2016年6月13日 星期一

F4 2nd Term EXP01: Practical 5.X


Practical 5.X
Comparison of the food substances
in two types of drinks

Objective:       To compare the food substances in milk and packaged fruit juice by using various food test.

Biological principle and experiment design:                                                                          (9 marks)
Different food tests are used to test for the presence of particular food substances. The following table shows different food tests and the corresponding food substances:
Food test
Food substances
Using Glucose test paper            (1)
Glucose
Benedict’s test
 Reducing sugar            (1)
Iodine test              (1)
Starch
Grease spot test
Lipid                (1)
Using protein test paper          (1)
Protein
Using DCPIP solution
Vitamin C               (1)

Milk and fruit juice contain different nutrients, so we can use the above food tests to compare the food substances they contain.
To ensure that the positive result of the food test is due to the presence of the food substance in the food
sample instead of the chemicals or contaminants in the container, Control (1) should be set up by using the same amounts of water to repeat all the food tests.
Expected results: (2)
Milk may contain … (1);

While the packaged fruit juice may contain … (1)

Results:                                                                                                                          (12 marks)
A  Using glucose test paper to test for glucose
Sample
Colour of the test end of the glucose test paper
Orginal
After the test
Milk
Pink
Pink
Fruit juice
Pink
Blue
Distilled water
Pink
Pink

B  Using Benedict’s test to test for reducing sugars
Sample
Observation
Relative amount of precipitate
(arbitrary unit*)
Before heating
After heating

Milk
Pale blue
Brown precipitate
+++
Fruit juice
Pale blue
Brick precipitate
++++
Distilled water
Pale blue
Pale blue
-
*Note:         “-” represents no brick-red precipitate
                     no. of “+” represents the relative amounts of brick-red precipitate.

C  Using iodine test to test for starch
Sample
Colour of the iodine solution
Orginal
After the test
Milk
Brown
Brown
Fruit juice
Brown
Brown
Distilled water
Brown
Brown

D  Using grease spot test to test for lipids
Sample
Appearance of translucent spot
Before adding the organic solvent
After adding the organic solvent
Milk
Yes
No
Fruit juice
No
No
Distilled water
No
No



E  Using protein test paper to test for proteins
Sample
Colour of the test end of the protein test paper
Orginal
After the test
Milk
Yellow
Green
Fruit juice
Yellow
Yellow
Distilled water
Yellow
Yellow

F   Using DCPIP solution to test for vitamin C
Sample
Colour of DCPIP solution
No. of drops needed for decolorization
(drops)
Original
After the test

Milk
Blue
Milky violet
> 50
Fruit juice
Blue
Original colour of the fruit juice
e.g. 2 - 10
Distilled water
Blue
Pale blue
> 50

Discussion:                                                                                                                              (5 marks)
1     Is it necessary to measure the amounts of food samples accurately for comparison of the food substances contained? Why? (2)
No, it is not necessary to measure the amounts of food samples accurately (1).
It is because the objective of the experiment is to compare the food substances in the two food
samples qualitatively only (1).
/ Yes (1). It is because we need to compare the food substances in the food samples quantitatively (1).

2     Explain why the use of DCPIP solution in the measurement of the amount of vitamin C of a food sample is not necessarily accurate. [Challenging question] (3)
DCPIP may react with substances other than vitamin C and decolourize (1). In fact,
substances other than vitamin C (e.g. anti-oxidant) can also decolourize DCPIP (1), that may interfere
with the measurement of the amount of vitamin C (1)

Conclusion:                                                                                                                            (2 marks)
The milk sample contains… (1) and the fruit juice sample contains … (1)

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