Anhydrous copper(II) sulfate turns from white to blue when water is added.

Anhydrous cobalt(II) chloride turns from blue to pink when water in an aqueous solution is added.

Clean and safe water supply is very important to mankind. Many problems arise in the event of an inadequate water supply, including:

- Food shortages and famine due to a lack of crops which cannot grow without a clean water supply.

- Poor sanitation leads to spread of bacteria and disease as drinking water becomes infected.

Filtration is the process used to remove large insoluble particles by passing the water through layers of sand and gravel filters that trap larger particles.

But bacteria and other microorganisms are too small to be trapped by the filters so chlorination is used.

This involves the careful addition of chlorine to the water supply which kills bacteria and other unwanted microorganisms.

- As a coolant to reduce the temperature of some industrial processes e.g: in nuclear power plants.

- Watering crops.

- As a solvent in many chemical production processes.

- Hydroelectric power stations to generate electricity.

- As a first raw material for many processes e.g: the production of ethanol from ethene and steam (water).

- Drinking, cooking and washing.

- General sanitation.

- In car radiators, for gardens and plants.

The composition of clean, dry air is approximately 78% nitrogen, 21% oxygen and the remainder is a mixture of noble gases and carbon dioxide.

- The air is first filtered to remove dust, and then cooled in stages until it reaches –200°C. At this temperature the air is in the liquid state.

- Water vapour and carbon dioxide freeze at higher temperatures and are removed using absorbent filters.

- The Noble gases are still in the gaseous state at -200ºC, leaving a mixture of liquid nitrogen and oxygen.

- The liquefied mixture is passed into the bottom of a fractionating column.

- Note that the column is warmer at the bottom than it is at the top.

- Oxygen liquefies at -183°C and nitrogen liquefies at -196°C.

- Nitrogen has a lower boiling point than oxygen so it vaporises first and is collected as it rises in the gaseous state to the top of the column.

- The liquid O₂ is then removed from the bottom of the column.

Carbon monoxide, sulfur dioxide, oxides of nitrogen and lead compounds.

Sources: incomplete combustion of fossil fuels e.g: incomplete combustion of gasoline.Adverse effects: poisonous, combining with hemoglobin in blood and prevents it from carrying oxygen.Sources: combustion of fuels, natural gas and sulfide ores e.g: zinc blende (ZnS) in the extraction of zinc.Adverse effects: acid rain which causes corrosion to metal structures, buildings and statues made of carbonate rocks, damage to aquatic organisms. Pollutes crops and water supplies, irritates lungs, throats and eyes.Sources: reaction of nitrogen with oxygen in car engines and high temperature furnaces and as a product of bacterial action in soil.Adverse effects: acid rain with similar effects as SO₂ as well as producing photochemical smog and breathing difficulties, in particular for people suffering from asthma.Sources: old water pipes, old paints, petrol in some kinds of racing cars and from very old engines.Adverse effects: causes significant damage to the central nervous system, young infants are particularly susceptible to lead poisoning.

- These compounds (NO and NO₂) are formed when nitrogen and oxygen react in the high pressure and temperature conditions of internal combustion engines and blast furnaces.

- Exhaust gases also contain unburned hydrocarbons and carbon monoxide.

- Cars are fitted with catalytic converters which form a part of their exhaust systems.

- Their function is to render these exhaust gases harmless.

- They contain a series of transition metal catalysts including platinum and rhodium.

- The metal catalysts are in a honeycomb within the converter to increase the surface area available for reaction.

- A series of redox reactions occurs which neutralises the pollutant gases.

Carbon monoxide is oxidised to carbon dioxide:

2CO + O₂ → 2CO₂

Nitrogen oxides are reduced to N₂ gas:

2NO → N₂ + O₂

2NO₂ → N₂ + 2O₂

Unburned hydrocarbons are oxidised to carbon dioxide and water:

C₈H₁₈ + 12½O₂ → 8CO₂ + 9H₂O

Both oxygen and water.

Rust can be prevented by coating iron with barriers that prevent the iron from coming into contact with water and oxygen.

