Activity Book

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Activity Book
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11.1
Detection of food substances by food tests
Questions
1 Why is a boiling water bath preferred to a
Bunsen flame for heating up the mixture of
glucose solution and Benedicts solution?
The water bath has a better control over the
temperature and can prevent bumping of the
mixture.
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11.1
Detection of food substances by food tests
2 What is the purpose of standing the test tube
of boiled mixture in a test tube rack in the
Benedicts test?
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To allow the brick-red precipitate to settle.
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11.1
Detection of food substances by food tests
3 How does the translucent spot caused by fats
differ from the one caused by water?
The translucent spot caused by fats is permanent.
On the contrary, the translucent spot caused by
water disappears as water evaporates.
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11.1
Detection of food substances by food tests
4 Explain the difference in results after
immersing the filter paper with the translucent
spots into an organic solvent.
Fats but not proteins dissolve in organic
solvent. Thus, the translucent spot caused by
fats disappear whereas the one caused by proteins
remains on the filter paper.
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11.1
Detection of food substances by food tests
5a Explain the difference in effects of vitamin
C solution and boiled vitamin C solution on
DCPIP solution.
Boiling destroys the reducing property of vitamin
C. Thus, only the vitamin C solution that has not
been boiled can reduce the blue DCPIP solution
and decolourize it.
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11.1
Detection of food substances by food tests
5b Based on your answer in a, how can we avoid
losing vitamin C in fruits and vegetables?
Do not boil fruits or overheat vegetables.
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11.1
Detection of food substances by food tests
6 Is DCPIP solution suitable for detecting the
presence of vitamin C in a sample containing
glucose? Why?
No. Glucose, like vitamin C, is reducing and will
decolourize DCPIP solution no matter vitamin C is
present in the sample or not.
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11.1
Detection of food substances by food tests
7 Do the amounts of the samples used in the food
tests need to be measured very accurately?
No. This is because these food tests are
qualitative tests for showing the presence of
certain food substances. They are not
quantitative tests.
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11.1
Detection of food substances by food tests
8 How should you perform Benedicts test on a
blood sample?
As the red colour of the blood will mask the
results of the Benedicts test, the blood sample
should first be diluted with distilled water.
Alternatively, the blood sample should be
centrifuged and the plasma collected for the
Benedicts test.
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11.2
Investigation of the food substances present in
common foods
Questions
1 Which of the food samples contain the most and
the least different types of food substances
respectively?
(Answer depends on the types of foods tested.)
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
Design and perform an experiment
1 Do you need to measure the volume of DCPIP
solution accurately in this experiment? Explain.
Yes. Accurate measurement is necessary because a
comparison of vitamin C content in different
fruits and vegetables is needed.
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
2 What are the controlled variables in this
experiment?
Amount of each type of fruits / vegetables used
to extract the juices, volume and concentration
of DCPIP solution used with each sample,
temperature of sample and DCPIP solution, etc.
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
3 What is the independent variable in this
experiment?
The type of fruits and vegetables.
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4 What is the dependent variable in this
experiment?
The number of drops of sample needed to
decolourize 1 cm3 of DCPIP solution.
ANS ?
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
5 What is the control in this experiment?
Distilled water.
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6 How are you going to record and present the
results?
(Answer varies with Ss.)
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Next
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
7 How can you obtain faster results?
Use less DCPIP solution or a more dilute DCPIP
solution.
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
8 How can you make sure that the results are
reliable?
Take any dilution factor of the juices into
consideration in the comparison of vitamin C
content. Repeat the experiment with more samples
from the same types of fruits and vegetables.
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
9 What precautions should be taken?
Use freshly prepared juices in the experiment.
Otherwise, vitamin C will be oxidized if the
juices are allowed to stand in the air for a long
time. Do not use hot water to extract the juices
from the fruits and vegetables. (To be
continued.)
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
continued from previous page
Otherwise, the high temperature will lower the
reducing power of any vitamin C in the samples.
When mixing the samples and the DCPIP solution,
do not shake the test tube too much. (To be
continued.)
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
continued from previous page
Otherwise, oxygen in the air bubbles produced by
shaking will lower the reducing power of any
vitamin C in the samples. Rinse the dropper
thoroughly for each sample.
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
Write an experimental report
Objective
To compare the vitamin C content in different
fruits and vegetables.
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
