Group 2
Group member: Chan Tsz Mei
Cheng Yu Sang
Lai Tsz Ching
Lam Wing Hon
Tsui Yuk Ping
South Tuen Mun Government Secondary School
Chemistry Project
Secondary 6
Chemistry and Society: Unit 46: Chemistry and the Environment
1. Define the terms pollution and pollutant.
Ans: Pollution is the introduction of waste matter or energy into the environment
by humans. It causes damages or deterioration to living systems and/or the
environment.
A pollutant refers to any substance or energy produced through human
activities which, at an unacceptable level, causes damages to the
environment or is harmful to living systems.
2. What are the major air pollutants?
Ans: There are six major air pollutants. They are carbon monoxide, sulphur
dioxide, nitrogen oxides, hydrocarbons, ozone and particulates.
3. What are their sources or formation with the help of equations?
Ans: Carbon monoxide
Carbon monoxide is the most common air pollutant in large cities. The
major source of this pollutant is motor car exhaust. It is produced through
incomplete combustion of carbon-containing fuels, such as petrol, coal and
wood. For example, octane may be burnt incompletely as follows:
C8H18 (g) + 9O2 (g) —> 3CO2 (g) + 3CO (g) +2C (s) + 9H2O (l)
Sulphur dioxide
Sulphur dioxide comes mainly from burning fossil fuels (e.g. fuel oil and
coal) which contain sulphur. A large proportion of sulphur dioxide is
produced during the operation in power stations and metal smelters in
which sulphur-containing coal is used as the fuel, and also the
manufacturing industries in which fuel oil is burnt.
Nitrogen oxides
Nitrogen oxides are produced whenever fuels are burnt at a very high
temperature. In Hong Kong, the level of nitrogen oxides is noticeably high
because of the high proportion of diesel-driven vehicles running in the
territory. Diesel engines produce much less carbon monoxide but a lot more
nitrogen oxides than petrol engines.
Hydrocarbons
Hydrocarbons are formed from the evaporation of organic materials such as
petrol, diesel and solvents. They are also found in car exhaust as unburnt
hydrocarbons.
Ozone
Ozone is produced by the reaction of oxygen gas with the free atoms of
oxygen which are formed from the reactions between nitrogen oxides and
hydrocarbons in sunlight.
O2 (g) +O•—> O3 (g)
oxygen
radical
Particulates
Particulates include black smoke (mainly fine carbon particles), dust and
soot (coarse carbon particles). They are produced during the operations in
incinerators, factories, diesel vehicles, construction sites and coal or
charcoal burners. They are solid or liquid particles and are so small that
they remain suspended in air for a long period of time.
4. What are the common effects of these pollutants?
Ans: Carbon monoxide
Carbon monoxide is a poisonous gas. When one inhales it, it will enter the
blood stream and bind preferentially to haemoglobin (the substance that
carries oxygen) in red blood cells, thereby replacing oxygen. This result in
the reduction of the amount of oxygen delivered to body organs and tissues.
Carbon monoxide can cause death even in relatively low concentrations. The
danger of carbon monoxides is heightened by the fact that the gas is
colourless, tasteless and odourless. People may suffer from its effects before
they are aware of its presence.
Sulphur dioxide
Sulphur dioxide is a colourless, toxic gas. It can cause impairment of the
respiratory function, aggravation of existing respiratory diseases (especially
bronchitis and asthma) and cardiovascular diseases. It is mostly the sulphur
dioxide in air that causes acid rain. Acid rain can damage buildings and is
also though to be responsible for the deaths of large numbers of trees in
Europe and America.
Nitrogen dioxide
An example of nitrogen oxide is nitrogen dioxide which is reddish brown
toxic gas with a very unpleasant pungent odour. It gives rise to
photochemical smog. It can also irritate the lungs and lower our resistance
to respiratory infections such as influenza. Individuals with respiratory
problems, such as asthma, are more susceptible to the effects. In young
children, nitrogen dioxide may also impair lung development. Nitrogen
oxides also cause the formation of acid rain.
Hydrocarbons
Hydrocarbons cause photochemical smog. Certain hydrocarbons (e.g.
benzene) are known carcinogens.
Ozone
Ozone has a great impact on the respiratory system. Symptoms associated
with exposure to ozone include cough, chest pain, and throat and eye
irritation. Ozone can also increase our susceptibility to respiratory
infections and damage plants.
Particulates
The environment effects of particles depend on both the size and the nature
of the particulates. Very fine particles (diameter less than about 1µm) are
the most hazardous. They cannot be filtered out by hairs and mucus in the
nose and the respiratory tract. They can be drawn deep into the lungs,
where they can remain indefinitely and impair the lung function. They can
hinder gas exchange, damage lung tissues, and cause respiratory illnesses.
Certain particles are suspected to cause cancer.
5. What are the factors that the harmful effects of pollutants depend on?
Ans: The harmful effects of pollutants depend on their concentrations and the
duration of exposure to them. Take carbon monoxide, a well- know
poisonous air pollutants, as an example. Of the concentration of carbon
monoxide is 600-700 ppm, inhaling it for an hour brings about barely
detectable effects. However, a concentration of 4000 ppm or higher can
cause death in less than one hour. If an adult is exposed to an atmosphere
containing 1000 ppm carbon monoxide for 4 hours, he is likely to die form
carbon monoxide poisoning.
