What is potable water?

Water that is safe to drink.

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What are the 3 criteria for water to be considered potable?

  1. Levels of dissolved substances (e.g. salts) must be fairly low.
  2. The pH must be between 6.5 and 8.5.
  3. There must be no microorganisms.

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Is potable water the same as pure water?

No, pure water must only contain water molecules, whereas potable water can contain dissolved substances that are not water (e.g. chlorine).

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What is surface water?

Surface water is water that is exposed to the air.


It includes lakes, rivers, and reservoirs. 

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What is ground water?

Ground water is water that is found under the ground, such as in aquifers.

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What are the benefits of relying on fresh water?

  1. It is easy to access.
  2. It is replaced frequently.
  3. It naturally has low levels of dissolved substances. 

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Where does the UK get most of its potable water?

From freshwater sources, such as surface and ground waters, which are supplied by rainwater.

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Outline the 3 stages of treating freshwater.

  1. Pass the water through a wire mesh. This will filter out any large objects like plastic bottles or leaves.
  2. Pass the water through a bed of sand and gravel. This will filter out smaller things like bits of rock.
  3. Sterilise the water to kill any microorganisms. There are three different ways to do this:

1) bubbling chlorine gas through it

2) exposing it to ozone

3) exposing it to ultraviolet radiation

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What is desalination?

The extraction of potable water from sea water.

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When is desalination required?

When supplies of fresh water are limited.

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What are the 2 methods of desalination?

  1. Distillation.
  2. Processes that use membranes (e.g. reverse osmosis).

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What is the disadvantage of desalination?

Desalination is very expensive, as it requires large amounts of energy.

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Outline the process of using membranes in desalination.

  1. Salty water is passed through a membrane so only water molecules can pass through.
  2. Ions and larger molecules are trapped and separated.

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How does desalination using distillation work?

  1. Test the pH of the water using a pH meter - use a titration to neutralise the water if it isn't already neutral.
  2. Test the water for sodium chloride (flame test and halogen test).
  3. Do simple distillation to collect the pure water.
  4. Retest the water to ensure sodium chloride has been removed, and that the pH is neutral (pH 7).

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Why is sea water not considered potable?

It has large amounts of dissolved solids.

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Why is chlorine added to water?

To kill microorganisms/bacteria.

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