Page 16 - 新思维科学学生用书9 样章

1.2 More about photosynthesis

Think like a scientist

Testing a leaf for starch

You are going to use a test you may already know – the iodine test for starch – to find
out if a leaf has been photosynthesising.

You will need:
• a healthy plant that has been kept where it gets plenty of sunlight (if possible,
use a plant whose leaves have some green parts and some white parts),
a burner (e.g. a Bunsen burner or a spirit burner), a tripod and gauze, a

medium-sized beaker (e.g. 250 cm ), a large test tube that can be safely
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heated, some ethanol, tongs or another way of handling a hot test tube,
forceps (tweezers), some iodine solution with a dropper, a tile.

Safety
• You will be using hot water, so make sure you keep safe. It is best to stand up
while you work, so that if something is spilt you can easily move away.

• Be very careful to turn off the flame before you use any ethanol.
Ethanol is very flammable (it catches fire easily).
Method

1 Put some water into your beaker. Stand the beaker on
the tripod and gauze and heat it until it starts to boil.
2 Take a healthy leaf from the plant. Carefully drop it

into the boiling water.
3 After about two minutes, turn off the burner. This is really
important, because you will use ethanol in the next step.
4 Collect some ethanol in the large test tube.
5 Use forceps to remove the leaf from the hot water.

Carefully drop the leaf into the ethanol in the test tube.
6 Stand the test tube in the hot water. This will safely heat the
ethanol. Watch what happens. You should see the green colour

dissolve out of the leaf into the ethanol.
7 When you think most of the green colour has come out of the leaf,
use forceps to gently take the leaf out of the tube. Be very careful –
the leaf will be brittle. Dip it into the warm water to soften it.
8 Spread the leaf out onto a tile. Use the dropper to

spread iodine solution over the leaf. If the leaf
contains any starch, it will go blue-black.

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