6. What does ppm stand for?
Ans: “ppm” represents “part per million” which is a unit for expressing
concentration. This is the number of molecules of pollutants per million
(106) molecules in air. It is thus a ration with no unit. Based on the
Avogadro’s law, ppm is also equal to the number of volumes of pollutants
per million volumes of air.
Parts per million (ppm) = Number of molecules of pollutants
106 molecules of air
= Number of volumes of pollutants
106 volumes of air
7. Express 0.03 ppm SO2 as µgm-3.
Ans: In 1 m3 of air, there is 0.03/106 sulphur dioxide.
Using the ideal gas equation,
PV = nRt
= (Mass of SO2/M) RT
where M is the molar mass of SO2.
Mass of SO2 = PVM/RT
= (101x103 Pa)(0.03/106 m3)(64.1 gmol-1)
(8.314 JK-1mol-1)(298K)
=79.9µg
→The concentration of sulphur dioxide is 79.9µgm-3.
8. Decide whether the unpolluted rainwater is acidic or alkaline? Explain with
the help of equations?
Ans: Unpolluted rainwater is slightly acidic, with a pH value of about 5.7. it is
because as it falls, the rainwater reacts with carbon dioxide in the air to
form carbonic acid.
CO2 (g) + H2O (l) —> H2CO3 (aq)
9. What are the two major air pollutants responsible for the formation of acid
rain? Write down the relevant equations.
Ans: Sulphur dioxide and nitrogen dioxide are the two major pollutants
responsible for the formation of acid rain. Sulphur dioxide is mainly
released from the burning of fossil fuels in power stations.
In the atmosphere, sulphur dioxide is slowly oxidized to sulphur trioxide
which dissolves readily in water droplets o form sulphuric(VI) aic.
2SO2 (g) +O2 (g) —> 2SO3 (g)
SO3 (g) +H2O (l) —> H2SO4 (aq)
The actual pathways are more complicated. The formation of sulphur
trioxide from sulphur dioxide is influenced by the prevailing atmospheric
conditions: sunlight, temperature, humidity and the presence of
hydrocarbons, nitrogen oxides and particulates in the atmosphere.
Sulphuric(VI) acid is also formed when sulphur dioxide dissolves in
rainwater.
SO2 (g) + H2O (l) —> H2SO3 (aq)
The sulphuric (IV) acid formed will be further oxidized to sulphuric (VI)
acid by atmospheric oxygen.
2H2SO3 (aq) + O2 (g)—> 2H2SO4 (aq)
Nitrogen oxides released from the burning of fossil fuels in automobiles and
power stations into the atmosphere also lead to the formation of acid rain.
When nitrogen monoxide is released to the atmosphere, it combines with
atmospheric oxygen to form nitrogen dioxide.
2NO (g) + O2 (g) —> 2NO2 (g)
In a series of complex reactions, nitrogen dioxide combines with oxygen and
water vapour to form nitric (V) acid.
4NO2 (g) + 2H2O (l) + O2 (g) —> 4HNO3 (aq)
10. What are the effects of the acid rain on the environment?
Ans: Acid rain causes a lot of harmful effects on the environment. In water of pH
lower than 4.5, calcium metabolism in freshwater fish will be affected,
leading to poor health and stunted growth. As a result, the diversity and
population of some freshwater species will be reduced. In soil of pH lower
than 4.5, absorption of essential nutrients (e.g. K+, Ca2+ and Mg2+) by
plants will be affected, resulting in the death of plants.
Acid rain also corrodes metals and accelerates the rate of deterioration of
buildings, rocks and statues.
11. What does it mean by photochemical smog?
Ans: The word “smog” comes from two words: smoke and fog. It is used to
characterize a visible combination of smoke and fog. Photochemical smog
is a mixture of pollutants including particulates, nitrogen oxides, ozone,
aldehydes, peroxyacrtyl nitrate (PAN) and unreacted hydrocarbons, etc. a
brownish haze that irritates our eyes is an indicator of photochemical smog.
Nitrogen dioxide is responsible for the brownish color of the haze.
12: how is the photochemical smog formed?
Ans: Reactions that lead to the formation of photochemical smog are initiated by
sunlight and involve hydrocarbons and nitrogen oxides emitted form
automobiles. The combination of sunlight, particulate matters and abundant
pollutants present in modern cities provide favourable conditions for smog
formation.
Nitrogen dioxide form automobile exhaust first absorbs sunlight and breaks
down into nitrogen monoxide and free oxygen atom which is very reactive.
UV light
NO2 (g) —> NO (g) + O (g)
The oxygen atom then reacts with other components of automobile exhaust (e.g.
unburnt hydrocarbons) and those of the atmosphere (e.g. oxygen and water
vapour) in a series of complex reactions to produce a variety of lachrymatory
and toxic chemicals such as peroxyacrtyl nitrate (PAN).
O
║
CH3─C─O─O─NO2
PAN molecule
Very much simplified, some of the principal reactions involved in the formation
of photochemical smog are illustrated below:
O (g) + O2 (g) —> O3 (g)
O (g) + hydrocarbons —> aldehydes
O3 (g) + hydrocarbons —> aldehydes
Hydrocarbons + O2 (g) + NO2 (g) —> lachrymatory substances, including PAN
The wavelength of ultraviolet radiation from the sun determines which bonds are
broken in the reactions that lead to the formation of photochemical smog.
