C11. Air & Water

Question 1

Composition of clean, dry air

Answer 1

78% nitrogen, 21% oxygen, 1% mixture of noble gases and carbon dioxide

Question 2

Fractional distillation of liquid air

Answer 2

1. Air is pumped into the plant and filtered to remove dust particles.
2. Water vapour, carbon dioxide and pollutants are removed.
3. Air is compressed and then cooled.
4. The cold, compressed air is passed through a jet, into a larger space. It expands rapidly and it gets very cold. After several cycles, the air liquifies at -200°C.
5. Liquid air is pumped into the fractionating column and it is warmed up slowly. The gases boil off separately due to their different boiling points. (Nitrogen boils off first.)
6. The gases are collected in tanks or cylinders.

Question 3

Uses of oxygen

Answer 3

1. In oxygen masks or oxygen tents in hospitals, for people with breathing difficulties
2. Used along with the gas acetylene in oxy-acetylene torches for cutting and welding metal; the burning mixture is so hot it can melt steel.
3. As an oxygen supply for astronauts and deep-sea divers
4. In steel works, oxygen is used to convert impure iron from the blast furnace into steels

Question 4

Uses of nitrogen

Answer 4

1. Inside food packaging, to remove oxygen and keep the food fresh
2. To freeze food, and keep containers of food frozen during transport
3. To freeze liquid in cracked pipes, allowing the pipes to be repaired
4. To store tissue samples in hospitals

Question 5

Carbon monoxide (a colourless gas with no smell) - source and harmful effect

Answer 5

Source: from the incomplete combustion of carbon-containing substances, eg. petrol in car engines
Effect: it binds to haemoglobin in blood, and prevents it from carrying oxygen to the body cells, so you can die from oxygen starvation

Question 6

Sulfur dioxide (acidic gas with a sharp smell) - source and harmful effect

Answer 6

Source: from the combustion of fossil fuels which contain sulfur compounds
Effect: causes respiratory problems, dissolves in rain to form acid rain, which damages crops and trees, lowers the pH in rivers and lakes, kills fish, and corrodes buildings and metal bridges

Question 7

Oxides of nitrogen (acidic gases) - source and harmful effect

Answer 7

Source: from hot car engines, and hot furnaces
Effect: causes respiratory problems, dissolves in rain to from acid rain

Question 8

Lead compounds - source and harmful effect

Answer 8

Source: leaded petrol (lead compounds were once added to petrol to help it burn smoothly)
Effect: causes brain damage, harms the body’s nervous system and damages the kidneys

Question 9

Chemical tests for water

Answer 9

1. Anhydrous copper (II) sulfate: changes from blue to white

2. Anhydrous cobalt (II) chloride: changes from blue to pink

Question 10

What are the 2 main processes involved in the treatment of the water supply?

Answer 10

1. Filtration to remove suspended solid particles

2. Chlorination to kill bacteria and other microbes

Question 11

Haber process: From where is nitrogen obtained?

Answer 11

From the air. Oxygen is removed by burning hydrogen in air (forming water), leaving nitrogen behind

Question 12

Haber process: From where is hydrogen obtained?

Answer 12

1. From the reaction between methane and steam (giving carbon dioxide and hydrogen)
2. From the cracking of hydrocarbons from petroleum (ethane –> ethene + hydrogen)
   * Both reactions require catalysts

Question 13

Haber process: Optimum conditions

Answer 13

To improve the yield:
1. Quite high pressure of 200 atmospheres
2. Remove ammonia to prevent it from breaking down to nitrogen and hydrogen
*Recycle unreacted gases to give another chance to react at the catalyst so that overall yield improves
To get a decent reaction rate:
3. Moderate temperature of 450°C
4. Use a catalyst (iron)

Question 14

Uses of nitrogen (plants)

Answer 14

1. Make proteins

2. Make chlorophyll

Question 15

Uses of potassium (plants)

Answer 15

1. Make proteins

2. Resist disease

Question 16

Uses of phosphorus (plants)

Answer 16

1. Help roots to grow
2. Help crops to ripen
3. Improve crop yields

Question 17

Formation of carbon dioxide

Answer 17

1. As a product of complete combustion of carbon-containing substances
2. As a product of respiration
3. As a product of the reaction between an acid and a carbonate
4. From the thermal decomposition of a carbonate (except sodium and potassium carbonates)

Question 18

Describe the formation of oxides of nitrogen and suggest how they can cause acid rain.

Answer 18

Nitrogen and oxygen from the air react together due to high temperature in the car engine to form nitrogen oxides that react with rainwater to form acid rain.

Question 19

3 industrial uses of water

Answer 19

1. As a solvent
2. Generating electricity
3. Contact process

Question 20

Four sources of methane

Answer 20

1. Animal waste
2. Decomposing vegetation
3. Decaying organic matter
4. Fractional distillation of petroleum

Question 21

Reasons why levels of carbon dioxide are rising

Answer 21

1. Combustion of fossil fuels and other carbon-based fuels in power stations, factories, car engines and homes
2. Cement production; limestone breaks down, giving off CO2

Question 22

Reasons why levels of methane are rising

Answer 22

1. Livestock farming

2. Putting decaying organic matter in landfill sites (methane forms as buried material rots)

Question 23

Why should carbon monoxide not be released into the atmosphere?

Answer 23

Carbon monoxide is toxic / poisonous