how can you show that oxygen is only one fifth of the components of the air???

Aim of the experiment - To show that oxygen occupies one-fifth of the air by volume
Materials required - candle, trough, water, glass tumbler 
Procedure - 
  1. Fill the trough with water. 
  2. Take a lighted candle and place it carefully at the centre of the trough.
  3. Now take a glass tumbler and use a marker pen to mark five equal portions on the tumbler. 
  4. Invert this tumbler over the burning candle.
Observation - 
  1. The candle burns for some time and is then extinguished. 
  2. The water rises from the edge of the inverted tumbler to occupy about one-fifth of it. 

Inference - 
The candle burnt till oxygen was present, Once the oxygen inside the tumbler was used up, it stopped burning. The air was devoid of oxygen inside the tumbler and as such the water from the trough moved up the tumbler and occupied its place, Since the candle was extinguished after about one-fifth of the air being used up, we know the gas used up was oxygen as it is the only gas in air which supports combustion. Thus, it is proved that oxygen occupies about one-fifth of the air by volume. 

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Oxygen is the first element in Group 16 (VIA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. The elements in Group 16 are said to belong to the chalcogen family. Other elements in this group include sulfur, selenium, tellurium, and polonium. The name chalcogen comes from the Greek word chalkos, meaning "ore." The first two members of the family, oxygen and sulfur, are found in most ores.

Oxygen is by far the most abundant element in the Earth's crust. Nearly half of all the atoms in the earth are oxygen atoms. Oxygen also makes up about one-fifth of the Earth's atmosphere. Nearly 90 percent of the weight of the oceans is due to oxygen. In addition, oxygen is thought to be the third most abundant element in the universe and in the solar system.

The discovery of oxygen is usually credited to Swedish chemist Carl Wilhelm Scheele (1742-86) and English chemist Joseph Priestley (1733-1804). The two discovered oxygen at nearly the same time in 1774, working independently of each other.

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