Point 4.8 – Identify the origins of chlorofluorocarbons (CFCs) and halons in the atmosphere.
Haloalkanes are carbon compounds that contain 1 or more halogen atoms in place of H atoms in the hydrocarbon. Chlorofluorocarbons (CFCs) are a class of haloalkanes in that they contain chlorine and fluorine atoms which substitute all of the hydrogen atoms in the compound. Example of a CFC include: CCl3F (trichlorofluromethane).
CFCs are synthetic in that they are produced from human activity. CFCs were developed to replace ammonia as a refrigerant in the 1930s. At the time, their properties were found to be ‘safer’ than the ammonia because they were much more stable. Gradually the uses for CFCs grew and there were large amounts of CFCs emitted into the atmosphere due to their extensive use. Some of its uses and hence origins were:
Refrigerants in air conditioners and refrigerators
Solvents in dry-cleaning
Propellants in aerosol spray cans (deodorants, insecticides)
They were used for these applications because they were non-toxic, inert and easily liquefied and returned to gas phase.
Halons are similar to CFCs except they include bromine atoms in addition to chlorine and/or fluorine atoms. Example of a halon is CCl2Br (dichlorobromomethane). Halons are also produced from humans and not from nature. They were used in fire extinguishers because they do not support combustion and thus gaseous halons from this application were emitted into the atmosphere.
Point 4.9 – Identify and name examples of isomers (excluding geometrical and optical) of haloalkanes up to eight carbon atoms.
Isomers are compounds with same molecular formulas but different structural formulas. This is the nomenclature of naming haloalkanes:
Use prefixes for the halogen groups (chloro, bromo, iodo, fluoro)
If more than one type of halogen atom is present, name them in alphabetical order.
Use di, tri, tetra when there is more than one atom of any halogen.
Identify the longest carbon chain
Number the carbon chain giving the lowest set of numbers to the halogens present
Consider the compound C5H10BrCl. These are some of its isomers:
Point 4.10 – Discuss the problems associated with the use of CFCs and assess the effectiveness of steps taken to alleviate these problems.
As outlined in dot point 4.8, the major uses of CFCs were: as refrigerants in air-conditioners and refrigerators, solvents in dry cleaning and as propellants in aerosol spray can (deodorants, insecticides). Gaseous CFCs are emitted into the atmosphere due to this extensive use.
When CFCs are emitted into the troposphere it does not react with surrounding substances because it is inert and extremely unreactive. Thus over a period of 3-5 years they slowly move up into the stratosphere, where they do the most damage. In 1974, scientists Mario Molina, Sherwood Rowland and Paul Crutzen (noble prize winners), demonstrated that CFCs were responsible for ozone depletion. Here is the process:
In the stratosphere, the CFC (e.g. CClF3) photo dissociates in UV radiation from the sun to produce a chlorine free radical:
CClF3 (g) + UV Cl .(g) + CF3 (g)
This chlorine free radical is extremely reactive because it contains 7 electrons in the outer shell and wants to gain one more electron to fill its outer shell. So what happens is that it reacts with an ozone molecule to produce a chlorine oxide radical and an oxygen molecule:
Cl .(g) + O3 (g) ClO. (g) + O2 (g)
Then this chlorine oxide radical reacts with an oxygen free radical (oxygen atom) to form a chlorine free radical and an oxygen molecule:
ClO. (g) + O. (g) Cl .(g) + O2 (g)
Overall reaction: O3 (g) + O. (g) 2O2 (g)
Note: Halons also cause destruction of ozone.
We can see that the ozone molecule has been destroyed in this reaction chain. The chlorine free radical which is produced in the last step, is free to destroy tens and thousands of more ozone molecules until it is destroyed by the reaction with methane. So this is the major problem of CFCs: they produce chlorine free radicals which are capable of destroying thousands of ozone molecules in the stratosphere. This is compounded by the fact that CFCs have a lifetime of about 150 years.
Because of the destruction of ozone by CFC’s in the stratosphere, there are major implications for life on earth. Ozone’s major role is to protect life on earth by absorbing dangerous and harmful UV radiation from the sun. Since it has been destroyed, the UV radiation is able to enter the lower atmosphere including the troposphere.
UV radiation has many devastating effects on life on earth:
Greater rates of skin cancer in humans
Eye irritations which lead to cataracts and retina damage
UV radiation is able to break covalent bonds of important biological molecules such as DNA and proteins which leads to gene mutations.
