A student sets up three jars. Each holds a water plant in water…
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A student sets up three jars. Each holds a water plant in water. Jar P is placed in bright light. Jar Q is kept in a dark cupboard. In jar R, the plant is a colourless fungus, which has no green parts, in bright light. After an hour, she counts the bubbles that rise. (a) In which jar do you expect bubbles, and what gas are they? (b) Which scientist did this kind of experiment first? (c) What does the result of jar Q add to the result of jar P? (d) What does jar R add?
Need a reminder? The key ideaShow
Priestley (1770): a mint plant restores the air that a burning candle or an animal makes unfit. Ingenhousz (1779): sunlight is needed and only green parts release oxygen. Sachs (1854): glucose is made in green parts and stored as starch. Engelmann: bacteria gather in the blue and red regions of a prism spectrum, the first action spectrum. Van Niel: bacteria using H2S give out sulphur; in green plants the donor is water, so the oxygen comes from water. Overall: . Water is oxidised, carbon dioxide is reduced.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Which jar. Jar P has green parts and light, so it gives bubbles of oxygen. Jar Q has no light, and jar R has no green parts.
Step 2. The scientist. This is the experiment of Jan Ingenhousz (1779).
Step 3. Jar Q. It shows that light is needed. The same plant gives no bubbles without it.
Step 4. Jar R. It shows that the green parts are needed. Light alone is not enough.
Answer: (a) jar P, oxygen; (b) Ingenhousz; (c) light is needed; (d) green parts are needed.
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