GCSE Chemistry revision

Potable water

Using the Earth's resources and obtaining potable water

AQA 4.10.1.2 Foundation & Higher

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Potable water is water that is safe to drink, but unlike chemically pure water (which contains only H₂O molecules), it may contain dissolved substances.
For water to be considered potable, it must have low levels of dissolved substances, a pH between 6.5 and 8.5, and no harmful microorganisms such as bacteria or fungi.

1. What Is Potable Water? AQA

Potable water is water that is safe to drink, but unlike chemically pure water (which contains only H₂O molecules), it may contain dissolved substances.
For water to be considered potable, it must have low levels of dissolved substances, a pH between 6.5 and 8.5, and no harmful microorganisms such as bacteria or fungi.

Fresh water is water that contains very little dissolved material, and in the UK it is largely sourced from rainfall.
Surface water sources include lakes, rivers, and reservoirs — bodies of water exposed at the surface that are easily accessible and regularly replenished by rain.
Groundwater sources are found underground, such as aquifers, which are areas of permeable rock that trap and store water.
Surface water can dry up in hot, sunny conditions due to evaporation, so warmer parts of the UK often rely more heavily on groundwater.

2. Fresh Water Sources AQA

Fresh water is water that contains very little dissolved material, and in the UK it is largely sourced from rainfall.
Surface water sources include lakes, rivers, and reservoirs — bodies of water exposed at the surface that are easily accessible and regularly replenished by rain.
Groundwater sources are found underground, such as aquifers, which are areas of permeable rock that trap and store water.
Surface water can dry up in hot, sunny conditions due to evaporation, so warmer parts of the UK often rely more heavily on groundwater.

Fresh water must be treated before it is safe to drink, typically through a series of three key steps.
The first step involves passing water through a wire mesh screen to remove large debris such as twigs and leaves.
The second step passes the water through a bed of sand and gravel, which filters out smaller solid particles.
The final step is sterilisation, which kills harmful microorganisms and can be achieved by bubbling chlorine gas through the water, or by exposing it to ozone or ultraviolet (UV) light.

3. Treating Fresh Water AQA

Fresh water must be treated before it is safe to drink, typically through a series of three key steps.
The first step involves passing water through a wire mesh screen to remove large debris such as twigs and leaves.
The second step passes the water through a bed of sand and gravel, which filters out smaller solid particles.
The final step is sterilisation, which kills harmful microorganisms and can be achieved by bubbling chlorine gas through the water, or by exposing it to ozone or ultraviolet (UV) light.

Countries with limited rainfall, such as those in the Middle East, lack sufficient fresh water supplies and must instead extract potable water from seawater.
Desalination is the process of removing dissolved salts and other substances from seawater to produce potable water.
Although seawater is abundant, desalination is expensive and energy-intensive, making it impractical for producing large quantities of water.

4. Why Some Countries Use Desalination AQA

Countries with limited rainfall, such as those in the Middle East, lack sufficient fresh water supplies and must instead extract potable water from seawater.
Desalination is the process of removing dissolved salts and other substances from seawater to produce potable water.
Although seawater is abundant, desalination is expensive and energy-intensive, making it impractical for producing large quantities of water.

In distillation-based desalination, large quantities of salty seawater are boiled so that water evaporates, leaving the dissolved salts behind.
The water vapour is then condensed and collected, producing pure distilled water that is safe to drink.

In reverse osmosis, salty water is forced through a partially permeable membrane that only allows water molecules to pass through.
Dissolved ions and larger molecules are unable to pass through the membrane and are therefore separated from the water, leaving pure water on the other side.
Like distillation, reverse osmosis requires a significant amount of energy, contributing to its high cost.

6. Desalination by Reverse Osmosis AQA

In reverse osmosis, salty water is forced through a partially permeable membrane that only allows water molecules to pass through.
Dissolved ions and larger molecules are unable to pass through the membrane and are therefore separated from the water, leaving pure water on the other side.
Like distillation, reverse osmosis requires a significant amount of energy, contributing to its high cost.

Treating fresh water is generally cheaper and less energy-intensive than desalination, making it the preferred method where fresh water sources are available.
Desalination methods (distillation and reverse osmosis) both produce very pure water but are costly due to their high energy demands.
The choice of water treatment method depends largely on the availability of fresh water in a given region.

7. Comparing Water Treatment Methods AQA

Treating fresh water is generally cheaper and less energy-intensive than desalination, making it the preferred method where fresh water sources are available.
Desalination methods (distillation and reverse osmosis) both produce very pure water but are costly due to their high energy demands.
The choice of water treatment method depends largely on the availability of fresh water in a given region.