Potable Water

• Potable Water

  • water that is safe to drink.
  • Is not pure because still contains dissolved substances.
• For humans, what makes drinking water safe?

  • additions of chlorine
  • resulting in sufficiently low levels of dissolved substances and microbes / pathogens
• Desalination is only used when there is no other source of potable water. Give one reason why

  • high energy requirement
  • very expensive
• The first stage is to filter the water from lakes and rivers.

  Why is the water filtered?

  • to remove (insoluble) solids
• Chlorine gas is then added to the filtered water.

  Why is chlorine gas used to treat water?

  • to sterilise the water and remove harmful microbes
• Test for presence of chlorine

  • Damp litmus paper
  • Bleaches litmus paper / turns white
• What are the two main steps used to treat water from lakes?

  1. Filtration
  2. Sterilisation = to kill microbes
• Which two substances are used to sterilise fresh water?

  • Ozone
  • Chlorine
  • Ultraviolet
• Explain how potable water is produced from fresh water

  1. Choose an appropriate source of fresh water such as lakes or rivers.
  2. Filtration = Pass through filter beds to remove insoluble solids
  3. Sterilise using chlorine / ozone / ultraviolet
  4. which destroys harmful microbes
• 2 processes of desalination

  1. Distillation
  2. Reverse osmosis
• Differences between potable water and pure water

  • It is not chemically pure because it contains dissolved substances/ treated water.
• A different country has:

  • very little rainfall
  • a long coastline
  • plentiful energy supplies.

  Suggest one process this country could use to obtain most of its potable water.

  • Desalination
• Which two methods show the presence of dissolved solids in the sample of drinking water?

  1. Evaporate all the water from the sample
  2. Measure the sample's boiling point (not pure if not 100 degrees)
• How do you distinguish potable water from pure water?

  How could the water be tested to show it is pure?

  1. determine boiling point
  2. should be at a fixed temperature of 100 degrees
• Explain why it is more difficult to produce drinking water from waste water than from water in lakes.


  1. water needs more / different purification processes
  2. because it contains more microbes and more toxic chemicals
• Desalination 


  1. Pure water can be obtained from sea water by desalination
  2. Distillation
  3. Reverse osmosis
• Boiling point of pure water

  100 degrees
• Boiling point of impure water / potable water

  • range of boiling points
  • contains dissolved substances
• State one health benefit of drinking hard water

  • maintain bones
  • maintain teeth
• Why must water undergo sterilization?


  • remove harmful microbes / pathogens
  • that may cause disease and harm to human health
• Analysis of Water

  • pH test
  • mass of dissolved solids
  • boiling points
• Problems with desalination

  • High carbon emissions due to energy demand
  • Salt waste can damage ecosystems
• Problems with chlorine 


  • Toxic gas so needs to be carefully monitored
• Desalination : distillation 


  • boiling seawater creates steam, which then condenses to give pure water
• What is reverse osmosis?

  • Sea water is passed through a partially permeable membrane that only allows through water molecules.
  • Requires high pressure to push the water through the membrane.
  • The high pressure requires a lot of energy to produce.