Air and Water - Chemistry
- Created by: gemmablacker
- Created on: 16-03-15 20:46
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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
- So first we have to get air cold enough for it to condense into a liquid.
- 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
- These are the boiling points of the main substances left in the liquid air mixture passed into the fractionating column
- 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
- 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
- and at -196'C nitrogen boils off first
- Fractional distillation is a process in which liquids with different boiling points are separated
- 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
- Nitrogen gas is very unreactive so we use it in sealed food packaging to stop food going off
- It is used to store sperm in hospitals to help in fertility treatment
- Oxygen is used to help people breathe. It is also used to help things react. E.g. high temperature welding
- In industry the gases in air are separated by the fractional distillation of liquid air
- 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
- These reactions with oxygen to form oxides are known as combustion reactions
- 2Mg(s) + O2(g) --> 2MgO(s)
- When non-metallic elements burn they often from oxides which are produced as gases for example...
- C(s) + O2(g) --> CO2(g)
- The metallic elements burn to form solid oxides for example...
- 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 are basic. Non-metal oxides are acidic
- Most non-metal oxides dissolve in water to form acids, for example...
- 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
- Amphoteric oxides behave like both acids and bases. They do not dissolve in water so we cannot test the pH of solutions formed
- 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
- Some metal oxides are called amphoteric, such as lead (II) oxide (PbO) and aluminium oxide (Al203)
- 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
- Rust forms on the surface of iron (or most steels). Unfortunately, this rust is a soft, crumbly substance
- 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
- The corrosion of iron is called rusting
- The Atmosphere
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