IGCSE science Chemistry study notes

C11- Air and water

Describe chemical tests for water using cobalt(II) chloride and copper(II) sulfate

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.

Discuss the implications of an inadequate supply of water, limited to safe water for drinking and water for irrigating crops

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.

Describe, in outline, the treatment of the water supply in terms of filtration and chlorination

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.

Name some of the uses of water in industry and in the home

Water in industry

- 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).

Water in homes

- Drinking, cooking and washing.

- General sanitation.

- In car radiators, for gardens and plants.

State the composition of clean, dry air as being approximately 78% nitrogen, 21% oxygen and the remainder as being a mixture of noble gases and carbon dioxide

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

Describe the separation of oxygen and nitrogen from liquid air by fractional distillation

- 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.