Chemistry 4H

Chemistry 4H

1. Key Processes

   • The amount of available fresh water is effectively limited, so it is important to conserve water. Over-use of water, especially in dry weather, can lead to water shortages and hosepipe bans. Conserving water also saves the materials and energy needed to treat and transport it
   • Water Pollutants
     • Nitrate fertilisers are very soluble in water and are easily washed off fields by the rain and then into rivers and reservoirs. Because nitrates are all soluble they cannot easily be removed from the water
     • Older houses and water supply systems still rely on lead pipes. Poisonous lead compounds formed on the inside of the pipes slowly dissolve into the water
     • Pesticides used by farmers to kill weeds or insects may be washed or blown into streams and rivers
     • Some soluble substances are not removed by the treatment processes. For example, nitrate fertilisers applied to farmland may be washed into rivers and lakes. Nitrates can be poisonous, particularly for young people or if they are in high concentrations, so care has to be taken to limit nitrates getting into water sources
   • Distillation of sea water
     • It takes a lot of energy to boil water for distillation
     • The fresh water produced is much more expensive than water from traditional sources
     • There are problems disposing of the salt-rich waste water left over by the process

2. There are three stages to water purification

   • 1. Sedimentation (bits drop to the bottom)
   • 2. Filtration using sand (removed fine particles)
   • 3. Chlorination (kills microbes)

3. Lead pipes lead to lead being in the water

4. Nitrates enter the water system from fertiliser run off

5. Pesticides enter the water system from pesticides spraying near water courses

6. Sea water is not used as drinking water as it will be too expensive to purify it

7. To test for the presence of sulphate ions add barium chloride – a positive result wil show a white precipitate

8. Barium chloride + magnesium sulphate --> barium sulphate (white ppt) and magnesium chloride

9. BaCl (aq) + MgSO (aq) --> BaSO (a) + MgCl (aq)

10. To test for the presence of halides add silver nitrate

    • Chloride = white precipitate
    • Bromide = cream precipitate
    • Iodide = yellow precipitate

11. Silver nitrate + sodium chloride --> siver chloride (white ppt) + sodium nitrate

12. NaCl (aq) + AgNO (aq) --> AgCl(a) + NaNO (aq)