Two bags, two leaves

You cannot cover a leaf to keep carbon dioxide out — it would keep the light out too. So instead of blocking the gas, this experiment removes it.

You need

A potted plant, the materials for the starch test, two polythene bags of equal size, potassium hydroxide solution, and water.

Method

From the same plant used in activity 2.3, choose two largely similar leaves — call them leaf C and leaf D.
Take two transparent polythene bags and put potassium hydroxide (KOH) solution in one and water in the other.
Then put leaf D inside the bag containing the KOH solution and leaf C inside the bag containing water, and tie them so that they are airtight.
Leave this plant for about 3 to 5 hours in a place that receives good sunlight. Then take leaves C and D and subject them to the starch test.

Figure to be added

Figure 2.7 — a plant in a pot with two of its leaves each enclosed in a transparent polythene bag tied shut at the stalk. The bag on the left, labelled C, holds water; the bag on the right, labelled D, holds KOH solution.

What the potassium hydroxide is for

Potassium hydroxide absorbs carbon dioxide. Sealed inside bag D with the leaf, it takes the carbon dioxide out of the air in the bag and goes on taking it out, so the leaf inside is left with none. You met KOH doing exactly this job in grade 10, in the experiment that showed carbon dioxide is released in respiration.

Bag C is the control, and water is doing nothing

The water in bag C absorbs no carbon dioxide worth speaking of. It is there so that leaf C is in a sealed bag with a liquid in it, exactly like leaf D, and differs from leaf D in one respect only — whether the air around it still contains carbon dioxide. Same plant, same sunlight, same bag, same few hours.