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Unit 31
Detergents
Suggested answers to in-text activities
Internet Search and Presentation (page 210)
Historical development of detergents
Soap making is one of the oldest known organic chemical reactions. Soaps are formed by the
reaction of fats or oils with an alkali. It is possible that the process could have been discovered in
prehistoric times when animal fat from cooking meat dripped onto wood ash (which is alkaline)
producing a crude soap. Archaeologists have found evidence that the Babylonians were making
soap around 3000 BC. Soap making was probably introduced to Europe by the Phoenicians
around 600 BC.
The Romans also produced soap and the word soap in many languages is derived from a
famous Roman legend. According to this legend women who washed clothes in the stream
below Sapo Hill noticed how much easier they were to clean than in other streams. It seemed
that the ashes and fats from sacrificial fires in temples on Sapo Hill mixed together to produce
soap which washed down from the hill. A soap-making factory, complete with soap moulds and
bars of soap, was discovered in the ruins of Pompeii. With the fall of the Roman Empire, soap
making declined and soap was used mostly for cleaning clothes and textiles rather than for
personal bathing. Soap making made a revival around the 8th century in Italy and Spain where
olive oil rather than animal fats were used to produce a higher quality soap more suitable for
bathing and washing. Throughout this period wood ash or even seaweed ash were the main
source of alkali used in the process of soap making.
During the Second World War, the shortage of fats and oils stimulated the development of
soapless detergents. After the war, the need for new types of detergents for automatic washing
machines accelerated the trend.
Discussion (page 214)
Tasks for student
Before the class
1
View the animation.
2
Check the meaning of scientific terms on the Worksheet from a scientific dictionary or other
sources.
3
Write down three questions they would ask their classmates regarding the emulsification
process.
During and after the class
4
Discuss questions they have prepared in a group, among groups and finally in the whole
class.
5
View the animation again and write a short essay describing the emulsification process.
Notes for teacher
Students may ask questions regarding
 the emulsification process;
 the structure of a detergent particle;
 the cleaning action of detergent.
The essay should include the following ideas:
 A detergent particle consists of an ionic head and a hydrocarbon tail.
 The ionic head is hydrophilic and the hydrocarbon tail is hydrophobic.

The hydrophobic tails of detergent particles dissolve in the grease and the hydrophilic ionic
heads remain in the water (outside).

Upon stirring, the greasy dirt breaks up into tiny oil droplets suspended in the water and thus
leaves the surface of the cloth.

The ionic heads bear like charges and repel each other, thus preventing the small oil droplets
from coming together again.
Worksheet - Emulsification process of a detergent
1
You are going to view an animation regarding the emulsification process of a detergent
when acting on greasy dirt.
2
Search, from a scientific dictionary or other sources, for the meaning of the following terms:
Soap ________________________________________________________
Detergent ____________________________________________________
Hydrophilic ___________________________________________________
Hydrophobic __________________________________________________
Emulsification _________________________________________________
3
Write down three questions you would ask your classmates regarding the emulsification
process.
Q.1 ________________________________________________________
________________________________________________________
Q.2 ________________________________________________________
________________________________________________________
Q.3 ________________________________________________________
________________________________________________________
4
Write a short essay about the emulsification process of a detergent when acting on greasy
dirt. Your target reader is a friend who has not viewed the animation and has no idea about
the structure of a detergent particle.
________________________________________________________
________________________________________________________
________________________________________________________
________________________________________________________
________________________________________________________
________________________________________________________
________________________________________________________
Internet Search and Presentation (page 219)
Environmental problems associated with the use of detergents
What does biodegradable mean?
The term biodegradable means the ability of a material to be broken down into its simplest
chemicals such as water, carbon dioxide and mineral salts by organisms called decomposers.
Although people may believe they are doing the right thing by using biodegradable
detergent, it is important to remember that these detergents can harm the environment too and
that waste water should never enter waterways.
Bacteria use up oxygen in the water during their decomposition of detergents. This makes
the water of some streams, rivers and seas smells badly due to oxygen depletion.
Phosphate-containing detergents
When phosphate detergents are used, disposal of the waste water is an environmental issue.
The breakdown of the phosphorus complexes in water containing phosphate detergents creates
biologically available phosphates. In waterways these can contribute to an oversupply of
phosphates. Low concentrations of plant nutrients, such as nitrogen and phosphorus, can limit
plant and algae growth. When phosphates from the sources mentioned above are introduced to
waterways, any phosphorus-limitation may be removed and may cause excessive algae growth.
This can lead to:

increased numbers of insects, crustaceans and fish in the waterway. When these and the
algae die and decompose, oxygen in the water can be reduced to such low levels that other
aquatic organisms may die;

the release of toxins by some cyanobacteria (blue-green algae) that can kill other organisms
and make the water unsuitable for humans, livestock and wildlife.
Phosphate-free detergents
Disposal of phosphate-free detergent waste water is also an environmental issue. Phosphates
are primarily used in detergents as ‘builders’ which remove calcium ions, and allow cleaning
agents to work more effectively. In phosphate-free detergents, the phosphates are replaced by
either:

alternative builders such as zeolites; or

alternative builders and an increased concentration of cleaning agents.
The problem with this is that cleaning agents are among the most toxic compounds in
detergents and have been implicated in decreasing the ability of aquatic organisms to breed. To
minimize the environmental harm caused by phosphate-free detergents, the waste water must
be treated in sewage treatment plants. Unfortunately, even after treatment the effects of some
alternative builders remain.
Check Your Understanding (page 222)
1
2

Soapless detergents do not form scum in hard water. Their cleaning action is not affect
by the hardness of water.