Decreases plant growth
Affects aquatic organisms
CFCs also contribute to the problem of the enhanced greenhouse effect. (The enhanced greenhouse effect is caused by gases, released by human activity, absorbing heat rays that come from the Earth's surface, and then emitting the heat rays. Many of these heat rays come back to the Earth's surface. This raises the temperature of the atmosphere.)
In the stratosphere, CFCs (as well as halons) absorb heat rays emitted from the earth’s surface – preventing them from being released into outer space. Thus the earth’s heat is trapped in the stratosphere, raising the temperature of the earth. This small increase in temperature can have huge effects on aquatic organisms such as plankton who cannot cope with this change. Thus organisms dependant on plankton (food chain) will also die.
These problems all originate from the emission of CFCs into the stratosphere.
Steps taken to reduce problems associated with CFCs
The international community has gradually begun to realise the damaging effects of CFCs on ozone. To combat the devastating effects of CFCs, many international agreements and treaties have been proposed. These are some of them:
In 1985 a number of governments adopted the Vienna Convention on Protection of Ozone Layer.
In 1987, a United Nations Convention established the Montreal Protocol which aimed to restrict and control global emissions of ozone destroying chemicals (such as CFCs and halons) and to protect the ozone layer. This protocol has brought about a large reduction in the amount of CFCs released. However not all countries ratified this protocol thus decreasing its effectiveness.
There have been many amendments to this protocol. By 1996, 155 countries had ratified the Montreal Protocol and its amendments.
The Kyoto Protocol in 1997 required all halon production to cease by 2010 as well as setting a 5% reduction target in emissions by developed countries in 2012. This has reduced halon emissions but some major countries including USA have not ratified it thus reducing the effectiveness of Kyoto Protocol.
The US and 12 nations in Europe agree to ban all use and production of CFCs by 2000. This is highly significant as these countries produce three quarters of CFCs in the world.
Increased research and development into new forms of UV protection such as new sunscreens.
Replace CFCs with compounds which have significantly lower ozone depletion potentials (ODP) such as hyfrochloroflurocarbons (HCFCs). These compounds are effective as they are attacked while in the lower atmosphere thus it does not reach the stratosphere to attack ozone. However these compounds are more expensive and in some cases less effective than CFCs in various applications.
These steps have been effective in combating the problems caused by CFCs, namely the major problem of ozone depletion. There has been a huge decrease in CFC production and emission due these steps. However more needs to be done to ensure the survival of ozone as it is vital in protecting life from harmful UV radiation from the sun.
Point 4.11 – Analyse the information available that indicates changes in atmospheric ozone concentrations describe the changes observed and explain how this information was obtained.
The concentrations of ozone in the atmosphere have been recorded since 1957. In the 1970’s, it was found that CFCs were depleting the ozone layer in the stratosphere. In 1974, scientists Mario Molina, Sherwood Rowland and Paul Crutzen (noble prize winners), demonstrated that CFCs were responsible for ozone depletion. In 1985, Joe Farman and his colleagues at the British Antarctic Survey (BAS) in Cambridge published a paper revealing a dramatic decline in stratospheric ozone over the Antarctic in spring.
These diagrams show that the concentrations of ozone in the stratosphere have decreased. This can be seen in the purple area getting larger which indicates a lower concentration of ozone in Dobson units.
FOR THE FORMATION OF OZONE SEE DOT POINT 4.5.
Ozone levels have decreased in the stratosphere, especially above Antarctica. In 1985, it was revealed that the concentrations of ozone had decreased 50% since 1975. This ‘hole’ is continuing to grow. Here is a brief timeline showing yearly changes of ozone concentration:
In 1993, reduction in ozone levels was about 25% - 35% depending on the area.
In 2000, the Antarctic ozone hole was 3 times larger than the USA
In 2001, ozone depletion of 10 to over 40% was observed over Antarctica. There was complete destruction of ozone at altitudes between 15 and 20km.
In 2002, ozone depletion of more than 40% was observed. However in this year, the duration of ozone hole was shortest since 1988.
In 2003, there was again ozone depletion of over 40% over Antarctica.
This monitoring of yearly changes of ozone concentrations in the stratosphere suggest that ozone levels are decreasing at a rapid rate and that the ozone hole has been increasing in area.
This ozone depletion is due to complex air motions. Air enters the stratosphere near the equator, where solar heating is highest. Then the air moves slowly towards both poles where it sinks.
Over Antarctica, cloud ice particles exist that are not present at warmer latitudes. Reactions occur on the surface of these particles that accelerate ozone destruction caused by chlorine.
