Potable Water (OCR Gateway GCSE Chemistry: Combined Science)

Fit for Human Consumption

• Potable water is water that has been processed and is safe for human consumption and daily use
• The difference between pure water and potable water is that pure water is solely made up of H₂O molecules, whereas potable water may contain different substances, usually dissolved minerals and salts
• Potable water should have the following characteristics:
  • Have a pH between 6.5 and 8.5.
  • The dissolved substances (e.g. salts) will be present is very small regulated quantities
  • Be free of bacteria or potentially harmful microbes

• Water is considered fresh when it is relatively free from dissolves substances e.g. rainwater
• Water can collect in reservoirs, lakes and rivers and is known as surface water
• In addition, it can collect in aquifers which are porous rocks that store water underground.
• This water is called groundwater

Water Treatment

• The origin of fresh water depends on the climate in the region in question
• In warmer areas, such as in the south-east of England, water primarily dries up before it can be collected so is found underground
• Despite being relatively low in dissolved substances, there is still a process in place to ensure it is safe and suitable for use.
• Two important steps in this process are:
  • Filtration:
    • Debris such as pieces of soil and dirt, small pebbles, twigs etc. are removed by a wire mesh screen
    • After this, other debris is filtered through sand beds and gravel

  • Sterilisation:
    • Ultraviolet light and ozone is used to sterilise water or alternatively chlorine gas is bubbled through the water
    • This removes any dangerous bacteria or microbes

• Where aquifers are not present and/or the collection of surface water is limited, the process of desalination must be used to provide potable water to the population
• Desalination involves the treatment of seawater to remove the salt by distillation or reverse osmosis, a process that involves the use of membranes
• When salt water is put through a semi-permeable membrane, only water molecules can pass through it. This happens as the membrane stops larger molecules and ions passing through
• Desalination is an expensive process as it consumes large amounts of energy and is not ideal when producing large quantities of fresh water
• This is used in regions with a very hot climate such as Saudi Arabia