Therefore, the reactions vary with altitude.
13. What are the effects of the photochemical smog?
Ans: Photochemical smog causes headaches, eye, nose and throat irritation,
worsening of the lung function, coughing and wheezing.
14. What is the formula of zone?
Ans:
UV light
O2 (g) —> 2O (g)
O (g) + O2 (g) > O3 (g)
15. What is the formation of the ozone in the lower part of atmosphere?
Ans: In the lower part of the atmosphere (i.e. the troposphere), ozone can be
formed in a number of ways: by the reaction between nitrogen oxides and
hydrocarbons in sunlight; by electric sparks which occur in car engines and
electrical appliances (e.g. photocopiers and electric motors); and by lightning. It
can also be formed by passing a stream of oxygen through an electric discharge.
However, ozone is very reactive (oxidizing). So, it does not stay in the atmosphere
for a long time.
16. What is the formation of the ozone in the stratosphere?
Ans: In the stratosphere, ozone is formed from atmosphere oxygen by absorption
of ultraviolet radiation of the right energy (wavelength 250 nm), which
dissociates oxygen molecules into free oxygen atoms.
O2(g) → 2O(g)
The oxygen atoms produced react with other oxygen molecules to form
ozone.
O(g) + O(g) → O3(g)
17. What happen to ozone if it absorbs UV radiation of λ 215 to 295 nm?
Ans: When ozone absorbs UV radiation of λ 215 to 295 nm, it will undergoes
photodissociation and splits up again.
O3(g) → O2(g) + O.(g)
18. What is the significance of the above reactions?
Ans: This reaction responsible for the vital screening effect of ozone. It screens
off the harmful ultraviolet radiation.
19. What are the effects of ozone to human?
Ans: Ozone has a great impact on the respiratory system. Symptoms associated
with exposure to ozone include cough, chest pain, and throat and eye
irritation. Ozone can also increase our susceptibility to respiratory
infections.
20. What would happen if the ozone layer becomes thinner?
Ans: The thinning of ozone layer may result in an increase in the amount of
ultraviolet light reaching the Earth’s surface, leading to increase in the
incidence of skin cancer, genetic mutation, and a decrease in the yield of
crops.
21. What substances would cause a decrease of ozone in the atmosphere?
Ans: CFCs would cause a decrease of ozone in the atmosphere.
22. Using one of the substances in Q21, write down the relevant equations for
depletion of the ozone in the atmosphere.
Ans: CCl3F(g) →
CCl2F•(g) + Cl•(g)
CCl2F2(g) → CClF2•(g) + Cl•(g)
Cl•(g) + O3(g) → ClO•(g) + O2(g)
ClO•(g) + O3(g) → Cl•(g) + 2O2(g)
23. Give some alternatives for CFCs compounds.
Ans: Low ozone depletion potential, hydrocarbons such as butane and propane
are possible alternatives for CFCs.
Water Pollution:
24. What are the causes of water pollution?
Ans: The adverse effects of each of the water pollution on the environment
includes:
Water pollution is caused by livestock waste, oil spillages, residues of
pesticides, detergents in the sewage and industrial effluents.
25. What are the effects of water pollution?
Ans:
1. Livestock waste
When there is an excessive amount of organic matter discharged into water,
the existing micro-organisms will grow and multiply rapidly and oxygen is
rapidly depleted. Because of the lack of oxygen, anaerobic micro-organisms
begin to partially break down the organic matter, releasing noxious gases like
methane, ammonia and hydrogen sulphide. As a result, aquatic organisms are
endangered.
2. Oil spillages
Oil may clog the feathers of sea birds and prevent them from flying or
swimming. It also interferes with the insulation provided by the feathers, so
affected birds may die of cold or pneumonia. When birds try to clean their
feathers with their beaks =, they may be poisoned through the ingestion of oil.
Furthermore, oil floating on water prevents the dissolution of oxygen. Besides,
oil deposited von beaches may drive away holiday-makers and spoils
recreational resorts
3. Residues of pesticides
Pesticides are synthetic organic compounds used to eliminate flying and crawling
pests. On entry into water pathways, pesticides cause immediate toxic effects
on the aquatic life. They are non-biodegradable and may accumulate along
the food chain until a toxic level is reached. DDT is an organochlorine
insecticide widely used previously.
DDT has caused a lot of ecological damages because
(a)
It resists degradation and remains in the environment.
(b)
It is very soluble in fat, including the fat in milk and the body fat of the
animals, with the potential for damaging internal organs.
(c)
It affects the reproduction of birds, fish, etc, and weakens the shells of
eggs
(d)
Many species of insects can develop resistance to this insecticide.
4. Detergents in sewage
Sewage is domestic waste water. A major pollutant present in sewage is synthetic
detergent. Discharges of detergents causes eutrophication brought about by
phosphates found in some detergents. It also causes foaming in rivers and lakes.
Ingestion of detergents by aquatic organisms can cause death.
5. Industrial effluents
The toxic materials presents in the industrial effluents vary with the types of
industries involved. They may contain heavy metals, cyanides, polychlorinated
hydrocarbons, etc. Of particular concern in aquatic environments is the
bioaccumulation of heavy metals. This is a process by which the heavy metals
become more concentrated as they move up food chains.
26. What does DO stand for?
Ans: DO stands for Dissolved Oxygen.