Winter in the Antarctic is a period of continuous darkness and the stratosphere is extremely cold. This is an equation which shows how cloud particles catalyse a reaction between HCl and ClNO3:
HCl (g) + ClNO3 (g) Cl2 (g) + HNO3 (g)
This has no effect upon ozone levels in winter but when arrives in spring, the UV rays from the sun splits the chlorine molecule to form 2 chlorine free radicals which are able to destroy thousands of ozone molecules. This is why there is increased destruction of ozone in spring which also means that the ozone hole is the biggest in spring.
How information is obtained
Information about ozone concentrations in the stratosphere was obtained by ground-based instruments. This includes UV spectrophotometers which point upwards through the atmosphere, measuring the intensity of light received at a wavelength at which ozone absorbs.
Another instrument used is the Total Ozone Mapping Spectrometer (TOMS) that measures concentration of ozone aboard several US satellites. They are able to scan through atmosphere and measure ozone levels in relation to altitude. However this has been replaced by another instrument – Ozone Measuring Instrument (OMI) which is a satellite that covers the whole area of the earth every day and accurately maps ozone levels at all altitudes.
Very large helium-filled balloons have been used to carry instruments including UV spectrophotometers up to the stratosphere to measure concentrations of ozone.
Point 4.12 – Present information from secondary sources to write the equations to show the reactions involving CFCs and ozone to demonstrate the removal of ozone from the atmosphere.
Secondary sources may include the internet, books, school notes, journals etc.
Ozone depletion by CFCs
Once the CFC has entered stratosphere, the UV radiation from the sun is absorbed by the CFC causing it to photo dissociate. This produces a chlorine free radical:
CCl3F (g) + UV Cl .(g) + CCl2F (g)
This chlorine free radical attacks an ozone molecule producing a chlorine oxide radical and an oxygen molecule:
Cl . (g) + O3 (g) ClO . (g) + O2 (g)
This chlorine oxide radical reacts with an oxygen free radical to form a chlorine free radical and oxygen:
ClO . (g) + O. (g) Cl .(g) + O2 (g)
Ozone is destroyed. The chlorine free radical is free to destroy thousands of more ozone molecules just like this until it reacts with methane. Ozone depletion is more frequent in winter and spring due to more ice particles. These provide a surface to act as a catalyst.
Ozone depletion by halons
Process is very similar to CFCs:
Once the halon has entered stratosphere, the UV radiation from the sun is absorbed by the halon causing it to photo dissociate. This produces a bromine free radical:
CF3Br (g) + UV CF3 (g) + Br. (g)
This bromine free radical attacks an ozone molecule producing a bromine oxide radical and an oxygen molecule:
Br. (g) + O3 (g) BrO. (g) + O2 (g)
This bromine oxide radical reacts with an oxygen free radical to form a bromine free radical and oxygen:
BrO. (g) + O. (g) Br. (g) + O2 (g)
Like the chlorine free radical produced, the bromine free radical is free to destroy thousands of ozone molecules. Halons are more dangerous than CFCs because they may contain bromine and chlorine atoms resulting in the formation of bromine and chlorine free radicals which destroy ozone.
Point 4.13 – Gather, process and present information from secondary sources including simulations, molecular model kits or pictorial representations to model isomers of haloalkanes.
Basically we used molecular model kits to construct different isomers of haloalkanes. To represent the atoms, there were different coloured balls (each representing a different atom) and there were rubber bonds which we attached between atoms to represent the covalent bonds. To make isomers, we pulled the balls apart and rearranged them to form a different structural formula. It is an isomer because it retains the same molecular formula but changed the structural formula. We drew the molecular formula, structural formula and 3D molecular model for each isomer.
Point 4.14 – Present information from secondary sources to identify alternative chemicals used to replace CFCs and evaluate the effectiveness of their use as a replacement for CFCs.
Alternative chemicals used to replace CFCs are HCFCs (hydrochlorofluorocarbons) and HFCs (hydro fluorocarbons).
CFCs had many useful applications: refrigerants in air-conditioners and refrigerators, a solvent in dry-cleaning and as a propellant in aerosol spray cans (insecticides, deodorants). They were used for these applications because they were inert and very stable. However the major problem of CFC is that they cause the depletion of ozone in the stratosphere. They do not react with substances in the troposphere so they move up into the stratosphere. They photo dissociate under UV light to produce chlorine free radicals which attack and destroy ozone molecules.
HCFCs were proposed as alternative chemicals to replace CFC in the above applications. Example of HCFC is: CF3CHCl2 (HCFC-123).
