Decolourising the leaf

A green leaf cannot be read. Before iodine will tell you anything, the green has to come out — and getting it out takes two separate heatings for two different reasons.

You need

A beaker, a test tube, a tripod, a Bunsen burner, water, ethyl alcohol, and a leaf.

Method

Take a leaf from a plant in a place that receives good sunlight, and boil it in water.
Then put that leaf into a boiling tube containing alcohol, stand the tube in a water bath, and boil it.
Then wash that leaf with water, add a few drops of iodine solution, and observe the colour change.

Figure to be added

Figure 2.4 — the two stages of the method, side by side. On the left, a beaker of water on a tripod over a Bunsen burner, with a leaf in the water. On the right, the same beaker of water on the tripod acting as a water bath, with a tube of alcohol standing in it and the boiled leaf inside the tube.

Why each step is there

Boiling in water first

This kills the leaf and increases the permeability of the cell membranes, so that the alcohol can reach the chlorophyll and the iodine can later reach the starch.

Then boiling in alcohol

Chlorophyll dissolves in alcohol. The alcohol turns green as the pigment comes out of the leaf, and the leaf itself turns white.

Why a water bath

Alcohol is flammable, so it must never be put over a flame directly. Standing the tube in hot water heats it safely.

Then washing with water

Alcohol leaves the leaf brittle and dried out. Washing softens it again so the iodine can spread across it evenly.

It is the alcohol that removes the colour, not the water

This is the step most often written down the wrong way round. Chlorophyll does not dissolve in water. Boiling in water makes the membranes permeable; boiling in alcohol takes the green out. Two steps, two different jobs, and swapping them in an answer loses the mark.