27. What is the indication of the DO value?
Ans: If the level of dissolved oxygen falls below 5.0 mg dm³, fish start to die. If
the concentration of dissolved oxygen continues to fall, other marine
animals and aerobic bacteria will be unable to survive. In the complete
absence of dissolved oxygen, decomposition of organic matter continues but
it is taken over by non-oxygen requiring bacteria. The water begins to smell
unpleasant as different pathways of decomposition are proceeding.
28. What does BOD stand for?
Ans: BOD stands for Biochemical Oxygen Demand.
29. What does it mean by five-day BOD?
Ans: Five-day BOD is BOD determined in the laboratory by incubating a water
sample for five days at 20℃ and measuring the amount of oxygen
consumed.
30. What is the indication of the BOD value?
Ans: If the water has a high BOD, this implies that a lot of organic waste is
present and much oxygen is required to break down the waste. On the
other hand, a low BOD indicates the presence of only a small amount of
organic matter and there is little organic pollution. Natural clean water has
a BOD of about 1 to 4 mg dm³. If the BOD is greater than the amount of
oxygen replenished, then some fish and aquatic life suffocate.
Solid Waste:
31. What are the major causes of the solid waste?
Ans: In Hong Kong, solid waste is produced as a result of the activities of
domestic households, commercial operations, manufacturing industries, and
construction sites in the territory.
32. How we can dispose solid waste?
Ans: There are mainly two methods to dispose solid waste. They are by
landfilling and incineration. For landflling, which is the major disposal
method in Hong Kong, the waste is compressed and covered with soil at
landfill sites. And incineration, it is more expensive than landfilling, and the
solid waste are burnt off during incineration.
33. What are the advantages and disadvantages of each type of disposal methods
of solid waste?
Ans: Both landfilling and incineration have its advantages and disadvantages.
For landfilling. Lnadfill gas, mostly methane, and leachate, which is highly
polluting water percolating through decaying waste at landfill. The landfill
gas contains chemicals which are flammable and harmful.
For incineration, it is a more expensive method. However, the volume of
waste can be reduced by approximately 55% and the high temperature kills
disease-causing organisms. Moreover, it will not cause ground water pollution
and the heat produced can be used as a source of energy. The disadvantages
of it are due to its potential for polluting the atmosphere with toxic chemicals,
particularly metals, and the large amount of carbon dioxide produced causes
the grrenhouse effect on the globe.
34. What are the pollution problems associated with the disposal of plastics?
Ans: As plastics are chemically tailored for long-life uses, they do not generally
undergo decomposition at landfill sites and incineration of plastic waste
produces air pollutants such as hydrogen chloride from polyvinyl chloride
and other chlorine-containing polymers. The hydrogen chloride produced
can cause acid rain which damages the environment. Therefore, disposal of
plastics by landfilling and incineration causes environmental problems.
However, disposal of plastic waste directly in sea causes direct danger to
fishes. Small fish have been found dead with their digestive tracts clogged by
fragments of plastic foam ingested. Sea animals have been suffocated to
death by plastic bags.
35. How we can solve the pollution problems by solid waste?
Ans: We should use them lesser. And by 3Rs, we should also reuse, reduce and
recycle. For the plastic, degradable plastics should be developed. These
plastics included biopolymers, photodegradable plastics and synthetic
biodegradable plastics. Plastics can also be recycle, which included direct
recycling, recycling of energy and recycling of chemicals. In direct recycling,
it can only apply to thermoplastics. The plastics in the waste are separated,
cleaned, pulverized, and remoulded into new plastic items. In recycling of
energy, incineration of plastics to produce energy is recycling f the energy
content stored in the materials. The energy obtained from burning plastic
waste in incinerators can be used for heating or for generating electricity.
However, it may cause toxic gases, such as polychloroethene gives hydrogen
chloride gas when burnt. The method of recycling of chemicals is by
catalytic cracking. The large plastic molecules are broken down on the
surface of a hot silica catalyst to smaller, more useful molecules. This
process is known as pyrolysis, which is the thermal decomposition of a
material at a high temperature and in the absence of air. The products are
separated by fractional distillation and can be used to make plastics and
other chemicals.
Pollution Control in H.K.:
36. Which department in H.K. is responsible for the pollution control?
Ans: The Environmental Protection Department, in short EPD established in
1986 is responsible for the pollution control.
37. What are the measures to improve air quality? Describe each of them briefly
with the help of equation(s).
Ans: The air quality can be proved by using unleaded petrol, installation of
catalytic converters in car exhaust systems, limitation of sulphur content in
fuels, desulphurization of flue gas, installation of electrostatic precipitators
in power plants and installation of low nitrogen oxide burners in power
plants.
Firstly, during combustion of lead contained petrol, the lead compounds
react with other additives in petrol to form volatile lead halides which pass
out with the exhaust gases. Using unleaded petrol helps reducing these toxic
lead compounds which causes anaemia, and damages the nervous system
and the kidney. Moreover, infants are susceptible to lead poisoning.
Secondly, installation of catalytic converters in car exhaust systems can
reduce the content of nitrogen oxides, carbon monoxide and unburnt
hydrocarbons present in car exhaust. The catalytic converter, the carbon
monoxide is oxidized to carbon dioxide, unburnt hydrocarbons are oxidized
to carbon dioxide and water, and nitrogen monoxide is reduced to nitrogen.
