Revision notes for AQA GCSE Chemistry Potable water. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Potable water
What you'll learn
- What potable water means, and why it is not the same as pure water.
- How fresh water is treated in the UK using filter beds and sterilisation.
- Why salty water needs desalination, such as distillation or reverse osmosis.
- How to analyse water samples for pH, dissolved solids, and purity in the required practical.
Why water quality matters
Water is essential for life, but not all water is safe to drink. Drinking water must contain only low levels of dissolved salts and low levels of microbes.
A substance is dissolved when its particles are spread throughout a liquid. For example, sodium chloride can dissolve in water to make salty water.
Microbes are tiny living organisms, such as bacteria, that may cause disease if present in drinking water.
Definition
Potable water
Potable water is water that is safe to drink because it has sufficiently low levels of dissolved salts and microbes.
Potable water is about safety, not chemical purity.
Definition
Pure water
Pure water, in the chemical sense, contains only water molecules and no dissolved substances.
So bottled tap water can be potable without being pure. It usually contains small amounts of dissolved minerals, but it is still safe to drink.
Common Mistake
Potable does not mean pure
Do not write that potable water contains “nothing except water”. Potable water can contain dissolved substances; they just need to be at safe levels.
Fresh water and water sources
Definition
Fresh water
Fresh water is water with low levels of dissolved salts. It is found in places such as rivers, lakes and groundwater.
In the UK, rainwater collects in rivers, lakes and underground rocks. Because this water is already low in dissolved salts, most UK potable water is made by treating fresh water, not sea water.
The method used to produce potable water depends on:
- the water source available
- the amount of dissolved salts
- the amount of insoluble solids, such as mud or grit
- whether harmful microbes may be present
- local conditions, such as cost, energy supply and climate
Key Idea
Treatment depends on the starting water
Fresh water usually needs filtering and sterilising. Salty water needs desalination first because filtration cannot remove dissolved salts.
Producing potable water from fresh water in the UK
In the UK, potable water is usually produced by three main stages:
- Choosing an appropriate source of fresh water.
- Passing the water through filter beds.
- Sterilising the water.
1. Choosing an appropriate source
An appropriate water source should have low levels of harmful substances and low levels of dissolved salts. A clean reservoir or groundwater source is easier and cheaper to treat than polluted water.
Groundwater is water stored underground in rocks. It may already have passed through layers of rock, so it can contain fewer insoluble particles than river water. However, it may still contain dissolved minerals.
2. Filtering through filter beds
Definition
Filter bed
A filter bed is a layer of material, often sand and gravel, that traps insoluble solid particles as water passes through it.
An insoluble substance does not dissolve in water. Mud, grit and small pieces of plant material are examples of insoluble solids that can be removed by filtration.
Filtering makes the water clearer, but it does not reliably kill microbes and it does not remove dissolved salts.
Common Mistake
Filtering out salt
Simple filtration removes insoluble solids, not dissolved substances. Salt dissolved in water passes through a filter with the water.
3. Sterilising the water
Definition
Sterilisation
Sterilisation is the process of killing microbes in water so that it is safe to drink.
Sterilising agents used for potable water include:
- chlorine
- ozone
- ultraviolet light
Chlorine is commonly used because it kills microbes and can remain in the water in small amounts to help prevent microbes growing again in the pipes. Ozone and ultraviolet light also kill microbes, but they do not stay in the water in the same way.
Example
Choosing the treatment steps
A town uses water from a lake. Tests show the water has low dissolved salt levels, contains mud after heavy rain, and contains bacteria.
- The water can be treated as a fresh water source because the dissolved salt level is low, so desalination is not needed.
- The mud is an insoluble solid, so passing the water through filter beds would remove the mud and other suspended particles.
- The bacteria are microbes, so the filtered water must be sterilised using chlorine, ozone or ultraviolet light before it is potable.
Groundwater compared with salty water
You need to be able to describe the difference in treatment between groundwater and salty water.
Groundwater
Groundwater is usually fresh water. It may need:
- screening or filtering to remove insoluble particles
- sterilisation to kill microbes
It normally does not need desalination unless the dissolved salt level is too high.
Salty water and sea water
Sea water contains high levels of dissolved salts. To make it potable, the salts must be removed by desalination.
Definition
Desalination
Desalination is the removal of dissolved salts from salty water or sea water.
Desalination can be done by:
- distillation
- membrane processes such as reverse osmosis
These processes require large amounts of energy, so they are often used where fresh water supplies are limited.
Distillation
Definition
Distillation
Distillation is a separation process where a liquid is evaporated and then condensed to collect it as a pure liquid.
