Composition of air.
Air is a mixture of different gases. The gases that make up the air include nitrogen, oxygen, carbon dioxide, noble gases (argon, helium, neon, krypton and xenon) and a little water vapor.
Air may also contain traces of impurities such as carbon monoxide (CO), sulphur dioxide (SO2), hydrogen sulphide (H2S) and other gases. The presence of these gases in air results in air pollution. Table below shows the composition of air by volume. The proportion of water vapour and impurities in air is very variable.
The Gases Present in Air and their Proportions.
The composition of air is not exactly the same everywhere. It changes slightly from day to day and from place to place. There is more water vapour in the air on a damp day and in air above water bodies such as oceans, seas, lakes, rivers, etc.Over busy cities and industrial areas there is more carbon dioxide. But the uneven heating of the earth's surface by the sun causes the air to move continually, resulting in winds. The resultant winds spread the pollutants around.
The percentage composition of air by volume.
The percentage composition of air by volume.
The Presence of Different Gases in Air
The determination of air by mass was carried out by Dumas in 1841.
The apparatus used consists of three units as shown below.
The three parts of the apparatus include the following:
1. Several U-tubes containing potassium hydroxide pellets to remove carbon dioxide (only one tube shown in the figure for simplicity).
2. Another set of U-tubes containing concentrated sulphuric acid to remove water vapour (only one tube shown in the figure).
3. A heated, weighed glass tube containing finely divided copper to absorb oxygen.
The three parts of the apparatus would, therefore, remove all carbon dioxide, water vapour and oxygen contained in air. The remaining gas which enters the weighted evacuated flask (globe) will be atmospheric nitrogen and, of course, plus the rare gases. The copper will have reacted with all oxygen to form copper (II) oxide.
The increase in mass of the copper will give the mass of oxygen. The increase in weight of the globe will be due to the weight of nitrogen and the rare gases. If we neglect the weight of carbon dioxide, the percentage of oxygen by mass (weight) in dry, pure air is 23.2% and the remaining 76.8% is the percentage of nitrogen and rare gases.
The presence of nitrogen in air
In order to demonstrate the presence of nitrogen in air, we need to carry out an experiment that will convert the nitrogen of the air into a chemically recognizable substance. This is easily done by strongly heating magnesium in the residual gas from the above experiment.Magnesium and nitrogen will react thus:
Upon treatment with water, magnesium nitrite gives ammonia gas. The gas can be recognized by its characteristic smell and its action of turning red litmus paper to blue.
Upon treatment with water, magnesium nitrite gives ammonia gas. The gas can be recognized by its characteristic smell and its action of turning red litmus paper to blue.
The presence of oxygen in air
Oxygen is known as the active portion of the air because it supports combustion and combines with many other substances. Its presence and composition in air can be determined by using these properties.
The presence of water vapour in air
Water vapour is present in air in varying quantities. It is given off by evaporation from the oceans, lakes and rivers. The presence of water vapour in air can be demonstrated by exposing deliquescent substances to the air on a watch glass. These are substances which when exposed to air tend to absorb much moisture from the air, dissolve in that moisture, and finally form a solution.Examples of deliquescent substances include calcium chloride, sodium hydroxide and phosphorous pentoxide.
The resulting solution is distilled. The colourless liquid obtained from distillation may be proved to be water by various water tests such as use of cobalt chloride paper or anhydrous copper (II) sulphate. The cobalt chloride paper turns from blue to pink in the presence of water. The white anhydrous copper (II) sulphate turns blue. Any of the two tests confirms the presence of water.
Alternatively, one may expose the anhydrous copper (II) sulphate salt to open air straight away for quite some time and then observe any change in its colour and/or form. Upon absorption of water vapour from the air, the white, powdery and anhydrous copper sulphate salt turns into hydrated blue crystals.
The noble (rare) gases
About 1% of the air by volume is made up of the noble gases. The most abundant of the noble gases is argon. Others are neon, xenon, krypton and helium. The proportion of these four is very minute. Argon and neon are used in “gas-filled” electric light bulbs and coloured “neon” electrical signs. They are obtained from liquefied air.Air pollutants.
The air always contains small quantities of many gases. Such gases include hydrogen sulphide, sulphur dioxide, as well as dust and other solid particles, especially in industrial areas. These gases are given off during the combustion of coal, and the fuels resulting from coal.SEPARATION OF AIR INTO ITS CONSTITUENT GASES
The air we breathe is necessary to keep us alive. It is also a chemical resource. Oxygen is used in steel making, and nitrogen is used in making fertilizers. To use these gases in this way, they must be separated from the atmospheric air. Air, as we studied, is a mixture of different gases.The method used to separate its constituent gases is fractional distillation. The gases have to be liquefied so that the mixture can be fractionally distilled. The process of separating the air into its constituent gases is difficult. It cannot be done in the laboratory. It is only done in industry. The chemical industry needs the gases from the air in their pure form.
The fractional distillation of air involves essentially two stages:
1. First, the air must be cooled until it turns into a liquid.
2. Then, the liquid air is allowed to warm up again. The various gases boil off at different temperatures
Air is filtered to remove any dust particles (purification).
The air is cooled to -180°C to remove the water vapour and carbon dioxide.
The air is then compressed to 100-150 atmospheres. As the compressed air gets very hot, it has to be cooled.
The compressed cooled air is allowed to expand rapidly. The rapid expansion cools the air to very low temperatures, and the liquid drops out. At -200°C, only helium and neon remain as gases. The cold gases are used to cool the compressed air.
Figure below illustrates all the steps that take place during the process.
The fractional distillation of air involves essentially two stages:
1. First, the air must be cooled until it turns into a liquid.
2. Then, the liquid air is allowed to warm up again. The various gases boil off at different temperatures
Stage 1: Liquefaction of air
The air is cooled to -180°C to remove the water vapour and carbon dioxide.
The air is then compressed to 100-150 atmospheres. As the compressed air gets very hot, it has to be cooled.
The compressed cooled air is allowed to expand rapidly. The rapid expansion cools the air to very low temperatures, and the liquid drops out. At -200°C, only helium and neon remain as gases. The cold gases are used to cool the compressed air.
Stage 2: Fractional distillation of liquid air
The air is cooled and compressed to form liquid air. The liquid air is allowed to warm up. Nitrogen boils off first because it has a low boiling point, -196°C. Argon follows by boiling at -186°C and finally oxygen at -183°CFigure below illustrates all the steps that take place during the process.
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