Advantages of HCFCs include:
Unlike CFCs, HCFCs are less stable and thus are broken down in the troposphere. They contain relatively reactive C-H bonds which are easily broken up by reactive radicals and atoms to form carbon dioxide, water and hydrogen halide. This nullifies the problem of it attacking ozone molecules in the stratosphere.
HCFCs have a much lower ODP (ozone depleting potential) than CFCs which basically means that there is less chance of the destruction of ozone caused by HCFCs. For example the CFC, trichlorofluoromethane (CFC-11) has an ODP of 1.0 whereas the HCFC, HCFC-123, has an ODP of only 0.05.
Shorter life spans than CFCs.
Used as foam blowing agents, solvents and refrigerants.
Disadvantages of HCFCs include:
They are more expensive to use in various applications than CFCs.
Some HCFCs still contain chlorine atoms in their structure thus they can still produce free chlorine radicals which contribute to the destruction of ozone.
It has been decided by the international community that HCFCs should be phased out by 2030 because of the above reason.
Not necessarily safe as its toxicity is not known.
Contributes to greenhouse effect by absorbing long wave radiation.
HFCs are the most widely used replacements of CFCs. Examples of HFCs are: CF3CFH2 and C2F4H2 (HFC-134a).
Advantages of HFCs include:
Unlike CFCs and HCFCs, HFCs do not contain chlorine atoms. This means they have an ODP of zero. Thus they cannot produce chlorine free radicals which destroy ozone molecules in the stratosphere.
They contain reactive C-H bonds which are broken up by atoms and free radicals in the troposphere.
HFCs are used in refrigeration and air conditioning as an alternative to CFCs.
Disadvantages of HFCs include:
Not as effective propellants and refrigerants than CFCs.
More expensive and less efficient than CFCs.
As with HCFCs, HFCs contribute to greenhouse effect by absorbing long wave radiation. For example: the greenhouse effect intensity of HFC-134a is about 3400 times that of carbon dioxide.
When HFCs decompose in the troposphere, they produce a harmful by-product – Trifluoroacetate, which when it accumulates in wetlands and there is poor drainage, could result in aquatic organism death. Its corrosiveness also adds to this.
For HCFCs and HFCs to be effective alternatives to CFCs, they need to demonstrate first and foremost that they cause no (or minimal) destruction of ozone molecules and that they are capable of being used in applications which previously utilised CFCs.
With the introduction of HCFCs and HFCs as alternatives to CFCs, the chance of the destruction of ozone molecules in the stratosphere has decreased. However these two compounds add to the other major environmental problem concerning the greenhouse effect. Thus their effectiveness as replacements for CFCs in a variety of uses is minimal. There needs to be more research conducted in the future to find compounds which effectively can be used for CFCs applications as well as having negligible impact on the environment.
Human activity also impacts on waterways. Chemical monitoring and management assists in providing safe water for human use and to protect the habitats of other organisms.
Point 5.1 – Identify that water quality can be determined by considering: concentrations of common ions, total dissolved solids, hardness, turbidity, acidity, dissolved oxygen and biochemical oxygen demand (BOD).
Water is an essential substance for humans and we depend on it for survival. Thus it is essential that we continuously monitor water quality in local waterways, dams and especially in our drinking water. Contaminants and pollutants may be present in the water thus immediate removal is required to prevent any adverse affects on the human body. There are a number of aspects of water quality. A description and how it is measured is outlined below:
Concentrations of common ions
In the ocean, the major cations present are Na+, Mg2+ and Ca2+. The major anions found are Cl-, SO42- and HCO3-. The concentrations of sodium and chloride ions are significant as they heavily contribute to the salinity of the water system. Salinity refers to the total concentration of dissolved inorganic salts in water. Magnesium and calcium ions are usually tested for water hardness (explained later). However besides these common ions, there may be other ions present – e.g. NO3- and PO43- from fertilisers in the soil. The concentrations of the sodium and chloride ions need to be monitored as excess can increase salinity levels and aquatic organisms may not be able to cope with this. The concentrations of nitrates and phosphates need to monitored as excess can cause algal blooms and lead to eutrophication.
MEASURING/TESTING FOR IONS:
The concentration of metal cations can be easily and quickly detected by using AAS.
Chloride ions can be detected by adding drops of silver nitrate. A milky white precipitate confirms chloride ions.
Carbonate ions can be tested by adding HCl and if it forms bubbles of CO2 then carbonate ions are present.