Results
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
Analysis and discussion
1 Which property of vitamin C causes the
decolourization of the DCPIP solution?
Reducing property.
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
2 Explain what may happen to the vitamin C
content if
a fresh fruit or vegetable juice is left in the
air for a week.
The vitamin C content will decrease as vitamin C
is oxidized by the air.
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
b fresh fruit or vegetable juice is boiled.
The vitamin C content will decrease as vitamin C
is unstable at high temperature.
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
3 What are the possible sources of errors in this
experiment? Suggest ways for improvement.
Oxidation of vitamin C in the juices. Work
quickly with the freshly prepared juices. (To be
continued.)
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
continued from previous page
The observation of complete decolourization of
DCPIP solution is subjective, especially when the
juices are coloured. There are other reducing
substances, e.g. glucose and fructose, in the
juices.
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
4 What is the limitation of using DCPIP solution
to compare the vitamin C content in different
fruits or vegetables?
Fruits or vegetables that have juices very dark
in colour cannot be used. The dark colour of the
juices masks the decolourization of DCPIP
solution.
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11.3
Design an investigation to compare the vitamin C
content in different fruits and vegetables
Conclusion
Vitamin C is present in the fruits and vegetables
tested. Among them, the green pepper has the
highest vitamin C content, followed by the lemon
and the orange.
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Exercise 11
Multiple-choice questions
D
C
B
D
B
D
C
C
D
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Exercise 11
Structured questions
1a i Give two uses of calcium to the body.
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For growth of teeth and bones / blood clotting /
muscle contraction (Any 2, 1m each)
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Exercise 11
1a ii If the daily requirement of calcium for a
3-year-old child is 600 mg, how much of this food
has to be taken to meet the need?
600/160 X 100 375 g (1m)
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Exercise 11
1a iii Name another food that is rich in calcium.
Milk / cheese / vegetables (Any other correct
answer) (Any 1, 1m)
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1b i Which nutrient on the food label cannot be
stored in the body when in excess?
Protein (1m)
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Exercise 11
1b ii What is the fate of the excess nutrient
stated in bi?
Excess amino acids will be broken down by the
liver through deamination (1m). The amino group
is changed into urea which is then excreted out
in urine (1m).
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Exercise 11
1c The words Extra vitamin C added are
highlighted in the package of the yoghurt. Name
the test that can be used to test for the
presence of vitamin C. State the result if the
test is performed on the yogurt.
Using DCPIP solution (1m). If the test is
performed on the yoghurt, the blue colour of the
DCPIP solution (1m) will be decolourized (1m).
ANS ?
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Exercise 11
2a Calculate the daily energy requirement per kg
of body weight for each person. Show your steps.
Energy requirement Pregnant woman 12 831/70
183.3 kJ kg-1 (1m) Housewife 8 400/55
152.7 kJ kg-1 (1m) Waitress 13 400/54 248.1
kJ kg-1 (1m)
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Exercise 11
2b Why does a pregnant woman need more energy and
protein than a housewife?
It is because the foetus (1m) of the pregnant
woman needs energy and protein for making new
tissues (1m).
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Exercise 11
2c The waitress wants to cut down her body
weight. Explain why the following methods are not
suitable for her.
i drink laxative tea
It may cause excessive loss of water (1m) and
affect the normal functioning of various organs
(1m).
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Exercise 11
ii forced vomiting after a meal
The acidic gastric juice (1m) may damage the
teeth / oesophagus / throat (1m).
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iii on diet of fruits and vegetables only
The diet is unbalanced (1m) and the waitress may
suffer from deficiency diseases (1m) .
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Exercise 11
3a Plot the above results as a graph. Draw a
vertical dotted line above day 18 to indicate the
exchange of diets.
See next page
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Exercise 11

• Correct line (0.5m)
• Correct labelling of axes (0.5m0.5m)
• Correct scale of axes (0.5m0.5m)
• Correct curves (1m1m)
• Correct vertical dotted line (0.5m)

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Exercise 11
3b Why were two groups of rats instead of two
individuals used?
It was because using two groups of rats will
give a more accurate result than using two single
rats / To ensure the experiment can be carried
out successfully if any rat dies during the
experiment. (Any 1, 1m)
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Exercise 11
3c Why were rats from the same mother used?
It can eliminate the genetic effect that arose
from rats of different mothers. (1m)
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3d Why were the diets exchanged?
To eliminate the influences of factors other
than milk. (1m)
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Exercise 11
3e What conclusion can be drawn from the results?
Milk contains some essential substances (1m), in
addition to proteins, glucose, starch, fats,
minerals and water, that help in the growth of
rats (1m).
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Exercise 11
3f What substance in milk affected the growth of
rats?
Vitamins (1m)
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