Identifying gases in the air   Air pollution   
Experiment to show the percentage of oxygen in the air

The air is a mixture of gases, some of which are elements and some compounds.

gases_in_the_air The approximate composition of air is:

  • 78% nitrogen
  • 21% oxygen
  • 1% argon and other noble gases
  • 0.03% carbon dioxidewith a varying amount of water vapour

Air also contains a number of pollutants (see separate table).

Identifying gases in the air:

Oxygen Relights a glowing splint
Carbon dioxide  Turns limewater cloudy
Water vapour: Condense it into colourless liquid.This can then be tested with anhydrous cobalt chloride which will turn from blue to pink (or with anhydrous copper sulphate which turns from white to blue)

Gases in the air are separated industrially by the fractional distillation of liquid air.

Oxygen is the most reactive gas in the atmosphere and the only one that supports combustion. Its main use is in the steel industry for the purification of iron. It is used in welding, hospitals and in rocket fuel.

Carbon dioxide Used in fire extinguishers and in fizzy drinks

Nitrogen Used in fertilizers (nitrogen is an important plant food) and in explosives.

Argon Used in light bulbs to stop the filament oxidising

Air Pollution

Pollutant Where it comes from Dangers/effect How to reduce it
Sulphur dioxide (SO2) Burning fossil fuels eg coal and oil. Dissolves in rain water to form acid rainwhich can harm animal and plant life. It also attacks limestone buildings. 1. Use chemical filters in chimneys.2. Burn less fossil fuels This can be done by using other energy sources eg nuclear energy or solar and wind power.
Carbon Dioxide (CO2) Burning coal and wood Carbon dioxide is a greenhouse gas.
It increases global warming due to the greenhouse effect.
Carbon dioxide is naturally present in the atmosphere anyway due to the bi-products of decomposition and respiration but we must do what we can not to increase it unnecessarily.
eg    Do not clear forests by burning them.
Burn less coal.
Carbon Monoxide (CO) Car exhaust fumes Very poisonous and it has no smell. It damages the blood by making it not able to carry oxygen around the body so efficiently. 1. Have fewer cars on the road. 2. Fit catalytic converters.3. Find alternative fuels for cars eg electricity or hydrogen
Nitrogen dioxide (NO2) Car exhaust fumes. Forms acid rain Fit catalytic converters to cars.
Methane (CH4) Mainly from land-fill waste tips. Methane is a greenhouse gas.
It increases global warming due to the greenhouse effect.
Dispose of rubbish in other ways.
Lead compounds Car exhaust fumes Poisonous. Can cause brain damage. Use unleaded petrol.
Soot Mainly car exhaust fumes also smoke Looks unsightly on buildings; damages lungs. Use fewer cars. Fix filters in chimneys.

Greenhouse effect
Certain gases like methane and carbon dioxide absorb heat energy from the Sun and this causes the atmosphere to heat up.
This is not good as it can effect the Earths weather and ocean currents.
It may also cause the polar ice caps to melt resulting in the sea water level rising.

Experiment to show the percentage of oxygen of air

100 cm3 of air is put in one syringe. When the copper is heated it reacts with the oxygen in the air inside the syringe.

The syringe plungers are moved in and out so that all of the air is passed over the hot copper.

When there is no further change the apparatus is allowed to cool (to allow the heated air to contract.

Some copper remains unreacted because there is no oxygen left in the syringe.

The gas remaining in the syringe is mostly nitrogen.

Volume of air in syringe at start = 100cm3
Volume of air in syringe after heating = 79 cm3
Volume of air used up by copper = 21cm3

Similar results would be expected with anything that reacted with oxygen, eg iron rusting or a candle burning, ie about 20% of the air would be used up.