GCSE Earth Science: The composition of Earth's atmosphere, determining % oxygen

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1A OUR ATMOSPHERE - Composition, uses of gases in air, experiment to determine the % of oxygen in air

See also Section 1B RECYCLING OF GASES - carbon cycle, global warming, oxygen balance and photosynthesis,

and past ancient atmospheres, changes due to man's activities

Section 1C EVOLUTION OF EARTH'S ATMOSPHERE - changes over billions of years, origin of life


More detailed pages on the chemistry of the atmosphere

(i) Air pollution, incomplete combustion, carbon monoxide & soot

(ii) Greenhouse effect, global warming, climate change, carbon footprint from fossil fuel burning

(iii) Air pollution, sulfur oxides, nitrogen oxides, acid rain

1A OUR ATMOSPHERE - composition

The Earth and its atmosphere provide everything we need with a little help from the sun

We extract minerals from the crust and gases from the atmospheric gases - air

The Earth’s atmosphere is a very dynamic and complex system and continuously changing. The causes of these changes are varied including man-made (anthropogenic) intervention (e.g. fossil fuel burning) and often the effects of natural cycles. Scientists use very complex computer models (modelling software) to try to predict weather and climate change, but there are many variables that can influence this. Environmental problems caused by increased levels of air pollutants require scientists and engineers to develop solutions that help to reduce the impact of human activity on out planet.


1(a)(i) Today's atmosphere consists of a mixture of various chemicals including well known elements and compounds. We think the current atmosphere composition has been around for about 200 million years (since the time of the dinosaurs!). Apart from the individual atoms of the noble gases (e.g. helium, neon and argon), they are all typical small covalent molecules of 2-5 atoms, there only very weak intermolecular forces between the particles, even in the liquid state, giving rise to low boiling points - which is why they exist as gases at room temperature.

The pie chart of the Earth's atmosphere is shown on the right.

Elements - only one type of atom in the particle

78% nitrogen N2 molecules (about 80% or 4/5ths) , important to plants if not of direct use to us!

21% oxygen O2 molecules (about 20% or 1/5th) , rather important for respiration!

1% argon Ar atoms (1/100th), plus traces of other Group 0 Noble Gases (He, Ne, Kr, Xe atoms)

Compounds - at least two types of element (atoms) in the particle, held together by strong covalent bonds

0.040% carbon dioxide CO2 molecules (400 ppm or parts per million and steadily rising!)

variable amounts of water vapour H2O molecules (depends on humidity)

This atmosphere has been around for about 200 million years (see section 1b about the evolution of the Earth's atmosphere).

There are also traces of many other gases from natural or man-made pollution sources e.g.

the compounds sulphur dioxide SO2 molecules, nitrogen monoxide NO molecules and nitrogen dioxide NO2 molecules,

and carbon monoxide CO molecules, unburned hydrocarbon molecules CxHy (x and y are variable)

all pollutants from fossil fuel combustion (see Air Pollution and Climate Change Notes)

and methane (greenhouse gas) from cows and decomposing plant material)

The composition of our atmosphere is thought to be relatively unchanged for about 200 million years principally due to the Carbon-Cycle balance and helps provide the conditions to sustain complex life on Earth..

1A(ii) An experiment to determine the % of oxygen in air

1(a)(ii) One simple method to determine the % of oxygen in air is to use two 100cm3 glass gas syringes connected on either side of a piece of heat resistant silica tubing (*) containing excess copper powder/granules of copper. (*) ordinary glass tubing glass would melt!

One syringe is empty and the other filled with exactly 100cm3 of air.

The silica tube/copper is strongly heated with a hot flame bunsen burner and the gas syringes 1 and 2 are moved to and fro to pass the air over the hot copper so ALL the air comes into contact with the hot copper.

The oxygen in the air reacts with the copper to form copper(II) oxide.

copper + oxygen ==> copper(II) oxide

2Cu(s) + O2(g) ==> 2CuO(s)

Copper oxide is a black solid of little volume (so little error) and none of the other gases in air react with copper, so only the oxygen gas is removed AND any reduction in gas volume is solely due to the removal of oxygen. There will be a small error due to the oxygen removed from the small volume of gas in the silica tube AND another source of error is reading the volume of gas before everything has cooled down. You must make sure the original and final readings are taken at the same ambient room temperature.

Eventually the total volume reading reaches a minimum value when all the oxygen in the air has reacted with the copper.

Therefore, 100 - total final volume reading gives the % oxygen in air and you should get 79-80 cm3 of air (mainly unreactive nitrogen) left.

This gives a percentage of oxygen in air of ~20%-21%, but remember this is quite a crude method, modern analytical instruments like a gas chromatograph or a mass spectrometer will give the result much more accurately.

You can do simple experiments like burning a candle in a bell jar of air inverted over water. If time is allowed for the carbon dioxide to dissolved in water, you may see the level rise by a 5th as the oxygen is used up by the combustion of the wax but its a NOT very accurate, but a nice simple demonstration. In the early days of 'modern' science (18th to early 19th century?) the same experiment was done with mice whose respiration gives the same result, but not a good end for the mouse! The nature and role of oxygen was not fully understood in those days!. The experiment should work better with a little sodium solution added to the water since acidic carbon dioxide reacts with the alkali and reduces the gas volume.

Simple chemical test for pure oxygen: It relights a glowing splint.

Bubbling air through limewater should eventually produce a white precipitate (solution goes 'milky') showing the presence of carbon dioxide in air.

An experiment to investigate the products of burning a candle.

doc b oil notes The products are carbon dioxide and water.

The carbon dioxide is chemically detected with limewater – with which it forms a white precipitate (milky appearance) of calcium carbonate. The water is chemically detected either by (i) anhydrous white copper sulphate turning blue or (ii) dried blue cobalt chloride paper turning pink.

1A(iii) Obtaining and Using Gases from Air

  • Air is not an obvious resource for the chemical industry and commerce, we take it for granted, and just expect it to be there for its oxygen needed for our respiration.

  • However, it is a most valuable resource in its own right.

  • How do we get useful products from air?

    • The air must be initially filtered to remove dust particles, before liquefaction.

    • It is then compressed and cooled to around -200oC to form a liquid.

    • The cooling process is performed in such a way that water vapour is condensed out first and then carbon dioxide frozen out before fractional distillation.

    • The liquefied air is now warmed before entering a fractionating column and fractionally distilled twice to separate the gases nitrogen, oxygen, argon and helium. The gases can be stored and delivered to customers in pressurised cylinders.

      • Nitrogen is used in the synthesis and manufacture of ammonia, 'fixation of nitrogen' via the Haber process.

      • Oxygen is used in oxy-fuel welding torches, respiratory systems in hospitals for people with breathing difficulties etc.

        • In both these uses the increased concentration of oxygen compared to air gives a more efficient reaction!

      • Carbon dioxide as used as 'dry ice' for stage effects and as a coolant (-78oC).

      • Argon is in filament bulbs and to provide an inert atmosphere to stop reactions with air eg in welding and steel making.

      • Helium is used in balloons and liquid helium is used to provide extremely low temperatures in technologies like cryogenics.

The evolution of the Earth's atmosphere is described on a separate page

More detailed pages on the chemistry of the atmosphere

(i) Air pollution, incomplete combustion, carbon monoxide & soot

(ii) Greenhouse effect, global warming, climate change, carbon footprint

(iii) Air pollution, sulfur oxides, nitrogen oxides, acid rain



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