Burning is one of the most familiar examples of chemical changes. The fact that air is essential for burning or combustion is a part of everyday experience. The main stuff in the air that supports burning is the oxygen of the air. Without oxygen, nothing can continue to burn. That is why, one of the ways of fire fighting is to exclude air from the burning object. This indirectly shows the presence of oxygen in air.
Here we demonstrate the presence of oxygen in air by the iron filings method. Moist iron when kept exposed to air gets rusted gradually to form hydrated ferric oxide (rust)
4Fe + 3O₂ + xH₂0 ————> 2Fe₂O3.XH₂O
The rate of rusting has been found to be rapid in presence of traces of acetic acid.
1) Measuring cylinder (25 ml capacity)
2) 500 ml beaker or water trough
1) Iron filings
2) Dil acetic acid
i) Take a measuring cylinder of 25 ml capacity, and add one ml of water at the bottom inside. Add into it 2-3 drops of dil acetic acid (2M) by means of a dropper. Shake it.
ii) Introduce about 1 gm of iron filings (or steel wool) into it, and wait for a while till the iron filings are wet. The iron filings should remain stuck up at the bottom.
iii) Invert the cylinder over water in the beaker in a slant position allowing the air inside the cylinder to escape as much as possible.
iv) Equalize the levels of water inside and outside the cylinder, and note the volume of air inside the cylinder.
v) Keep the cylinder in the slant position for some time when you will find that water rises to some extent in the cylinder.
vi) Read the volume every 10 minutes each time equalizing the two levels. Do so only for an hour.
vii) Draw a graph of volume left vs time, and calculate the percentage of O₂ by volume in air.
viii) You will find that one fifth of the original volume of air is lost. This experiment shows that air contains approximately 20% oxygen by volume.
ix) The remaining gas in the cylinder is mostly nitrogen which does not support combustion nor is combustible.
To test it, slowly raise the cylinder in the inverted position, and quickly introduce a burning candle high up into it.
The candle extinguishes. And the gas also does not burn showing that the gas left is the inactive or inert part of the air.
Note: This experiment may be used for demonstration by the teacher. In this experiment, the final volume remains constant for one hour or so, but may increase after a long time due to H₂ evolution.