Sulfate ions can be tested by adding barium ions to produce white precipitate of barium sulfate.
Nitrate and phosphate ions tested using colorimetry. (Phosphate ions can also be tested by adding lead nitrate).
Total Dissolved Solids (TDS)
Total Dissolved Solids refers to the total mass of all solids dissolved in a specified volume of water. Usually freshwater systems such as rivers and lakes have much less concentrations of ions than oceans as these systems continuously deposit the ions into the oceans whilst at the same time being replenished. TDS are mainly composed of salts – ionic compounds. A large amount of TDS is indicative of unclean water as clean water has very low concentrations of TDS.
MEASURING TDS:
A sample of water is filtered leaving behind suspended solids as the residue and the total dissolved solids pass through filter paper with the water. Then solution is evaporated to dryness and the solid left behind is the dissolved solid. Now gravimetric analysis can be conducted to determine the mass of the dissolved solids. This procedure is laborious and not fully valid as solid could be lost through bubbling and spitting. Thus this process is extremely slow.
Electrical conductivity meter, since TDS mostly contains ionic salts. However it is unable to detect any non ionic salts which also make up TDS. Relatively accurate approximation of TDS.
Hardness
Hard water refers to water which contains significant amounts of calcium and magnesium ions. Hard water is an issue in water quality as it aids in the formation of scale which are deposits of CaCO3 and MgCO3. This can occur in kettles. Also hard water reduces the cleaning action of soap by forming a grey precipitate called scum as the calcium and magnesium ions react with the anion in soap.
MEASURING HARDNESS:
The water sample can be titrated against a chemical compound called EDTA (ethylene-diamine-tetra-acetic-acid) and the concentration determined is expressed as mg/L of CaCO3.
Levels of calcium and magnesium ions can be determined using AAS.
Can qualitatively determine if water is hard by adding soap to it. Shake and if soap does not lather then the water is hard.
Turbidity
Turbidity is the measure of suspended solids (cloudiness) in water. It is caused by suspended solids which are too small to settle to the bottom + are able to scatter light instead of letting it to pass through. Water which has large amounts of turbidity has low quality as turbidity gives water an undesirable appearance + taste as well as blocking sunlight for underwater plants which need sunlight for photosynthesis.
MEASURING TURBIDITY:
Lower a turbidity tube (long, narrow tube which has a small black cross etched onto the bottom) into the water until the cross cannot be seen. Record the water level and the turbidity result is obtained (measured in NTU units).
Acidity
The pH of water systems generally lies between 6.5-8.5 and this usually represents a healthy water system. The natural buffer system – HCO3-/H2CO3 works effectively in minimising pH changes in the water. A low pH (very acidic) or a high pH (very basic) may result in aquatic organisms’ death as they cannot cope with this sudden rise/fall in pH.
MEASURING ACIDITY (pH)
pH meter + data logger
universal indicator + pH strips
Dissolved oxygen
Molecular oxygen in the air dissolves in the water to form dissolved oxygen. Although dissolved oxygen exists in low concentrations in water, this is essential for aquatic organisms and plants which use this dissolved oxygen in water for respiration. A concentration of 5ppm or greater is suitable for waterways.
MEASURING DISSOLVED OXYGEN
Using a chemical titration called the Winkler Method. Here dissolved oxygen oxidises Mn2+ to Mn4+ in alkaline solution. This mixture is then acidified with iodide in the solution and Mn4+ oxidises iodine ions to iodine gas. Finally iodine can be titrated against sodium thiosulfate in the presence of a starch indicator.
An electronic oxygen sensor can also be used.
Biological Chemical Demand (BOD)
Biological Chemical Demand (BOD) refers to the concentration of dissolved oxygen which is needed by anaerobic bacteria to break down organic matter in the water. BOD is a vital determinant in the quality of water as a high BOD indicates that there are large amounts of organic matter in the water which drain supplies of dissolved oxygen in the water. Suitable BOD level in unpolluted natural waterways should be < 5ppm.
MEASURING BIOCHEMICAL OXYGEN DEMAND (BOD)
Two samples of water are taken with one of them placed in a sealed air-free container and incubated at 20ºC for 5 days in the dark. Meanwhile the other sample’s dissolved oxygen concentration is immediately determined upon collection by one of the methods explained above. After the 5 days, the dissolved oxygen concentration of the water sample which was in the sealed container is measured and the difference between this reading and the reading at the start of the experiment is the BOD level for the water.
Point 5.2 – Identify factors that effect the concentrations of a range of ions in solution in natural bodies of water such as rivers and oceans.