The converter contains rhodium and platinum as catalysis. When car
exhaust containing carbon monoxide, unburnt hydrocarbons and nitrogen
oxides is passed through the converter, rhodium catalyzes the reduction of
nitroden monoxide. Platinum catalyzes the oxidation of carbon monoxide
and unburnt hydrocarbons such as heptane.
2NO(g) + 2CO(g) → N2(g) + 2CO2(g)
2CO(g) + O2(g) → 2CO2(g)
C7H16 (g)+ 11O2(g) → 7CO2(g) + 8H2O(g)
Thirdly, by limiting the sulphur content in fuels can reduce the sulphur
dioxide emission from factories and power plants in which
sulphur-containing fossil fuels are burnt. Fourthly, as sulphur dioxide is
released during the combustion of coal which contains sulphur, using coal of
low sulphur content, the quantity of sulphur dioxide released is much
reduced. The desulphurization of flue gas utilizes the “wet
limestone-gypsum scrubbing process” which is capable of removing up to
90% of the sulphur dioxide emitted from the boiler plant when burning
coals with a wide range of sulphur contents. Flue gas from the boiler is
directed to the absorber which contains a recycled slurry of limestone for
removal of sulphur dioxide. The calcium sulphate(IV) formed is oxidized to
calcium sulphate(VI) by the air blown into the bottom part of the absorber.
The chemical reactions of the absorption and oxidation processes can be
summarized as:
SO2(g) + H2O(l) → H2SO3(g)
Absorption
CaCO3(s) + H2SO3(aq) → CaSO3(s) + CO2(g) +H2O(l) Neutralization
CaSO3(s) +1/2 O2(g) → CaSO4(s) Oxidation
CaSO3 + 1/2 H2O(l) → CaSO3‧ 1/2H2O(aq) Crystallization
CaSO4(s) + 2H2O(l) → CaSO4‧ 2H2O(aq) Crystallization
Fifthly, as coal contains a certain proportion of ash which cannot be burnt,
installing electrostatic precipitators reduced the ash to clean the flue gas
before its release into the atmosphere. In electrostatic precipitation, flue gas
from the boiler is passed through a chamber where particulates become
charged. The charged particles finally get attracted on the collecting wall.
The charges on the particles are neutralized on the wall where they are
deposited and removed. The cleaned flue gas is then discharged through the
chimney. Finally installation of low nitrogen oxide burners in power plants
lowering the content of nitrogen dioxide. Nitrogen oxides (NOx) usually
include nitrogen monoxide and nitrogen dioxide by combustion. Nitrogen in
the air reacts with oxygen at a high temperature near the combustion
chamber to form nitrogen monoxide in a series of complex reactions:
N2(g) + O2(g) → 2NO(g)
When released into the atmosphere, it reacts with oxygen to form nitrogen
dioxide.
2NO(g) + O2(g) → 2NO2(g)
Another way to form nitrogen dioxide is that fossil fuels may contain
nitrogen compounds. When these fuels are burnt, the nitrogen compounds
are oxidized to nitrogen oxides, which are known as fuel nitrogen oxides.
The low nitrogen oxides burner system utilizes a combustion control method
in which the operating conditions for combustion are modified to reduce
nitrogen oxides formation. These include a reduction in the flame
temperature in the combustion zone and a reduction in the availability of
oxygen in the nitrogen oxides formation zone. Reductions in the production
of nitrogen oxides up to 50% have been demonstrated in these systems.
39. What does CWTC stand for?
Ans: CWTC stands for Chemical Waste Treatment Centre.
40. What is the mission for the CWTC? Explain briefly.
Ans: The mission for the CWTC is to provide proper waste treatment services
for local waste producers. There are three major waste treatment systems in
the Centre, ranging from oil/water separation system, physical/chemical
treatment system and incineration. For oil/water separation system deals
with oil/water mixtures. It recovers waste oil from oily water mixture and
separation of water from all residue formed in neutralization and
precipitation processes. The physical/chemical treatment system treats
water-based chemical waste such as acids, alkalis, and wastewater
containing heavy metal salts by neutralization of acids and bases,
precipitation reactions and reduction reactions. Incineration system is used
for destructing hazardous organic waste such as pesticides and cyanide
waste by decomposing them to carbon dioxide, water and various gases.
41. What does it mean by 3Rs? Explain briefly.
Ans: 3Rs refers to reduce, reuse and recycle. It is introduced by the HKSAR to
arise the public environmental awareness and promote waste avoidance,
minimization, recovery for reuse and recycling. For example, recycling bins
for plastic bottles, aluminium cans and waste paper. For reducing, reusing
or recycling of plastics, we have to reduce our uses of plastic bag after
shopping in the supermarket. For the reusing and recycling of paper, we
have to use the recycled paper produced by local recyclers is used to make
corrugated paper and duplex papercard for packaging. Those computer
printout waste papers, old corrugated cardboard, newspaper and other
mixed paper, have to be collected for recycling. The metal articles such as
furniture, kitchen ware and aluminium foils can by reused. Metal waste can
be recycled by smelting used metals and using them again. For example,
ferrous metal waste is processed in a local foundry to produce reinforcing
bars for use in the construction industry. By recycling, reuse and reduce, not
only the pollution problem of solid waste can be solved, it can also save fuels,
energy, and other chemicals in the solid.
