Question 21
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 cm³), a large test tube that can be safely 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 light 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.
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 cm³), a large test tube that can be safely 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 light 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.