The ion concentrations in rivers are much less than that of oceans thus the concentrations of ions in freshwater systems is such that rivers are more vulnerable to change. The following are factors which determine the concentrations of particular ions in a body of water such as a river or ocean:
Natural sources of ions
Depending on the natural body of water’s location, there are different possibilities of ions which could be present in that particular water system. For example if rain falls on farms and bushland and then runs off into the water, there would be high concentrations of NO3-, PO43- (fertilisers). However if rain soaks into the ground and flows into aquifers (layers of permeable rock) and then into the river, there would be increased levels of calcium, magnesium, sulfate and chloride ions. If the river was situated near a limestone deposit or cave, the water would leach the limestone and thus there would be a high concentration of calcium and carbonate ions.
Also the effect of acid rain has significant impacts on the ions found in water as sulfur and nitrogen oxides dissolve in the rain to form weak acids which when entering water increases the concentrations of sulfate, nitrate and hydrogen ions (decreasing pH).
Land clearing
When the land is cleared of vegetation, the soil loses its stability as there are no plant roots to support it. When water flows over it, sediments and minerals will flow into the river thus increasing concentrations of Na+, K+, and nitrate + phosphate ions from run off fertiliser and animal faeces. This would increase turbidity.
Agriculture
When heavy rain falls on farm land, the soil is washed into the river thus increasing the concentrations of nitrate + phosphate ions. This would increase turbidity and TDS in the river system.
Industrial waste + sewage
Every year tonnes of industrial wastes are disposed carelessly without carefully monitoring the concentrations of the ions present. The huge influx of industrial waste from factories and sewage from homes greatly increase the concentrations phosphates and nitrate ions as well as heavy metal ions such as cadmium, lead and mercury. Because the level of organic matter has increased, the BOD also increases thus meaning depleting levels of dissolved oxygen available for aquatic organisms. Sewage + industrial waste greatly increase turbidity and TDS.
Point 5.3 – Describe and assess the effectiveness of methods used to purify and sanitise mass water supplies.
The water collected in dams from rainfall cannot be immediately pumped into households for consumption because it contains pollutants and contaminants which need to be removed to make it safe for drinking. The following steps are undertaken by authorities such as Sydney Water to purify and sanitise water supplies for domestic use:
Monitoring catchment area
Authorities monitor the catchment area carefully to ensure that local soil run offs are avoided as this can increase the concentrations of unwanted ions. Thus this may mean the closure of many industries in the area (if situation is terrible) in an attempt to prevent the discharge of heavy metal ions (in the waste) into the waterways.
Advantage: This process is effective as removing possible sources of contamination ensures that relative purity can be assured.
Screening
Once the water is collected from the dams, it is screened by passing it through huge metal plates which prevent large debris in the water (e.g. sticks, rocks, branches) from passing through.
Advantage: This step is suitable for its purpose but more treatment is needed as there are much more pollutants too small to be trapped.
Flocculation
This step involves the removal of the colloidal and particulate impurities. These particles are too small to be removed by conventional filtration. The particles are coagulated together to form large particles which then can be removed. Firstly the pH of the water is increased hence made alkaline. This is achieved by adding lime and these conditions promote the formation of precipitates. Chemicals such as alum (aluminium sulfate) and iron (III) chloride are added to form a gelatinous precipitate which causes the particles to stick to it and coagulate to form larger particles called flocs which are easy to filter.
Advantage: This step is efficient as it removes smaller colloidal particles and other particulate matter from the water by causing them to form large clumps. It is cost-effective method and is rapid.
Sedimentation (clarification)
The water now passes into large tanks where the flocs are allowed to settle to the bottom of the tank to form a sludge which is dried and used for composting.
Advantage: This process utilises a natural force (gravity) to separate the sludge from the water and thus reduces costs. However this process is time consuming which reduces its effectiveness.
Filtration
The water from the settling tanks is pumped into filtration tanks which contain layers of sand and gravel. The water passes through the fine sand + gravel filter bed; this removes any remaining particulate matter in the water that did not settle in the tank in the previous step. Charcoal filters may also be used to remove coloured substances.
Advantage: This step removes most of the particulate matter but does not trap bacteria and viruses.
Chlorination
The filtered water is disinfected with chlorine gas and various hypochlorites (OCl-). Chlorine gas is bubbled through water to form this hypochlorite ion which is mainly responsible for killing micro-organisms and bacteria such as E.Coli.
Advantage: An effective method for killing harmful bacteria but is not so effective at killing viruses.
