water

• water of appropriate quality is essential for life

  for humans, drinking water should have sufficiently low levels of dissolved salts and microbes
• water that is safe to drink is called potable water
• potable water is not pure water

  it contains dissolved substances (small amounts)
• In the UK, rain provides water

  with low levels of dissolved substances

  it collects in the ground/lakes/rivers

  FRESH water
• to produce potable water:

  • choose an appropriate source of fresh water (e.g. river)
  • pass the water through filter beds (removes material like leaves)
  • sterilise water to kill microbes (by chlorine / ozone / UV light)
• sterilising agents for potable water:

  • chlorine
  • ozone
  • UV light
• if supplies of fresh water are limited, provide potable water =

  salty water or seawater, which contain high levels of dissolved minerals,

  are desalinated
• desalination reduces the levels of dissolved minerals to an acceptable level for potable water

  can be done via distillation

  or passing the water through membranes - reverse osmosis

  these requires large amounts of energy which makes them expensive
• pure water contains no dissolved solids and has a ph 7 (neutral)

  potable water contains dissolved solids