Project work: Collect information about the experiments of Joseph Priestly. Write a two page report describing Priestly’s experiments proving that oxygen is needed for burning.

Joseph Priestly, a British clergyman, performed series of experiments which led to the discovery of oxygen. Some of his experiments also involved the process of photosynthesis. He conducted a burning candle experiment which can be described as follows:

1. Joseph Priestly lit a candle.

2. He then covered the burning candle with a glass jar.

3. He observed that the flame of the candle immediately died out.

Inference: He reasoned that something in air was necessary to keep the flame burning.

He then made some changes to the first experiment:

1. He placed fresh mint leaves underneath the jar and left the entire setup undisturbed for two days.

2. He found that he could relight the candle and it burnt for a while.

Inference: He concluded that the plant had produced the substance required for burning.

So the conclusions that came out of these two experiments were:

● Due to the absence of oxygen, candle flame died out in first case.

● Plant must have generated oxygen which was used to relight the candle.

He also performed an experiment which showed that oxygen supports combustion:

● He focused sunlight on mercuric oxide inside a glass tube.

● A gas was liberated.

● He observed that the candle burnt brighter in the presence of this gas.

Conclusion: The liberated gas was oxygen which made the candle to burn brightly.