Fluoridation
Fluoride compounds e.g. sodium fluoride or sodium hexaflurosilicate are added to the water to help prevent tooth decay by interacting with the tooth enamel to produce a denser lattice, making the teeth more stronger and resistant.
Advantage: Prevents tooth decay.
Point 5.4 – Describe the design and composition of microscopic membrane filters and explain how they purify contaminated water.
Microscopic membrane filters are an alternative to large scale chemical treatment of water to remove harmful microorganisms (outlined above).
Basically a membrane filter is a thin film of synthetic polymer (e.g. polypropylene, PTFE) throughout which there are tiny pores of fixed uniform size. Its main use is to remove solutes, colloidal particles and microorganisms which cannot be removed from processes outline above.
The polymer sheets are wound around a central rigid core to form a cartridge that can be replaced. The water is pumped from one end and flows over the surface of the polymer sheets. The pressure (gravity, vacuum or pressure pumps) forces the clean water filtrate through the core and out the other end. This results in colloidal particles, microorganisms and solute particles to be trapped and removed.
Other microscopic micro filtration membranes consist of fine, hollow capillaries that are present inside a filtering unit. Here the filtered particles are trapped on the outside of the capillaries and the clean water filtrate passes through the centre of the capillary.
Water that is to be filtered by the microscopic membrane is made to flow across the surface to prevent clogging of the pores. The filters can be cleaned by back flushing.
There are 3 types of membranes:
Micro filtration (MF) membranes
These membrane filters can remove microscopic parasites such as Crytptosporidium and Giardia as well as viruses and fine colloidal particles (200-500 nm).
Ultrafiltration (UF) membranes
These remove particles in the size range of 2-100nm. Paint particles and large organic molecules can be removed by UF membranes.
Nanofiltration (NF) membranes
These filters have pore sizes less than 1nm and can be used to remove ions from water (including heavy metal ions). They can also be used to remove small molecules.
Point 5.5 – Perform first-hand investigations to use qualitative and quantitative tests to analyse and compare the quality of water samples.
Qualitative tests refer to tests which aim to determine the composition of a substance i.e. what molecules, ions or atoms are present in the given sample.
Used precipitation reactions to deduce which ions were present in the sample. For the presence of heavy metals, the sulfide test was used where a precipitate indicated the presence of a heavy metal such as cadmium, lead or mercury.
Quantitative tests refer to tests which aim to determine the amount of different substances in a given sample.
For the turbidity test, we lowered long narrow turbidity tube into the water until the black cross on the bottom was not visible. Then the height of the water was calculated by reading the units on the side of the tube. The result was in NTU units.
For temperature we measured it using a thermometer.
For pH levels, we used universal indicator (or pH meter + data logger)
For TDS, we used an electrical conductivity meter.
Dissolved oxygen levels were measured using an oxygen sensing meter.
BOD was measured by collecting 2 water samples from the same area. Of one, the DO level was immediately recorded and for the other one, it was placed in a capped air free container in the dark where it was incubated at 20ºC for 5 days. Then DO level measured and difference from 1st and 2nd DO was the BOD level.
Hardness – titration method used where to a 250mL sample, 1mL of buffer solution + 2-3 drops of Eriochrome Black T indicator was added. This red/violet solution was titrated against EDTA and hardness was calculated.
Reliability – Repeat experiment to get consistent results.
Validity – Reasonably valid as we monitored water quality and tested the aim. However we did not use controls.
Accuracy – Accurate as we used sensitive instruments.
Point 5.6 – Gather, process and present information on the range and chemistry of the tests used to: identify heavy metal pollution of water, monitor possible eutrophication of waterways.
Heavy metal pollution
Heavy metals are toxic and have a large atomic mass. Examples of heavy metals include cadmium, lead and mercury. The concentrations of these heavy metals need to be constantly monitored as they can have significant impacts on surrounding aquatic organisms in waterways. Since they bioaccumulate in food chains, heavy metals are very dangerous to humans in large amounts.
The identification and quantity of heavy metals in a water sample can be determined by conducting the following tests:
Atomic Absorption Spectroscopy (AAS). This can accurately determine extremely low concentrations of heavy metals down to parts per billion (ppb).
Another quantitative test which can be used to identify heavy metals is the sulfide test. First of all a water sample is acidified and then a few drops of sodium sulfide is added. If a precipitate forms then one of the following ions are present: lead, silver, mercury, arsenic, copper or arsenic. However if a precipitate does not form, then the sample is made alkaline. If this produces a precipitate then one of the following ions are present: zinc, iron, nickel, and cobalt.