However, if the coatings are washed away or scratched, the iron is once again exposed to water and oxygen and will rust.

Iron can be prevented from rusting using the reactivity series.

Galvanising is a process where the iron to be protected is coated with a layer of zinc. ZnCO₃ is formed when zinc reacts with oxygen and carbon dioxide in the air and protects the iron by the barrier method.

If the coating is damaged or scratched, the iron is still protected from rusting by the sacrificial method (magnesium can also be used). This is because zinc is more reactive than iron and so it loses its electrons more readily:

Zn → Zn²⁺ + 2e⁻

The iron stays protected as it accepts the electrons released by zinc, remaining in the reduced state and thus it does not undergo oxidation. The electrons donated by the zinc react with hydrogen ions in the water producing hydrogen gas:

2H⁺ + 2e⁻ → H₂

Zinc therefore reacts with oxygen and water and corrodes instead of the iron.

Fertilisers contain nitrogen, potassium and phosphorus.

Nitrogen promotes healthy leaves, potassium promotes growth and healthy fruit and flowers and phosphorus promotes healthy roots.

Fertiliser compounds contain the following water-soluble ions: Ammonium ions, and nitrate ions, are sources of soluble nitrogen. Phosphate ions, are a source of soluble phosphorus. Most common potassium compounds dissolve in water to produce potassium ions.

- Ammonia can be displaced from its salts by the addition of an alkali substance.

- Farmers regularly add basic substances such as calcium hydroxide to their soil to neutralise any excess soil acidity.

- If too much of the basic substance is added or if it has been added too soon after fertiliser has been added, then an ammonia displacement reaction may occur.

- This involves the loss of nitrogen from the fertiliser, nullifying its effectiveness as a fertiliser.

Ammonium chloride + calcium hydroxide ---> calcium chloride + ammonia + water

Ammonia is manufactured using which occurs in five stages:

Stage 1: H₂ and N₂ are obtained from natural gas and the air respectively and are pumped into the compressor through pipes.

Stage 2: The gases are compressed to about 200 atmospheres inside the compressor.

Stage 3: The pressurised gases are pumped into a tank containing layers of catalytic iron beads at a temperature of 450°C. Some of the hydrogen and nitrogen react to form ammonia:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Stage 4: Unreacted H₂ and N₂ and product ammonia pass into a cooling tank. The ammonia is liquefied and removed to pressurised storage vessels.

Stage 5: the unreacted H₂ and N₂ gases are recycled back into the system and start over again.

- A higher temperature would favour the reverse reaction as it is endothermic (takes in heat) so a higher yield of reactants would be made.

- If a lower temperature is used it favours the forward reaction as it is exothermic (releases heat) so a higher yield of products will be made.

- However at a lower temperature the rate of reaction is very slow.

- So 450ºC is a compromise temperature between having a lower yield of products but being made more quickly.

- A lower pressure would favour the reverse reaction as the system will try to increase the pressure by creating more molecules (4 molecules of gaseous reactants) so a higher yield of reactants will be made.

- A higher pressure would favour the forward reaction as it will try to decrease the pressure by creating less molecules (2 molecules of gaseous products) so a higher yield of products will be made.

- However high pressures can be dangerous and very expensive equipment is needed.

- So 200 atm is a compromise pressure between a lower yield of products being made safely and economically.

- When shortwave radiation from the sun strikes the Earth's surface it is absorbed and re-emitted from the surface of the Earth as infrared radiation.

- However much of the I.R. energy is trapped inside the Earth's atmosphere by Greenhouse gases which can absorb and hold the radiation.

- Two such gases are carbon dioxide and methane.

- They both lead to climate change as they trap heat energy from escaping the Earth's atmosphere, leading to global warming.

- a product of complete combustion of carbon-containing substances

- a product of respiration

- a product of the reaction between an acid and a carbonate

- from the thermal decomposition of a carbonate

Sources: digestive processes of animals, decomposition of vegetation, bacterial action in swamps and in rice paddy fields.