Take a potted plant having a long and narrow leaves
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Destarch it by placing a dark room
(leaf can be destarch by keeping this plant in dark room for at least 3 days. The plant will use up all the starch stored in its leaves in about three days as in dark room the plant cannot make more starch because there is no sunlight)
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Take a wide mouth glass Bottle and put potassium hydroxide in it
(Potassium hydroxide solution absorb Carbondioxide gas present in air)
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In glass Bottle no Carbondioxide is left
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Take the rubber cork and cut it into two halves
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Put the destarch leaf inside the bottle in between the two halves
(Leaf is still attached to the plant)
As half part of leaf is inside the bottle and remaining half remains outside the bottle
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Now keep this potted plant in sunlight for 3-4 days
(During this period the upper half of leaf gets Carbondioxide from the air and lower half leaf cannot get Carbondioxide
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Now pluck the leaf and remove chlorophyll as chlorophyll is a green colour pigment it interferes in the starch test.
(Chlorophyll can be removed from the leaf by boiling it in alcohol. Alcohol is flammable this we add water and then boil it)
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Boil the leaf untill it becomes pale yellow and the alcohol becomes green
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Leaf is washed with water to remove extra chlorophyll stuck to it
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Now after removing chlorophyll, the leaf is tested for the presence of starch
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To check the presence of starch we drop the iodine solutions on leaf
(Starch gives blue black colour with iodine)
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The one part of decolourised leaf becomes blue black in colour means starch is present here.
(the part of leaf which is outside or in contact with Carbondioxide become blue black in colour)
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The other part of leaf remain colourless on adding iodine drop
(The part of leaf which is inside the bottle or not in contact with Carbondioxide remain colourless)
As the start gives blue black colour with iodine here it doesn't give that type of colour, it shows that starch is not present here
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As starch is only formed in presence of Carbondioxide, thus the experiment shows that Carbondioxide is necessary for photosynthesis.
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