Chemistry and Society : Unit 47 : Chemistry and Food:
1. What is the product formed when protein undergoes hygrolysis?
Ans: constituent amino acid
2. What are the classification of carbohydrates?
Ans: monosaccharide, disaccharide, polysaccharide
3. What is the general formula for carbohydrates?
Ans: (C6H10O5)n
4. What does it mean by the acyclic and cyclic forms of glucose molecule?What
does it mean by glycosidic linkage in carbohydrates?
Ans: The acyclic form of glucose molecule is an aldohexane as it contains an
aldehyde group in its acyclic structure. The cyclic forms of glucose are
dissolved in water, either of them will be converted to the other form, until
an equilibrium mixture is formed, together with a minute amount of the
acyclic form.
5. Illustrate by an example.
Ans: the bond formed between two monosaccharide is called a glycosidic linkage.
It is formed from the condensation reaction between two –OH groups of two
monosaccharides with the elimination of a H2O molecule. For example, a
maltose molecule is built up from two-glucose molecules with the
elimination of a H2O molecule.
6. What is/are the factor(s) affect the hydrolysis of sucrose and starch?
Ans: Sucrose: hydrolyzed by dilute mineral acids or catalyzed by maltase
Starch: catalyzed by enzymes (amylase) or boiled with dilute sulphuric(VI)
acid.
7. What are the basic structural units for the fat?
Ans: H
O
∣
‖
H—C—O—C—R1
O
∣
‖
H—C—O—C—R2
O
∣
‖
H—C—O—C—R3
∣
H
8. Write down a general equations for the formation of soap by using natural
fat.
Ans: H
O
∣
‖
H—C—O—C—R1
O
∣
‖
H—C—O—C—R2
∣
O
‖
H—C—O—C—R3
∣
H
+ 3NaOH 
H
∣
H—C—OH
∣
+
H—C—OH
∣
H—C—OH
∣
H
O
‖
R1—C—O-Na+
O
‖
R2—C—O-Na+
O
‖
R3—C—O-Na+
9. Define the term “iodine value”?
Ans: Its determination is based on the reaction between iodine and the C=C
bonds in fats or oils. The iodine value of a fat or an oil is defined as the
number grams of iodine that reacts with 100 grams of the fat or oil.
10. What is the indication of the iodine value?
Ans: The higher the iodine value, the greater is the degree of unsaturation in the
fat or oil.
11. What does it by hardening of vegetable oil?
Ans: The C=C double bonds in these unsaturated oils may be converted to single
bonds by the addition of hydrogen in the presence of a catalyst. The process
is called hydrogenation. Hydrogenation of some of the carbon-carbon
double bonds in these bonds in these oils thus converts them into solid fats.
12. What does it mean by rancidity? How many types are there? Describe each of
them with the help of equations.
Ans: Fats and oils are liable to spoilage which results in the production of an ‘off’
odour and a flavour described as rancidity. It is mainly caused by hydrolytic
or oxidative reactions which release foul smelling aldehydes and fatty acids.
Hydrolytic rancidity is due to the presence of moisture in fats and oils,
which causes the hydrolysis of the glyceride molecules to propane-1,2,3 triol
and free carboxylic acids.
Food Preservation:
13. What are the causes of food spoilage?
Ans: Food spoilage is due to the activities of micro-organisms (bacteria, moulds
and yeasts) and, to a lesser extent, chemical reactions which are often
catalyzed by enzymes.
14. Give some principle of food preservation.
Ans: The food preservation is commonly by slowed down the deterioration in
quality, and to increase the food’s shelf-life. The principle is that microbial
or chemical changes resulting in food spoilage are suppressed, such as
removal of moisture inside food, altering temperature, changing pH, use of
osmotic process and use of chemical additives. Both removal of moisture
and use of osmotic process are useful as their principle is by removing the
water inside food since the presence of water is one of the necessary
conditions for microbial growth in food. Using altering temperature is by
killing the bacteria and micro-organisms under high temperature or by
retarding the growth of micro-organisms under low temperature. As most
micro-organisms prefer a neutral or slightly acidic medium for growth.
Increasing or decreasing the pH value creates an unfavorable condition for
their growth. Finally, adding chemicals can inhibit or slow down microbial
activities in food.
15. Give some common techniques of food preservation. Briefly describe the
principle behind each of them.
Ans: There are several methods in preserving foods. The most common
techniques of them involve the killing of micro-organisms, inhibition of
microbial growth, or the retardation of chemical changes. Some common
techniques of food preservation used include heat treatment, drying and
dehydration, irradiation, refrigeration, sugaring, salting, canning and some
chemical techniques. The heat treatment kills micro-organisms, alters
protein structures, and destroys enzymatic activities of micro-organisms in
food during the process. All forms of cooked food are preserved by this
method. Another method is by irradiation. During this process, food to be
preserved can be exposed to ionizing radiation. The radiation destroys
enzymes and micro-organisms in food. It is very common in preserving
fruits. The drying and dehydration is the oldest practice of food
preservation. Water is extracted and the absent of water makes chemical
reactions cannot occur and micro-organisms cannot grow. The refrigeration
method involves chilling and freezing. The chilling slows down microbial
and enzymatic activities, as well as chemical and biochemical changes, so
food spoilage can be avoided. And in freezing, the temperature of food is
reduced to about -18C oC. Freezing turns the water present in the food to ice
and no water is provided anymore. Moreover, sugaring and salting both
treated the food with concentrated solution in order to remove the water in
food by osmosis to the concentrated solution. Hence, as no water is available
longer for enzymatic action and microbial growth. In canning, food is
cooked under pressure in hermetically sealed containers. Cooking destroys
enzymes and micro-organisms in food. As the cans are sealed, there is no air
and no micro-organisms can get in. Meat-curing is done by adding salt to
meat or treating the meat with concentrated salt solutions containing
nitrates(V) and nitrates(III). The salt ties up the water in food so that
micro-organisms cannot multiply and grow, and chemicals such as nitrates
(III) prohibit their growth. Picking in vinegar can be another effective
method as microbial growth is inhibited in acidic solutions. Finally, use of
food additives can control the growth of micro-organisms and chemical
spoilage are added to food. The added substances inactivate or kill
micro-organisms, retard chemical spoilage, but the considered relatively
safe to humans.
