Water Flashcards
hard water
water that will not form a lather easily with soap. forms a scum instead. caused by the presence of Ca+2 and Mg +2
what is the chemical formula of sodium searate
C17H35OONa
what is the most common substance found in soap
sodium stearate
what happens when sodium stearate is added to hard water
a grey insoluble scum
what is the name of the substance that forms when sodium stearate is added to hard water
calcium stearate
when will a lather form with soap
when all the calcium ions in the water have formed calcium stearate.
write the balanced equations for what happens when sodium stearate is added to hard water
C17 H35COONa –> C17H35COO- + Na+
Ca+2 + 2C17H35COO- —> (C17H35COO)2Ca(down arrow)
why are modern detergents not affected by water hardness
they do not contain soap they come from crude oil
test for water hardness
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
advantaged of hard water
provides calcium for healthy teeth and bones
tastes nicer
good for brewing industry
disadvantages of hard water
blocks pipes causes limescale on kettles and washing machines
wastes soap
produces scum
temporary hardness
hardness that can be removed by boiling
caused by calcium hydrogencarbonate
chemical formula of calcium hydrogencarbonate
Ca(HCO3)2
what forms carbonic acid
rain water is slightly acidic - CO2 dissolves in rainwater forming a weak acid ie carbonic acid
write the balanced equation for the formation of carbonic acid
H2O + CO2 (equilibrium) H2CO3
write the balanced equation of the reaction between carbonic acid and limestone
CaCO3 + H2CO3 —> Ca(HCO3)2
how do we remove temporary hardness
boiling
how does boiling remove temporary hardness
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
how can you remove limescale
reacting it with hydrochloric acid
write the balanced equation by which boiling removes temporary hardness
Ca(HCO3)2 —-> CaCO3 (down arrow) + CO2 + H2O
write the balanced equation for removing limescale with HCl
CaCO3 + 2HCl —-> CaCl2 + H2O + CO2
how do you confirm CO2 production
bubble gas through limewater. if gas is CO2 the limewater changes colour from colorless to milky white
permanent hardness
hardness that cannot be removed by boiling
caused by calcium or magnesium sulphate
what happens when water containing Calcium or magnesium sulphate is boiled
there is no chemical reaction that causes the precipitation of the calcium or magnesium ions
differences between distilled and deionised water
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
how is distilled water formed
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
what are the 4 methods of removing water hardness
add soap
distillation
washing soda
ion exchange
explain how adding soap is a method of removing water hardness
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
how does distillation remove water hardness
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
what is a disadvantage of distillation
expensive
how does washing soda remove water hardness
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.
what is ion exchange
swap ions that cause hardness for ions that do not cause hardness
explain household ion exchange
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
give the balanced equation of household ion exchange
Ca+2 + 2RNa–> R2Ca + 2Na+
explain deionised water ion exchange
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
write the balanced equations for a mixed bed resin
cation exchange: RH + Na+ —> RNa + H+
anion exchange: ROH + Cl- —> RCl + OH-
H+ + OH- —> H2O
what does EDTA stand for
ethylenediaminetetraacetic acid
criteria for drinking water
colourless, safe to drink( chlorinated), fluoridated
what are the 7 steps of water treatment
screening
flocculation
sedimentation
filtration
chlorination
fluoridation
pH adjustment
explain screening
water from a river or lake is passed through a wire mesh.
acts like a large sieve and removes large pieces of floating debris
explain flocculation
is the coagulation of small suspended particles in water to form larger particles
what chemicals are used in flocculation
aluminium sulphate
explain sedimentation
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
explain filtration
water from the sedimentation tanks is passed through beds of sand to filter any remaining suspended solids
explain chorination
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
explain fluoridation
1ppm of fluoride components are added to water.
what are examples of fluorine compounds added to water during treatment
NaF, H2SiF6
why is only a small amount of fluorine added to water during treatment
too much can stain teeth
why is fluorine added to water
it strengthens enamel of teeth
explain pH adjustment
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.
