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Activity Book

Activity Book. 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 Benedict’s solution?. ANS: .

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Activity Book

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  1. Activity Book

  2. 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 Benedict’s solution? ANS:  The water bath has a better control over the temperature and can prevent bumping of the mixture.

  3. 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 Benedict’s test? ANS:  To allow the brick-red precipitate to settle.

  4. 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. ANS: 

  5. 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. ANS:  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.

  6. 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. ANS:  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.

  7. 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. ANS: 

  8. 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? ANS:  No. Glucose, like vitamin C, is reducing and will decolourize DCPIP solution no matter vitamin C is present in the sample or not.

  9. 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. ANS: 

  10. 11.1 Detection of food substances by food tests 8 How should you perform Benedict’s test on a blood sample? ANS:  As the red colour of the blood will mask the results of the Benedict’s test, the blood sample should first be diluted with distilled water. Alternatively, the blood sample should be centrifuged and the plasma collected for the Benedict’s test.

  11. 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? ANS:  (Answer depends on the types of foods tested.)

  12. 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. ANS:  Yes. Accurate measurement is necessary because a comparison of vitamin C content in different fruits and vegetables is needed.

  13. 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? ANS:  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.

  14. 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? ANS:  The type of fruits and vegetables. 4 What is the dependent variable in this experiment? The number of drops of sample needed to decolourize 1 cm3 of DCPIP solution. ANS:  Home Back Next

  15. 11.3 Design an investigation to compare the vitamin C content in different fruits and vegetables 5 What is the control in this experiment? ANS:  Distilled water. 6 How are you going to record and present the results? ANS:  (Answer varies with Ss.) Next

  16. 11.3 Design an investigation to compare the vitamin C content in different fruits and vegetables 7 How can you obtain faster results? ANS:  Use less DCPIP solution or a more dilute DCPIP solution.

  17. 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? ANS:  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.

  18. 11.3 Design an investigation to compare the vitamin C content in different fruits and vegetables 9 What precautions should be taken? ANS:  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.) Home Back Next

  19. 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.)

  20. 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.

  21. 11.3 Design an investigation to compare the vitamin C content in different fruits and vegetables Write an experimental report Objective ANS:  To compare the vitamin C content in different fruits and vegetables.

  22. 11.3 Design an investigation to compare the vitamin C content in different fruits and vegetables Results

  23. 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. ANS: 

  24. 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. ANS:  The vitamin C content will decrease as vitamin C is oxidized by the air.

  25. 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. ANS: 

  26. 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. ANS:  Oxidation of vitamin C in the juices. Work quickly with the freshly prepared juices. (To be continued.)

  27. 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.

  28. 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? ANS:  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.

  29. 11.3 Design an investigation to compare the vitamin C content in different fruits and vegetables Conclusion ANS:  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.

  30. Question No. Answer Question No. Answer 1 6 2 7 3 8 4 9 5 Exercise 11 Multiple-choice questions D C B D B D C C D

  31. Exercise 11 Structured questions 1a i Give two uses of calcium to the body. ANS:  For growth of teeth and bones / blood clotting / muscle contraction (Any 2, 1m each)

  32. 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? ANS:  600/160 X 100 = 375 g (1m)

  33. Exercise 11 1a iii Name another food that is rich in calcium. ANS:  Milk / cheese / vegetables (Any other correct answer) (Any 1, 1m) 1b i Which nutrient on the food label cannot be stored in the body when in excess? ANS:  Protein (1m)

  34. Exercise 11 1b ii What is the fate of the excess nutrient stated inbi? ANS:  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).

  35. 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. ANS:  Using DCPIP solution (1m). If the test is performed on the yoghurt, the blue colour of the DCPIP solution (1m) will be decolourized (1m). Home Back Next

  36. 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) ANS: 

  37. Exercise 11 2b Why does a pregnant woman need more energy and protein than a housewife? ANS:  It is because the foetus (1m) of the pregnant woman needs energy and protein for making new tissues (1m).

  38. 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). ANS: 

  39. Exercise 11 ii forced vomiting after a meal ANS:  The acidic gastric juice (1m) may damage the teeth / oesophagus / throat (1m). iii on diet of fruits and vegetables only ANS:  The diet is unbalanced (1m) and the waitress may suffer from deficiency diseases (1m) .

  40. 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 ANS: 

  41. Exercise 11 • Correct line (0.5m) • Correct labelling of axes (0.5m+0.5m) • Correct scale of axes (0.5m+0.5m) • Correct curves (1m+1m) • Correct vertical dotted line (0.5m)

  42. Exercise 11 3b Why were two groups of rats instead of two individuals used? ANS:  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)

  43. Exercise 11 3c Why were rats from the same mother used? ANS:  It can eliminate the genetic effect that arose from rats of different mothers. (1m) 3d Why were the diets exchanged? ANS:  To eliminate the influences of factors other than milk. (1m)

  44. Exercise 11 3e What conclusion can be drawn from the results? ANS:  Milk contains some essential substances (1m),in addition to proteins, glucose, starch, fats, minerals and water, that help in the growth of rats (1m).

  45. Exercise 11 3f What substance in milk affected the growth of rats? ANS:  Vitamins (1m)

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