Water

Water

1. Summary of water treatment

   • 1. Aeration
     • Oxygen is added.
     • Removes iron(III) irons.
   • 2. Sedimentation
     • Larger solid particles are allowed to settle at the bottom of the sedimentation tanks.
     • Aluminium sulphate is added.
     • Coagulates the clay.
   • 3. Filtration
     • Filter beads can contain carbon slurry.
     • Removes substances that would give the water an odd smell or taste.
   • 4. Disinfection (sterilisation)
     • Chlorine is added.
       • Kills bacteria to prevent diseases.
     • Sulphur dioxide is added.
       • Removes excess chlorine.

2. Hard water

   • Advantages
     • Some people prefer the taste.
     • Calcium in the water is good for children's teeth and bones.
     • Helps to reduce heart illness.
     • Some brewers like hard water for making beer.
     • A coating of scale inside copper or lead pipes stops poisonous salts dissolving into our water.
   • Disadvantages
     • Difficult to form lather with soap.
     • Scum (calcium stearate) forms in a reaction, which wastes soap.
     • Scale forms inside kettles, which wastes energy when you boil your kettle.
     • Hot water pipes 'fur up' on the inside. The scale formed can even block up pipes completely.
   • Calcium or magnesium compounds dissolved in water make it hard.
     • Some compounds dissolve when water is flowed through rocks such as chalk or limestone (calcium carbonate) or gypsum ( calcium sulphate)

3. Removing hardness

   • 3. Temporary hardness
     • When boiled, the calcium and hydrogen carbonate ions react, turning back into the insoluble calcium carbonate.
     • Calcium ions are removed from the water.
     • Softened by boiling.
   • 1. Washing soda (sodium carbonate)
     • Calcium ions react with carbonate ions from the washing soda.
     • Forms insoluble calcium carbonate precipitate.
   • 2. Ion-exchange column
     • More suitable for large-scale treatment of hard water.
     • Ion-exchange resin contains sodium ions, which take the place of calcium ions. Sodium ions, which don't cause hardness, come out in the water at the bottm.

4. Desalination

   • Produces usable water from sea water by separating water from the dissolved salts.
   • Flash distillation
     • The pressure above the water is reduced.
     • The boiling point of the sea water is lowered.
   • Reverse osmosis
     • Uses a membrane to separate the water and the salts.
     • No heating involved so it uses less energy than distillation, but energy is still needed to pressurise the water.

5. Solubility is usually measured in grams of solute per 100 grams of solvent at a given temperature.

   • A saturated solution can dissolve no more solute at a given temperature.

6. The solubility of gases in water:

   • Decreases as temperature increases (e.g. at higher temperatures less oxygen can dissolve in water).
   • Increases as pressure increases (e.g. carbonated drinks contain carbon dioxide dissolved under high pressure and the dissolved gas comes out of the solution as bubbles at ordinary pressure.