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Experiment 7.4A:
The
iodine test (碘液試驗) for starch (澱粉)
Objective: To test for starch using iodine solution (碘液)
Procedures:
1. Add
a drop of starch solution (澱粉溶液) and a drop of water side by side
onto a white tile (瓷磚).
2. Add
a drop of iodine solution to each of them. Observe the colours of the mixture.
Results:
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Liquid
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Colour after adding iodine solution
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Starch solution
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Dark blue
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water
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Brown
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Conclusion:
Ø
Iodine solution can be used to test for starch.
Ø
A colour change from brown to dark blue shows
the presence of starch.
Experiment
7.4X: To study the necessary conditions (必須條件) for photosynthesis to
take place
Objective: To
study the necessary conditions (light光 and chlorophyll葉綠素) for photosynthesis to take place
Design
of Experiment:
To test whether light and chlorophyll are necessary
for photosynthesis to take place, a variegated
leaf (斑葉) covered with aluminium
foil (鋁箔)
is used as shown below:
Consider of “3Cs” of a fair test (公平測試)
n
Variable
to be changed: The presence of chlorophyll and light
n
Variable(s)
to be kept constant保持不變: (Put a “Ö” in the appropriate boxes.)
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The presence of chlorophyll or not
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Ö
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The supply of carbon dioxide
or not
|
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The supply of light or not
|
Ö
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Type of plant / leaf
|
|
Ö
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Temperature
|
Ö
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The supply of water or not
|
n What to compare: The
presence of starch
Procedures:
Preparation of the covered variegated leaf
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1.
Destarch (脫澱粉) a potted plant with variegated leaves (斑葉) by putting it in the
dark for 1 or 2 days.*
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2. Cover some part of a leaf with aluminium foil. Then, put the
plant under bright light for several hours.*
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3. Remove the leaf from
the plant.
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Test
for starch in the covered variegated leaf
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4. (a) Boil half a beaker of water.
(b) Remove the covered variegated
leaf from the potted plant.
(c) Put the leaf in the
boiling water for about two minutes.
(d) Turn off the Bunsen burner.
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5. (a) Half-fill a boiling tube with alcohol (酒精).
(b) Using a
glass rod, transfer the leaf to the boiling tube.
(c) Put the boiling tube
into the beaker of hot water for about 10 minutes.
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6. (a) Take the
leaf out of the alcohol. Observe what happens to the colours of the leaf and
the alcohol.
The leaf
becomes pale green and the
alcohol becomes green in colour.
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(b) Put the
leaf in the beaker of hot water for half a minute. Observe what happens to
the leaf.
The leaf
becomes soft.
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7. Carefully spread the leaf on a white tile. Add a few drops of
iodine solution to it. Observe what happens.
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*No need to be done by the students.
Results:
Draw a labelled diagram to show the result of the
iodine test on the leaf.
 |
The colours of different parts of the leaf after
the iodine test:
|
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Green part
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Non-green part
|
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Part
without cover
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Dark blue
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Brown
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Covered part
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Brown
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Brown
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Discussion:
(P.30)
Starch is found in the green parts where chlorophyll is present /
absent, but not in the non-green parts
where chlorophyll is absent.
Therefore, chlorophyll is necessary for photosynthesis to take place.
The part of the leaf without cover (i.e. exposed to light) is dark blue, but
the covered part
is brown. This
shows that starch is found in the part exposed
to light but
not in the part not exposed to light.
What is the purpose (目的) of each step below?
Write the letters representing the purposes on the blanks below.
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Step
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Purpose
|
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1. Put the leaf in boiling water.
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(c)
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(a) To test for the presence of starch in the
leaf.
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2. Put the leaf in hot alcohol.
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(d)
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(b) To wash away the alcohol and to soften軟化 the
leaf.
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3. Put the leaf in hot water again.
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(b)
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(c) To kill the leaf cells so that iodine
solution can enter the cells later on.
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4. Add iodine solution to the leaf.
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(a)
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(d) To remove the chlorophyll (葉綠素) to make the leaf lighter in colour for easy
observation of the results of the iodine test.
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Conclusion: (P.30)
Light and chlorophyll are necessary for photosynthesis to take
place.