what is Al2(SO4)3 or aluminium oxide used for in water treatment
floculating agent
what happens if you add too much Al2(SO4)3 or aluminium oxide in water treatement
affects taste and corrodes pipes
what is chlorine gas used for in water treatment
chlorination
what happens if to much chlorine is added in water treatment
affects taste and smell
what is sodium fluoride used for in water treatment
fluoridation
what happens if to much sodium fluoride is added in water treatment
stains teeth
what is calcium hydroxide used for in water treatment
pH adjustment increase
what happens if to much calcium hydroxide is added in water treatment
causes hard water
what does sulphuric acid do in water treatment
pH adjustment decrease
what happens if to much sulphuric acid is added in water treatment
corrosion of pipes
what does sodium carbonate do in water treatment
softens the water
what happens if to much sodium carbonate is added in water treatment
affects taste
biochemical oxygen demand
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
what is the bod test used for
used to measure the amount of dissolved oxygen in water
why is dissolved oxygen vital
to maintain life in oceans and lakes
what does the solubility of oxygen depend on
temperature - as temp increases solubility decreases
what affects dissolved oxygen in lakes or oceans
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
what else can affect dissolved oxygen levels
photosynthesis- o2 levels will increase
respiration- o2 levels will decrease
why do we dilute samples for bod tests with pure water
ensures dissolved oxygen remains present throughout the 5 day incubation period an ensures we can measure a value for O2 concentration
procedure for bod test
-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
what are the conditions at which the water being BOD tested is stored
dark, for 5 days, At 20 C
what are the 3 precautions when carrying a bod test
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
- 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
- immediately place the second bottle in the dark to prevent photosynthesis
what does a high bod value mean
lots of organic material is present so more oxygen is removed from water
bod value of clean drinking water
1-2
bod for polluted water ie the level that kills fish
100
bod for raw sewage
300
bod for silage effluent
54000
how to calculate bod
amount of o2 consumed = initial - final O2 conc
calculate dilution factor
multiply amount of O2 consumed by dilution factor
eutrophication
the enrichment of water with nutrients (nitrogen and phosphorus) which can lead to excessive growth of algae and other plants
how can bod be reduced
if there is excess of plant nutrients in the water eg nitrate and phosphate ions
what is natural eutrophication
occurs in lakes an is cause by nitrogen and phosphorus gradually increasing over time as sediment builds up in the lake
artificial eutrophication
caused suddenly, e.g. fertiliser run off from a farm or domestic waste
what causes algal blooms
nitrates and phosphates are absorbed by plants, this causes plants and floating algae to undergo rapid growth which leads to algal blooms
what are algae
tiny plants that live in wet environment
how do algae affect the dissolved oxygen content in lakes
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
what are high levels of nitrates in water associated with?
stomach cancer
how are heavy metals removed from water
precipitation reactions
examples of heavy metals
lead cadmium and mercury
why are heavy metals heavy
the have high relative atomic masses
how is heavy metal concentration in water measured
atomic absorption specectroscopy
what type of poison are heavy metals
commulative poisons- the build up in the body over time
where do heavy metals come from
discharge from factories
batteries not disposed of correctly
old houses had lead pipes
is mercury dangerous?
v dangerous if ingested or vapours are inhaled
what are the effects of lead poisoning
birth defects and death
how are lead ions removed
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)
how is mercury removed from water
reacted with sodium sulphide to form mercury sulphide
Hg+2 + S-2 –> HgS (down arrow)
primary sewage treatment
a mechanical process in which large solids are removed by screening and some suspended solids are removed by sedimentation
explain primary sewage treatment
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
secondary sewage treatment
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
explain secondary sewage treatment
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
tertiary treatment
a process that involves the removal of phosphorus compounds by precipitation and the removal of nitrogen compounds by biological and ion exchange methods
describe how phosphates are removed by precipitation
aluminum sulphate forming aluminium phosphate
Al+3 + PO4-3 —> AlPO4 (down arrow)
how is nitrogen present in tertiary sewage treatment
present as ammonia or nitrate compounds
why is a biological process used to remove nitrogen in tertiary treatment
removal is expensive
how is nitrogen removed in tertiary treatment
denitrifying bacteria converts nitrate ions to nitrogen gas or ion exchange is used
how is water analysed
pH measurement
Atomic absorption spectroscopy
colorimetry
explain the principle of colourimetry
the amount of UV light absorbed by a coloured solution is proportional to the concentration of the solution
what does colorimetry measure
conc of coloured substances in solution
explain the use of aas with water
detects presence of heavy metals
explain the use of pH measurement and water
place a pH probe in the water and read the pH from the digital display