The equilibrium reaction is shown below:
S2- (aq) + 2H3O+ (aq) H2S (aq) + 2H2O (l)
If acidic conditions are utilised, the above equilibrium shifts to the right to use up some hydronium ions (Le Chatelier’s principle). Thus the concentration of sulfide ions decrease which means that there is only a small amount of it. However this small amount of sulfide ions is able to precipitate the 1st group of heavy metals.
If alkaline conditions are utilised, the above equilibrium will shift to the left to produce more hydronium ions to neutralise the alkalinity in the reaction (Le Chatelier’s principle). Thus the concentration of the sulfide ions increase and therefore the 2nd group of heavy metals are able to be precipitated out.
Flame tests e.g. copper
Precipitation reactions
Volumetric + gravimetric analysis, colorimetry and chromatography.
Eutrophication of waterways
Eutrophication is the situation where a waterway experiences a significant rise in nutrients (excess concentrations of nitrates + phosphates) which cause an algal bloom resulting in depleting levels of dissolved oxygen in the water. The algae which are formed also release poisonous toxins which can kill humans and aquatic organisms. When they are broken down by decomposers such as bacteria, more dissolved oxygen is used up in the process thus causing the deaths of fish, crab and other organisms. Because eutrophication is a significant issue, the concentrations of phosphate and nitrate ions (which cause eutrophication) needs to be monitored carefully. Recommended levels of nitrogen and phosphorus levels are:
N: P ratio > 10:1
Nitrogen level: 0.1 – 1ppm
Phosphorus level: 0.01 – 0.1ppm
TEST FOR NITRATES
There are 3 methods for testing nitrates:
Brown ring test – Iron (II) sulfate is added to concentrated sulfuric acid and this produces a brown ring at the junction of the 2 solutions. This is nitrate.
Kjeldahl digestion – A sample is heated with concentrated sulfuric acid to ‘digest’ any nitrogen compounds into ammonium sulfate. This is now reacted with sodium hydroxide to form ammonia. Finally the levels of ammonia are measured by back titration against a standardised HCl solution.
Colorimetry – treated with Nessler’s reagent that reacts with nitrogenous compounds to form a yellow compound.
TESTS FOR PHOSPHATES
Colorimetry – Ammonium molybdate is added to the sample and dissolved. This forms a pale yellow complex. Then solid, powered ascorbic acid is added and this forms intensely blue complex of a compound known as ‘molybdenum blue’. This intensity of blue colour is measured by a colorimeter and compared to a series of standards to determine the concentration.
Add lead (II) nitrate to form white precipitate.
Point 5.7 – Gather, process and present information on the features of the local town water supply in terms of: catchment area, possible sources of contamination in this catchment, chemical tests available to determine levels and types of contaminants, physical and chemical processes used to purify water, chemical additives in the water and the reason for the presence of thee additives.
Catchment area:
Warragamba Catchment; has an area of around 9000km2 and extends from south of Goulburn, north to Lithgow, east to Mittagong and west to Crookwell.
Sources of contamination
Logging and land clearing activities – releases ions into water via run offs. This results in higher levels of TDS and turbidity.
Agriculture activities – using excess fertilisers which are rich in phosphate and nitrate fertilisers can enter waterways via run offs causing eutrophication and algal blooms. This causes low levels of DO and high levels of BOD.
Mining in local areas near the dam have caused leaching of ions e.g. calcium + carbonate ions from limestone and zinc + copper ions also released. Disposal of mining wastes directly into waterways.
Industrial waste + sewage may release bacteria + variety of ions + organic matter. Decreases DO levels, increases BOD, increases turbidity and TDS of the waterway.
Testing contaminants
Heavy metal ions and other cations using AAS or sulfide test.
Precipitation reactions + flame tests
Colorimetric analysis (nitrates + phosphates)
Water purification methods
Large scale industrial treatment of water – screening, flocculating, sedimentation, filtering, chlorination, fluoridation.
Membrane filters
Chemical additives
Chlorine bubbled in water to produce hypochlorite which is used as a disinfectant in water killing microorganisms and bacteria such as E.Coli.
Fluoride compounds (e.g. sodium fluoride) added to prevent tooth decay.
A note of thanks to:
Dot point Chemistry
Spotlight Chemistry
Conquering chemistry
HSC Online
Chemistry contexts 2
Macquarie revision guides: Chemistry
Ahmad Shah Idil
by
ssantarelli (student)
