Collecting the gas

Every experiment so far has tested a factor going in. This one tests a product coming out — and to catch a gas, it helps to work underwater.

You need

A trough, a boiling tube, a funnel, and an aquatic plant.

Method

Take a trough and put water in it.
Then hold a few aquatic plants such as Vallisneria or hydrilla inside the funnel.
Fill a boiling tube with water, turn it upside down and stand it on the funnel.
Place this arrangement in a place where there is good sunlight.

Figure to be added

Figure 2.10 — a glass trough of water with a funnel standing upside-down in it, a clump of pondweed held under the funnel, and a water-filled boiling tube inverted over the funnel stem. Bubbles rise from the plants, up the funnel and into the tube, where gas is collecting at the closed upper end and pushing the water down. Arrows at the top left show sunlight falling on the apparatus.

Why the whole thing is set up under water

A gas given off in air simply mixes with the air and is gone. Under water it has nowhere to go but upwards, and the inverted funnel channels every bubble into one place. The tube starts completely full of water so that anything found in it afterwards must have come from the plant — there was no air in there to begin with.

An aquatic plant is used for a reason

It is not that pondweed photosynthesises differently. It is that a plant already living under water can be left there without harm while the apparatus does its work. Vallisneria and hydrilla are the same plants used in activity 2.1 to watch chloroplasts move — convenient organisms, chosen for the experiment rather than for themselves.