Lesson 4 ways of Determining the percentage of oxygen in air

• Method for Rusting iron in a gas jar

  1. Set up apparatus as shown
  2. Mark the water level on scale
  3. Leave until the iron rusts
  4. Mark the new water level on scale
• When the iron rusts, it reacts with the oxygen and water vapour from air. As the oxygen is used up a vacuum is created. The water level inside the gas jar rises upwards to take the place of the used up oxygen.

  The water level at the start and end of the experiment is marked on the scale. This shows how much oxygen is used up and can be used to calculate the % of oxygen in air.

  Note – When the iron rust it changes from a shiny grey metal to a dull red/brown colour.
• Method of Burning a candle in a gas jar

  1. Mark a scale on the gas jar.
  2. ½ fill the water bowl with water.
  3. Place the candle in a crucible on the water.
  4. Light the candle.
  5. Gently place the inverted gas jar over the candle.
  6. Mark the starting water level.
  7. When the candle goes out mark the new water level on the scale.
• The results of burning a candle in a glass jar?


  The candle will burn until the supply of oxygen runs out. As the oxygen is used up burning the candle, the water level inside the gas jar will rise upwards.

  The rise in the water level can then be used to calculate the % of Oxygen in air.

  For example:

  On a scale marked from 0 cm to 100 cm:

  Start Level of water on Scale  =  0 cm

  Finish Level of water on Scale = 18 cm

  Formula to Calculate the % of Oxygen in Air:

  % Oxygen  =  Finish Level – Start Level / total on scale x 100
• What is the chemical name for rust?

  Hydrated iron (III) oxide
• A candle is made from a compound that contains elements of carbon and hydrogen.

  Name two gases that could form when a candle burns.

  Carbon: carbon dioxide

  Hydrogen: hydrogen oxide (water)