hardness

115
Q

nutrients that cause eutrophication

A

nitrates, phosphates

116
Q

how student could determine total dissolved solids in water

expressed in ppm

A
  • evaporate known volume to dryness
  • in weighed container (dish,etc)
  • cool, reweigh and find increase (mass of solids)

-express mass in mg/L

117
Q

formula of substance that causes temp hardness

A

Ca(HCO₃)₂

118
Q

formula of substance that causes perm hardness

A

CaSO₄ or MgSO₄

119
Q

how hard water due to Ca(HCO₃)₂ is softened by ion-exchange

A

each calcium ion (Ca²⁺) replaced by 2 sodium ions (Na⁺) from resin

2RNa + Ca(HCO₃)₂ –> 2NaHCO₃ + R₂Ca

120
Q

what is meant by flocculation

A

clumping of suspended particles in the water

121
Q

what is free chlorine?

A

chlorine available as oxidising agnet

122
Q

property of water that makes it useful in human body as medium in which chemical reactions occur + allows it to become polluted or contaminated easily

A

good solvent

polar

123
Q

elimination reaction

A
  • dehydration of an alcohol is an example
  • In this reaction, a larger alcohol molecule reacts to form a smaller alkene molecule and an even smaller water molecule
  • change in structure is from tetrahedral to planar
124
Q

dehydration of ethanol

A

ethanol is dehydrated to ethene

reaction is used in prep of ethene

125
Q

reaction conditions for elimination reactions

A

heat

aluminium oxide catalyst

126
Q

unit of BOD

A

ppm

127
Q

reaction between caco3 and bcl

A

CaCo3 + 2HCl –> CaCl2 + H2CO3

LC Chemistry (26 decks)

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