The production of soaps requires oils or fats. Soapless detergents are produced from
hydrocarbons obtained from petroleum. Vegetable oils and animal fats are more
expensive than chemicals for making soapless detergents. The natural fats and oils can
be saved and used as food.
a)
Detergent I
It is made from natural fat or oil.
b)
c)
i)
A scum would form.
ii)
Lather would form.
Detergent II
It is non-biodegradable.
Suggested answers to exercise
1
a)
B
b)
A
2
3
4

Bacteria use up oxygen in the water during the decomposition of detergent. This makes
the water of some streams, rivers and seas smells badly due to oxygen depletion.

The detergents often contain phosphate salts as ‘builders’ When the phosphates go
into rivers and seas, they become the nutrients of the algae. As a result, algae will grow
suddenly. Sometimes algae may grow so fast that they form a thick layer on the water
surface. Light cannot enter the deeper level of the water. As a result, plants underneath
cannot carry out photosynthesis and die.
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Option B - The ionic head of a soap particle is usually a
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group.
Option C - Soaps are made from fats or oils.
Option D - Soaps have been used for four and a half thousand years. The ancient
Egyptians may have used natural deposits of soda ash (sodium carbonate),
spices and animal or vegetable oils to produce perfumed soaps.
Soapmaking was common in Italy and Spain during the 8th century.
In 1791, a French chemist found a way to produce soda ash from sodium
chloride. This led to large-scale commercial soapmaking and soaps became
generally affordable.
During the Second World War, the shortage of fats and oils stimulated
the development of soapless detergents. After the war, the need for new
types of detergents for automatic washing machines accelerated the trend.
8
D
When fats or oils are heated with an alkali, they are hydrolysed first to form glycerol and
alkanoic acids. The acids then react with the alkali (sodium hydroxide or potassium
hydroxide) to form sodium or potassium salts which are soaps.
9
B
10 C
11 C
12 B
13 C
14 A
15 When a detergent solution and water are added to a table cloth, the ‘tails’ of the detergent
particles dissolve in the grease. The ‘heads’ of the detergent particles are insoluble in the
grease and remain outside. The surrounding water molecules attract the ‘heads’ and lift
the grease off the surface.
By stirring, the grease (which carries the dirt particles) breaks up into tiny droplets
suspended in the water. These tiny droplets cannot come together again due to the
repulsion between ‘heads’ of detergent particles on their surfaces. An emulsion forms.
Rinsing washes away the greasy suspension, and leaves the surface clean.
16 a)
b)
Triester
i)
Saponification
ii)

Pour 25 cm3 of concentrated sodium hydroxide solution into a small beaker.

Add 2 cm3 of castor oil.
 With a small Bunsen flame, gently heat the mixture for about 20 minutes. Stir
the contents with a glass rod.
 When the oily layer has disappeared, add about 25 cm 3 of brine. Continue
stirring.

Continue to heat gently for 2 - 3 minutes. Then allow to cool.

Filter the mixture.

17 a)
b)
c)
sh the residue (soap) 2 or 3 times with a little distilled water. Allow to dry.
Detergents I and II
i)
Detergent II
ii)
It is non-biodegradable.
Scum would form.
18 19 20 The formation of lather is a useful way of measuring the cleaning ability of a detergent. The
cleaning abilities of soaps and soapless detergents in hard and soft water can be compared
by measuring their lather forming abilities in the water.
First measure 20 cm3 of each type of water, hard water and soft water, into two
separate conical flasks. Then put some soap solution in a measuring cylinder. Add soap
solution using a dropper, 1 cm3 at a time, to each of the two flasks. Cork and shake the flask
after each addition of soap solution. Add soap solution until a permanent lather forms.
Record the volume of soap solution required to give a permanent lather in each case.
Repeat the experiment using soapless detergent solution instead of soap solution.
Record the volume of soapless detergent solution required to give a permanent lather in
each case.
A lot of soap solution is required to give a permanent lather in hard water while only a
little soapless detergent solution gives a permanent lather. A little soap solution or soapless
detergent solution gives a permanent lather in soft water. Both soaps and soapless
detergents are good cleaners in soft water. However, a very important advantage of
soapless detergents is that they are good cleaners in hard water as well while soaps are not.