Question

1. In 1772, Joseph Priestley ran experiments with a candle, a bell jar, and healthy leaves plucked from a mint plant. He ran the two experiments summarized below:

Experimental setup		Results
Lighted candle placed under a bell jar	Light present	Flame goes out after a minute or two.
Lighted candle placed under a bell jar containing healthy mint leaves	Light present	Flame continues for hours.

These experiments inspired another scientist, Jan Ingenhousz, to run related experiments. Ingenhousz added an additional variable—he ran the experiments in the presence and absence of light. His results are summarized below:

Experimental setup		Results
Lighted candle placed under a bell jar containing healthy mint leaves	Light present	Flame continues for hours.
Lighted candle placed under a bell jar containing healthy mint leaves	Light absent	Flame goes out after several minutes.

a. Explain why the mint leaves sustained the candle flame in Priestley’s experiment.

b. Explain what Ingenhousz’s experiment demonstrated about the interdependence of energy-producing processes and living organisms.

Answer 1

a. Joseph Priestley's experiment demonstrates the importance of the air purification process performed by plants.
He stated that plants purify the air by absorbing dephlogiston and releasing phlogiston.
He said that plants replenish the compound that is utilized by a breathing animal or a burning candle.
It was later identified that plants utilize CO2 for photosynthesis and release oxygen.

b. Ingenhosez demonstrated the importance of light for photosynthesis.
In the absence of light, a plant cannot perform photosynthesis. So, oxygen is not released to sustain a burning candle.
In the presence of light, the plant can perform photosynthesis. So, oxygen is released to sustain a burning candle.
i.e. photoautotrophic organisms utilize light energy to perform photosynthesis.