Water Flashcards
What is meant by hard water?
Water that will not easily form a lather with soap
What is hard water caused by?
The presence of calcium ions ( Ca 2+) or Magnesium ions (Mg2+)
Explain with the aid of a balanced chemical equation why does hard water not form a lather with soap?/ How does hard water waste soap?
Soap is a salt of a fatty acid.
Sodium stearate (C17H35COONa)
the Stearate ion in soap reacts with the Ca+ / Mg+ ions in hard water forming calcium/magnesium stearate, an insoluble grey ‘scum’
The soap does not form a lather until all of the Ca/Mg ions are used up, soap is wasted
Ca2+ + 2C17H35COO – –>
(C17H35COO)2 Ca
Chemical equation for Sodium stearate
Sodium stearate (C17H35COONa)
Name the two types of hardness in water
tempoary hardness
permanent hardness
Compare Tempoary hardness and Permanent hardness
Tempoary Hardness
Hardness that can be removed by boiling the water
caused by The presence of calcium hydrogencarbonate (Ca(HCO3)2)
or magnesium hydrogencarbonate (Mg(HCO3)2)
Permanent hardness
Hardness that cannot be removed by boiling the water
caused by the presence of calcium sulfate (CaSO4) or magnesium sulfate (MgSo4)
How does tempoary hardness in water arise?
Rainwater is slightly acidic due to carbon dioxide from the air dissolving it, forming a weak carbonic acid
as this acidic rainwater flows over limestone rocks, it reacts with it, forming calcium hydrocarbonate
How does boiling remove tempoary hardness?
The soluble calcium hydrocarbonate breaks down into calcium carbonate, CO2 and water
Calcium carbonate is insoluble and precipitates out of solution, the water is softened
Whats the common name for Calcium Carbonate (CaCo3)
Limescale
Give two disadvantages of hard water
causes limescale which builds up on and damages machines that use hot water- kettles
Hard water wastes soap - soap wont form a lather until the ions causing the hardness have been used up
What is total hardness?
The sum of permanent hardness and tempoary hardness
How can the total hardness in a water sample be removed?
Distillation
washing soda tablets (they use sodium carbonate)
Using an ion-exchange resin
Outline how water can be softened by an ion-exchange resin, so it is suitable for use in the home
a cation exchange resin containing Na= ions is used
as the hard water passes through the resin, each Ca/Mg ion is replaced by two Na ions
the softened water passes through
Outline how water how water can be softened by an ion-exchange resin
so it is suitable for use as deionised water in the laboratory
A mixed bed resin is used
cation-exchange resion w/ H+ ions
anion exchange resin w/ OH- ions
as water passes through
each Ca/mg ions is replaced by two H+ ions from resin
any negatice ions are replaced by OH- ions
the H+ and OH- ions form water
Compare deionised water and distilled water
deionised water
contains ni ions, but can contai suspended solids, dissolved organic material and dissolved gasses
not as pure as distilled water
distillled water
contains no ions, no suspended solids, no dissolved organic material
most pure form of water
Name the stages to process in water treatment to provide water for households and factories
Screening
Flocculation
Sedimentation
filtration
Chlorination
Fluoridation
PH adjustment
explain the process of Screnning
water is passed through a wire mesh, removing twigs, plastic bags and debris
explain the procces of flocculation
Flocculation is the clumping together of small, suspended particles into larger particles ( also known as coagulation)
What is a flocculating agest/coagulant
A flocculating agest is a chemical added to water to clump together small, suspended particles into larger particle, aiding sedimentation
name an example of a flocculating agent/ coagulant
Aluminium sulfate Al2(SO4)3
in Flocculation, why must the amount of aluminium sulfate added be controlled
causes corrosiom of pipes
affects the taste of water
describe the process of Sedimentation
water is passed into settling tanks and usupended solids settle to bottom
clean water overflows into channel
90% of particles removed
describe the process of filtration
water from top of the settlng tanks is passed through neds of sand and gravel underneath
all suspended solids removed
describe the process of chlorination
chlorine is added in small quantites to kill harmful microrganisms and sterilise it
why must the amount of chlorine added in Chlorination be carefully controlled?
Too much chlorine added is toxic and gives the water an unpleasant taste and smell
describe the process of Fluoridation
small amounts of fluoride compounds are added to strenghten enamel and help prevent tooth decay.
name an example of a fluoride compound
Sodium fluoride NaF
in Fluoridation why must the amount of fluoride compounds be carefully controlled
too much fluoride in water causes florosis which stains teeth
describe the process of Ph adjustment
Ph of water to be distruted to homes and factories is 7.2
Describe PH adjustment when water is too acidic, and why must it be controlled?
add Calcium hyproxide Ca(OH)2 to raise PH, toomuch can cause the water to become hard
Describe PH adjustment when water is too hard, and why must it be controlled?
add Sodium Carbonate to soften water, too much affects the taste of water
Describe PH adjustment when water is too basic , and why must it be controlled?
add Sulfuric acid to lower the Ph, too much causes the corrosion of pipes
Describe the procedure to determine the concentration of total suspended soilds of water bu filtration (in ppm)
- Water sample is added to volumetric flask of known volume
- measure mass of clean filter paper
- water sample is filtered through into a Buchner funnel and vacuum filtered
- filter paper is dried in a oven to remove water
- take away intial mass from final mass
- Mass of suspended solids in the known volume of water is then converted to the concentration of suspended solids in ppm
Describe the procedure to determine the concentration of total dissolved solids of a sample of water by evaporation
- Find mass of a clean beaker
- a known volume of a previously filtered water sample is placed in beaker
- using a hot plate the water is evaporated
- mass of beaker is found again
- take away initial mass from final mass
- Mass of dissolved solids in the known volume of water is then converted to the concentration of dissolved solids in ppm
Describe the procedure to determine the PH of a sample of water
- Use a Ph meter
- Place the probe into the water sample and record the PH
- also can be done with universal indicator and the PH scale
What is meant by eutrophication?