In desalination by distillation:
- Sea water is heated.
- Water evaporates to form water vapour.
- Dissolved salts are left behind because they do not evaporate with the water.
- The water vapour is cooled and condenses to form liquid water.
Distillation can produce very pure water, but heating large volumes of water uses a lot of energy.
Reverse osmosis
Definition
Reverse osmosis
Reverse osmosis is a desalination process where pressure forces water through a partially permeable membrane, leaving many dissolved salts behind.
A membrane is a thin barrier. A partially permeable membrane allows some particles through but not others. In reverse osmosis, water molecules pass through more easily than dissolved salt ions.
Reverse osmosis still needs a lot of energy because high pressure is required to force water through the membrane.
Tip
Desalination comparison
Distillation needs energy mainly for heating. Reverse osmosis needs energy mainly for pumping water at high pressure.
Required practical: analysing and purifying water samples
In the required practical, you compare water samples from different sources. You may test:
- pH
- amount of dissolved solids
- purity after distillation
Testing pH
Definition
pH
pH is a measure of how acidic or alkaline a solution is. A pH of 7 is neutral at room temperature.
You can test pH using universal indicator paper, universal indicator solution, or a pH probe. Natural water samples may not be exactly pH 7 because they can contain dissolved substances.
Measuring dissolved solids
A dissolved solid is a solid substance dissolved in the water. If you evaporate the water, the dissolved solid is left behind.
A typical method is:
- Measure a known volume of water sample.
- Weigh an empty evaporating basin.
- Add the water sample and gently evaporate the water.
- Heat until dry, then allow the basin to cool.
- Weigh the basin and solid residue.
- Calculate the mass of dissolved solids.
Example
Calculating dissolved solids
A student evaporates 50.0 cm³ of water. The empty evaporating basin has a mass of 32.64 g. After evaporation, the basin and dry solid have a mass of 32.70 g. Calculate the concentration of dissolved solids in grams per cubic decimetre.
-
Find the mass of dissolved solids by subtracting the mass of the empty basin:
mass of solids=32.70 g−32.64 g=0.06 g\text{mass of solids} = 32.70\ \text{g} - 32.64\ \text{g} = 0.06\ \text{g}mass of solids=32.70 g−32.64 g=0.06 g -
Convert the water volume from cm³ to dm³, using 1000 cm³ = 1 dm³:
volume=50.01000 dm3=0.0500 dm3\text{volume} = \frac{50.0}{1000}\ \text{dm}^3 = 0.0500\ \text{dm}^3volume=100050.0 dm3=0.0500 dm3 -
Calculate concentration using concentration=massvolume\text{concentration} = \frac{\text{mass}}{\text{volume}}concentration=volumemass:
concentration=0.06 g0.0500 dm3=1.2 g dm−3\text{concentration} = \frac{0.06\ \text{g}}{0.0500\ \text{dm}^3} = 1.2\ \text{g dm}^{-3}concentration=0.0500 dm30.06 g=1.2 g dm−3
Purifying water by distillation
In the lab, you can purify a water sample using simple distillation. The water evaporates, then condenses in the condenser and is collected as the distillate. The dissolved solids remain as the residue in the flask.
Definition
Distillate and residue
The distillate is the liquid collected after condensation. The residue is the material left behind after the liquid has evaporated.
For safety and accuracy:
- wear eye protection
- do not heat a sealed apparatus
- use anti-bumping granules if instructed
- do not let the flask boil dry
- keep the condenser water flowing, with water entering at the lower end
Common Mistake
Distilled does not automatically mean drinkable in real life
In the school practical, distillation shows purification of water. Real drinking water also needs careful treatment and monitoring to make sure microbes and harmful chemicals are at safe levels.
Reasons for each treatment step
A good exam answer should link each step to its purpose:
- Choose a suitable source: starts with water that is easier and safer to treat.
- Filter: removes insoluble solids such as grit, mud and plant material.
- Sterilise: kills microbes that could cause disease.
- Desalinate salty water: removes dissolved salts that filtration cannot remove.
Exam technique
In the exam
- If asked for the difference between potable and pure water, say: potable is safe to drink; pure contains only water molecules.
- If asked why filtration is used, link it to insoluble solids, not dissolved salts or microbes.
- If the water source is sea water, mention desalination and explain that it needs lots of energy.
Self review
Check yourself
- Why can potable water contain dissolved substances but still be safe to drink?
- What are the purposes of filtration and sterilisation in UK water treatment?
- Why is desalination more energy-demanding than treating fresh water?
Using the Earth's resources and obtaining potable water
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