Air and water


Gases in the atmosphere

Air is a mixture of gases with different boiling points:

  • Oxygen: 21%
  • Nitrogen: 78%
  • Carbon dioxide (0.04%)
  • Argon (0.9%)
  • Small percentages of pollutant gases such as carbon monoxide, sulfur dioxide, and oxides of nitrogen aswell as water vapour 


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Separating the gasses in air

Fractional distillation of liquid air - liquids with different boiling points are separated 

  • Air has to be compressed to about 150 times atmospheric pressure
  • It is cooled to normal temperatures by passing it over pipes carrying cold water
  • It is cooled to a temperature below -200 degrees C by releasing the pressure so it expands rapidly and the temperature drops far enough for the gases in the air to condense to liquids
  • The carbon dioxide and water can be removed easily from the mixture before fractional distillation takes place as they are solids at this low temperature
  • Main substances left in the liquid air mixture & their boiling point: 
  • Nitrogen (-196) Argon (-186) Oxygen (-183)
  • They are passed into the fractioning column which is colder at the top than at the bottom and at -196 nitrogen boils off first - passes out of the top of the column as nitrogen gas
  • At -185 in the fractioning column the oxygen is still cold enough to be a liquid as it boils at -183 so the oxygen can be separated off as a liquid from the bottom of the column
  • The small amount of argon can be collected from the column below the nitrogen outlet but above the oxygen
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Fractional distillation of liquid air


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Uses of gases in the air in industry

Liquid nitrogen is used to cool things down to very low temperatures. At these temperatures most things solidify. It is used to store sperm in hospitals to help in fertility treatment.

Nitrogen gas is very unreactive so we use it in sealed food packaging to stop food going off. It is also used on oil tankers when the oil is pumped ashore to reduce the risk of explosion. In industry, nitrogen gas is used to make ammonia which we convert to fertilisers 

Oxygen is used to help people breathe, often at the scene of an accident or in hospital. It is also used to help things react. Examples include high temperature welding and in the steel making process

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Oxygen and oxides

  • When any substance burns, it reacts with the oxygen in the air to form
  • The metallic elements burn to form solid oxides
  • When non-metallic elements burn they often form oxides which are produced as gases
  • These reactions with oxygen to form oxides are known as combustion reactions
  • They are examples of oxidation in which oxygen is added to an element
  • Reactions in which oxygen is removed from an oxide are known as reduction

Metal oxides: dissolve in water to make an alkaline solution

  • Some metal oxides do not dissolve in water - they test neutral with universal indicator however they do react with acid. Insoluble metal oxides are called bases
  • Some metal oxides are called amphoteric. Amphoteric oxides behave like both acids and bases. They do not dissolve in water so we cannot test the pH of solutions formed
  • However they do react and dissolve in acids and alkalis

Non-metal oxides: dissolve in water to form acids 

In general metal oxides are basic & non-metal oxides are acidic

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Hardness of water

  • A hard water does not form a lather with a soap but forms scum 
  • Hard water contains dissolved salts of magnesium and calcium and these ions (mg2+ & ca2+) form an insoluble substance with soap molecules called scum
  • Temporary hardness is caused by the hydrogencarbonate ion HCO3- in Ca(HCO3)2
  • Permanent hardness is caused by dissolved calcium sulfate
  • Temporary hardness is removed by boiling. 
  • When heated the calcium hydrogencarbonate decomposes to form calcium carbonate which is insoluble - this solid is limescale

             Ca(HCO3)2                    -->                 CaCO3                  + CO2 + H20

  • Both types of hardness can be softened by adding washing soda (sodium carbonate - Na2CO3) 
  • The added carbonate ions react with the Ca2+ and Mg2+ ions to make an insoluble precipitate of calcium carbonate and magnesium carbonate
  • Both types of hardness can also be removed by running water through 'ion exhcange columns' which have lots of sodium or hydrogen ions and 'exchange' them for calcium or magnesium ions in the water
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  • The corrosion of iron is called rusting
  • Both air and water are needed for iron to rust

iron + oxygen + water --> hydrated iron(111) oxide

Preventing rust

Coating with:

  • Paint
  • Oil
  • Plastic
  • A less reactive metal
  • A more reactive metal - zinc is often used to protect iron which is known as galvanising / sacrificial protection - water and oxygen react with the zinc not the iron
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