The over enrichment of water with nutrients, leading to the excess growth of algae and other plants
Why is eutrophication problamatic/ what is a harmful effect of eutrophication
Leads to excess algal blooms and plant growth, depleating oxygen from the water and killing aquatic life
Name two nutrients, whose over enrichment in water can cause eutrophication. Give a major source of these nutrients entering waterways
Nitrates (NO3-)
Phosphates(PO4-)
sources: fertilisers, domestic sewage or far, silage or slurry entering the water
Describe the process that occours in water leading to eutrophication
Nitrates and phosphates are absorbed by algae in the water causing algal blooms
- Algal blooms blocksunlight into the water, preventing, cause underwater plants die
- As dead plants and algae are decayed by micro-organisms, the dissolved oxygen in
the water is used up due to respiration by microorganisms
- This decreased dissolved oxygen can cause much of the aquatic life to die
What is BOD and how is it measured
Biochemical Oxygen demand
it is the amount of dissolved oxygen comsumes by biological action when a sample of water is kept at 20C in the dark for 5 days
How to measure BOD
First sample oxygen concentration - second sample
Explain why samples of effluents/slurry’s are often diluted and aerated before analysis?
The solubility of oxygen in water is low, and effluents have a high BOD level
* If not diluted with well aerated distilled water, all of the oxygen in the water will have been used up before the 5 days ,no oxygen left to measure
* Diluting and aerating the water ensures a high enough level of oxygen to begin with,
meaning there will be a measurable amount of oxygen left after 5 days
Name 3 heavy metal ions that are another source of water pollution
Lead ions (Pb2+)
Mercury ions (Hg 2+)
Cadmium ions (CD2+)
Give three ways in which these heavy metal ions can enter water suppies
Discharge of industrial waste
Dumping of batteries containing these metals
Old houses containing lead plumbing
Why is it bad to drink water containing heavy metal ion
high levels of heavy metal ions consumed are toxic
Name an instrumental method used to analyse water for the presence and concentration of
heavy metals.
Atomic absorption spectrometry AAS
How are heavy metal ions removed from water, name an example
by precipation
Lead ions are reacted with dilute hydrochloric acid – the lead is precipitated as insoluble lead
chloride
Name the stages in volved in sewage treatment.
Primary Treatment, Secondary Treatment,Tertiary treatment
What is primary sewage treatment?
A mechanical process in which large solids are physically removes by screening and some suspended solids are removes by settlement
what does primary treatment involve?
Screening- removes twigs and debris
Grit channels- pebbles and grit settle and are removes
settlement/sedimentation - sewage allowed to settle in settling tanks
What is secondary sewage tratment?
A biological process involving the breakdown of suspended and dissolved organic material by the aerobic oxidation of the sewage by microorganisms
Why is secondary treatment known as a biological process?
The sewage is broken down/ decomposed by microrganisms
what does secondary sewage treatment involve
activated sluge process
The sewage is pumped into an aeration tank and provided with oxygen
- Micro-organisms break the sewage down by aerobic oxidation
- The sewage flows into a settling tank; sludge is removed
What is tertiary sewage treatment?
Tertiary sewage treatment is a process involving the removal of nitrates and phosphates
Why is the tertiary sewage treatment stage carried out?
To remove nitrates and phosphates to avoid eutrophication downstream from sewage
works
Why is tertiary sewage treatment not carried out in all sewage treatment plants?
The process is very expensive
How are nitrates removed from the water in tertiary treatment?
Biologically, adding denitrifying bacteria
How are phosphates removed from the water in tertiary treatment??
Precipitation by adding aluminium sulfate
Whay property of water makes it very useful in the human body as a medium in which chemical reactions occour
Water is a polar covalent molecule - make it a excellent solvent
it will redily dissolve ionic compounds and other polar molecules
Name three instrumental methods that are used when analysing water
pH measurement - a pH meter is used
Atomic absorption spectrometry – Can detect the presence of heavy metal ions
Colorimetry - measures the concentration of coloured substances in solution
Name two species in swimming pool water known as free chlorine
Hypochlorous acid (HOCl)
Hypochlorite ion (OCl –)
What is meant by free chlorine?
Free chlorine is chlorine present in water in the form of hypochlorous acid (HOCl) and as
hypochlorite ions (OCl –)
What is the principle on which the measurement of free chlorine present in a water sample
using a colorimeter is based? (and of all colorimetric experiments)
The absorbance of light of free chlorine is directly proportional to concentration of free
chlorine
Name a reagent suitable to test for free chlorine in a water sample
A DPD tablet
How does this reagent indicate the presence of free chlorine in a sample?
The solution will turn pink
The more free chlorine present, the more intense the pink colour
What is the relationship between absorbance and concentration of free chlorine?
Absorbance of light is directly proportional to concentration of free chlorine
A straight-line graph through the origin (0,0) is obtained for a directly proportional
relationship
Give a reason why the concentration of free chlorine in treated drinking water is usually
between 0.2-0.5 ppm whereas in swimming pool water it should be between 1-5 ppm
Swimming pool water has more bacteria added by people swimming in it
Therefore the concentration of chlorine must be greater in swimming pools than in drinking
water
