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Changing states and the water cycle

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Obtaining potable water

Potable water is water that is safe to drink.  It is not 100% pure, as this would mean that it contains H2O only, and it usually contains other dissolved substances too.  

The criteria that must be met for water to be potable include: 

  • Low levels of dissolved substances 
  • A pH of 6.5-8.5 
  • No microorganisms  

To achieve this, potable water can be obtained from fresh water. This is either surface water (water from lakes, rivers or reservoirs) or ground water, which comes from rocks underground. The surface water is replenished with rain and is easily accessible, and so is used often.  

To make freshwater potable, it is first passed through a wire filter to remove larger items that are trapped in it. It is then filtered through a bed of sand and gravel to remove the smaller particles, and is sterilised with either ultraviolet light, ozone or chlorine to remove any microorganisms that could make humans ill.  

However, in areas that experience frequent droughts, desalination of sea water must be performed. This means that the sodium chloride in the sea is removed to make the water potable. However, this is very energy expensive and costly compared to using fresh water as a source. 

To perform desalination, reverse osmosis and distillation can be undertaken. 

Distillation uses changing states to make sea water potable. The sea water is heated to generate water vapour via evaporation and the sodium chloride remains in the liquid. The water vapour can therefore be condensed back into liquid form, and this will no longer contain salt.  

In reverse osmosis, pressure is applied to the water and it is passed through a membrane which contains pores. The pores prevent salt from passing through, but the water molecules can. Therefore, the water is desalinated.