16. What are the functions of BHA / BHT?
Ans: The butylated hydroxyanisole, in short BHA and butylated hydroxytoluene
(BHT) are antioxidant used in food to retard the development of oxidative
rancidity in unsaturated fats and oils. They appear to work by donating the
H atom of the ─OH group to the free hydroperoxide radical (ROO ●)
involved in the autoxidation of fat and oils, thereby stopping the chain
reactions in oxidative spoilage:
AH + ROO ● → ROOH + A ● where AH represents the antioxidant, and A ●
is a radical derived from the antioxidant. These antioxidants are added to
vegetable oils, shortening and products that contain unsaturated oils and
fats.
17. What are the possible menace of using too much food additives?
Ans: Too much food additives cause allergies, hyperactivity and other long-term
illnesses. Some people are allergic to certain additives. It is suspected that
MSG and tartrazine may cause rashes, stomach upsets and asthma. And
food additives was suggested to the diseases hyperactivity and attention
deficient disease (ADD) in children. Some other additives, such as sodium
nitrate (III), are suspected to be carcinogens. Some additives are believed to
be the causes of some long-term illnesses. Small amount of them can prevent
cancer, however, larger amounts of it can cause cancer. When sulphur
dioxide is poisonous and when it is ingested, sulphur dioxide salts attack the
respiratory system if it is used as food additive.
18. What is the use of the monosodium glutamate (MGS) in food processing?
Ans: Monosodium glutamate (MGS) does not add any taste to food but it is a
flavour enhancer. It is the monosodium salt of glutamic acid which is an amino
acid. It is found naturally in many fresh meats and vegetables such as
mushrooms, peas and tomatoes. It enhances the flavour of food and makes the
food taste better. One explanation of how MSG works is that MSG stimulates
glutamate receptors in the tongue and thus enhances meat-like flavours. Another
explanation is that MSG increases the sensitivity of our taste buds and stimulates
the flow of saliva, which starts the breakdown of food, thus releasing its flavour.
19. What are the side effects of MSG?
Ans: MSG may cause some possible symptoms and side effects. These include
burning sensation, thirst, headache, chest pain, vomiting and abdominal
discomfort. The side effects are more prominent in some individuals than others
and are dose-related.
20. What is the use of nitrate (Ⅲ)/ nitrites or nitrate (Ⅴ) in food processing?
Ans: Nitrates (Ⅲ) and nitrates (Ⅴ) are often used as preservatives in many cured
and canned meats. In the meats, nitrates(Ⅴ) are reduced to nitrates(Ⅲ). The
nitrates (Ⅲ) are actually responsible for developing the cured aroma and
fresh pink colour in meats and they are also for inhibiting bacterial growth.
Nitrates (Ⅲ) are particularly effective in preventing botulism.
21. What are the side effects of nitate(Ⅲ)/ nitrites of nitrate(Ⅴ)?
Ans: Excessive ingestion of nitrates (Ⅲ) causes decreased levels of haemoglobin
in the blood. Long-term ingestion may even lead to malnutrition, growth
retardation, impairment of reproductive capacity and reduced lifespan.
Nitrates(Ⅴ) are fairly harmless, but dietary nitrates(Ⅴ) ingested from
vegetables and meats are also reduced to toxic nitrates(Ⅲ) in our body.
Nitrates(Ⅲ) in the body may give rise to the formation of some
carcinogenic nitrosamines. In the stomach nitrates(Ⅲ) are first converted
to nitric(Ⅴ) acid.
NaNO2 (aq) + HCl (aq) → HNO2 (aq) + NaCl (aq)
Stomach juice
Then, nitric(Ⅲ) acid can react with secondary amines, which are released
during digestion of proteins, to form carcinogenic nitrosamines under
certain conditions.
HNO2 (aq) + R2NH (aq) → R2N-N=O (aq) + H2O (l)
Secondary amine
nitrosamine
22. What is the use of SO2 in fruit juice?
Ans: The food additives, sulphur dioxide, is a collective term for sulphur dioxide
gas and its salts, namely sulphates (Ⅳ), hyhdrogen-sulphates(Ⅳ) and
disulphates(Ⅳ). Both sulphur dioxide and sulphates(Ⅳ) help maintain the
colour and the vitamin C of the fruit juice. They are used as bleaches and
antioxidants to prevent browning in fruit juices and also other foods, such as
alcoholic beverages, dried fruits and vegetables.
