Air and Water - Chemistry

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  • Air and Water
    • The Atmosphere
      • Nitrogen 78%
      • Oxygen 21%
      • Argon 0.9%
      • Carbon Dioxide 0.04%
      • Trace amounts of other gases
      • There are also small percentages of pollutant gases, such as carbon monoxide, sulphur dioxide and oxides of nitrogen, in the air, as well as varying proportions of water vapour
    • Separating the gases in air
      • In industry the gases in air are separated by the fractional distillation of liquid air
        • Fractional distillation is a process in which liquids with different boiling points are separated
          • So first we have to get air cold enough for it to condense into a liquid.
            • It has to be cooled to temperature below -200'C
        • In industry they do this in a two-stage process. Firstly, they compress the air to about 150 times atmospheric pressure
          • This actually warms the air up. So it is cooled down to normal temperatures by passing the air over pipes carrying cold water
        • The second stage provides the main cooling, which takes place when the pressure is released. As this happens, the air is allowed to expand rapidly
          • This is similar to what happens in an aerosol can when pressure is released as the aerosol is sprayed
          • The temperature drops far enough for even the gases in the air to condense to liquids.
            • The carbon dioxide and water can be removed easily from the mixture before fractional distillation take place as they are solids at this low temperature
              • These are the boiling points of the main substances left in the liquid air mixture passed into the fractionating column
                • Nitrogen; boiling point is -196'C
                • Argon; boiling point is -186'C
                • Oxygen; boiling point is -183'C
        • The column is colder at the top than the bottom
          • and at -196'C nitrogen boils off first
            • It is passed out of the top of the tall fractionating column as nitrogen gas
          • At -185'C in the fractionating column the oxygen is still cold enough to be a liquid, as it boils at -183'C
            • 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
      • 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 into fertilisers
      • Oxygen is used to help people breathe. It is also used to help things react. E.g. high temperature welding
    • Oxygen and Oxides
      • The reactive gas in the air is oxygen. When any substance burns, it reacts with the oxygen gas in air to form compounds called oxides
      • The elements burn more vigorously in pure oxygen that in air, which only contains 21% oxygen
        • The metallic elements burn to form solid oxides for example...
          • 2Mg(s) + O2(g) --> 2MgO(s)
            • These reactions with oxygen to form oxides are known as combustion reactions
              • C(s) + O2(g) --> CO2(g)
              • They are examples of oxidation, in which oxygen is added to an element
              • Other reactions, in which oxygen is removed from an oxide, are known as reduction
                • This is important in the extraction of metals from their ores
        • When non-metallic elements burn they often from oxides which are produced as gases for example...
          • C(s) + O2(g) --> CO2(g)
      • Non-metal oxides
        • Most non-metal oxides dissolve in water to form acids, for example...
          • CO2 (g) + H20 (l) --> H2CO3 (aq)
        • In general we can say:
          • Metal oxides are basic. Non-metal oxides are acidic
            • Some oxides, such as water (H2O) and carbon monoxide (CO) are neutral
      • Metal oxides
        • Some metal oxides are called amphoteric, such as lead (II) oxide (PbO) and aluminium oxide (Al203)
          • 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, thus behaving like a base, but they also react and dissolve in alkalis, thus behaving like an acid
        • If a metal oxide dissolves in water, it makes an alkaline solution, for example...
          • Na2O + H2O --> 2NaOH
          • Some metal oxides do not dissolve in water. Copper (II) oxide is insoluble
            • These metal oxides test neutral with universal indicator. However, they do react with acid
            • Insoluble metal oxides (or hydroxides) are called bases
    • Rusting
      • The corrosion of iron is called rusting
        • Rust forms on the surface of iron (or most steels). Unfortunately, this rust is a soft, crumbly substance
          • It soon flakes off, exposing fresh iron, so that more iron can rust
      • Air and water are needed for iron to rust
        • Rust is a from of iron (III) oxide. It has water loosely bonded to it. It is called hydrated iron (III) oxide

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