23. What are the side effects of SO2 in fruit juice?
Ans: SO2 is poisonous. When sulphur dioxide and its salts are ingested, they
attack the respiratory system. Individuals who suffer from chronic
respiratory diseases such as bronchitis of asthma are much more susceptible
to the attack. Therefore, the daily intake of SO2for humans should not
exceed 0.7 mg per kg body weight.
24. What is the use of saccharin?
Ans: Saccharin is an organic compound about 500 times sweeter than ordinary
sugar. Although it has no food value, it is used as a sweetener due to its
intense sweetness. It is used as a sugar substitute and diet sugar, especially
for patients of diabetes, since it has a low calorific value.
25. What are side effects of saccharin?
Ans: According to some studies, high doses of saccharin in second generation
rats are linked to bladder cancer. Consequently, saccharin has been banned
as and additive in Canada and it is allowed to be sold in pharmacies as a
table-top sweetener only. Nevertheless, whether or not saccharin is
carcinogenic in humans is still an open question.
26. What is the use of E number in food additive?
Ans: E number is a coding system that is used on food manufactured in
European Union (Eu) countries. The letter E tells us that the addictive is
approved by Eu and the first digit tells us what class of additive it is. The
following table shows the E numbers of some food additives.
Type of additive
E number
Colourings
Most begin with 1
Preservatives
Most begin with 2
Flavourings
Not numbered
Antioxidants
E300-321
Emulsifiers and stabilizers
E322 and some numbers between
E400 and E495
Acids, bases and buffers
Most begin with 5
Sweeteners
Most begin with 4 or 6
27. Are food additives being used extensively, and why?
Ans: Yes, food additives are being used extensively due to their uses. They are
present in food as a result of any aspect of production, processing, treatment,
packaging, transportation or storage. Most additives are added intentionally
so that food can last longer and the original characteristics of the food are
retained. This plays a key role in maintaining high qualities and constancy
of characteristics demanded by consumers nowadays.
28. To what extent do food additives threaten our health and general well-being?
Ans: Food additives threaten our health and general well-being to certain extant.
Although food additives seem to be a necessary part of the modern society, it
cannot be denied that there are hazards associated with their uses. One of
the possible harmful effects of food additives to our health is allergies. Some
people are allergic to certain additives and it is suspected that MSG and
tartrazine may cause rashes, asthma and stomach upsets. Moreover, it was
suggested by some scientists in the 1970s that food additives may be related
to hyperactivity and attention deficient disease (ADD) in children.
Furthermore, some additives, such as sodium nitrate(Ⅲ), are suspected to be
the causes of some ling-term illnesses.
29. Are there effective solutions to these problems?
Ans: Yes, there are some solutions to these problems. The use of food additives
can be monitored by research and legislation. Also, we should not take more
than the maximum amount of individual additives which are suspected to be
harmful suggested by scientists. As a result, our health and general
well-being will not be threatened by the food additives.
30. What is being done to tackle these problems?
Ans: The use of food additives is monitoring by research and legislation to tackle
the problems. Substances, such as salt, sugar, vitamins and some minerals,
which have long been used as additives to food, are generally recognized by
experts as safe to use, based on their extensive history or on published
scientific evidence. Apart form them, no food additive may be used unless
it has been granted approval. Studies using animals to show the effects
caused by the intake of certain food additives are often carried out at
expected levels of human consumption. The data obtained are then
compiled and lists of permitted additives are published. In the mean time,
scientists are continuing to study the potent effects of the additives on
humans, while watching out for any adverse ones.
Moreover, the use of food additives is governed by legislation in many
countries. In the USA, the Food and Drug Administration is responsible for
monitoring the safety, purity, and wholesomeness of food and food
additives. In the United Kingdom, the Food Advisory Committee compiles
lists of food additives permitted by the government. For countries in the
European Union (Eu), a permitted food additive is assigned a E number.
In Hong Kong, the department of Health is responsible for the monitoring
and legislation of food additives. The department is empowered under the
Public Health and Municipal Services Ordinance to:
(i) legislate permissible additives and maximum limits of additives in
particular food;
(ii) require supply of information on the composition of substances used in
the preparation of food;
(iii) check food labelling;
(iv) inspect food-processing industries;
(v) take samples of foods and food additives for chemical analysis;
(vi) seize and destroy food;
(vii) prosecute for false labeling or advertisement, using non-permitted
additives or using additives beyond the permissible limits.
31. Can we, or can we not, do away completely with food additives?
Ans: No, we cannot do away completely with food additives. If food additives are
not used, much of our food will spoil and many people will die of starvation.
Besides, diseases due to vitamin and mineral deficiencies may flourish.
Food additives seem to be a necessary part of the modern society.
32. Should all food additives be banned by legislation?
Ans: No, not all food additives should be banned by legislation, but only the toxic
and harmful food additives should be banned by legislation. It is because
food additives are useful and they play an important role in our daily life.
They have been used for centuries in preserving and improving the
appearance and colour of food. For example, salt, sugar and vinegar are
used to preserve meat, fruits and vegetables respectively; herbs and spices
are added to improve the flavour of food. Moreover, food additives are
widely used in the food processing industry. They play a vital role in
reducing diseases due to deficiency in nutrients. Besides, additives help
assure the availability of wholesome, appetizing and affordable foods that
meet consumer demands from season to season.