Finite and renewable resources, sustainable development, how drinking water is made from fresh and salty water, and how sewage is treated.
Objectives
Tell finite resources from renewable ones, and say what sustainable development means.
Explain the difference between potable water and pure water, and describe how potable water is made from fresh water and from sea water.
Describe the stages of sewage treatment.
Describe how to test the pH and dissolved solids of a water sample and how to purify it by distillation.
Specification references this topic covers: AQA 5.10.1; Edexcel C2.11, C2.12.
Earth's resources, potable water and waste water explained
Start here
Everything we use comes from the Earth, the oceans or the air. Some of it will run out. Some of it can be replaced. Chemists help us to use these resources carefully and to make clean water to drink.
This matters because there is a limited supply of fresh water and many materials. Treating water properly stops disease. Using resources wisely leaves enough for people in the future.
By the end you will be able to sort resources, describe how drinking water is made, and describe how sewage is treated.
Finite and renewable resources
A finite resource is one that will run out because it takes millions of years to form. Crude oil, coal, natural gas and metal ores are finite. A renewable resource can be replaced as fast as we use it. Wood from replanted forests and crops grown on farms are renewable.
Natural resources are supplemented by farming. Some natural products are also replaced by synthetic ones. For example, cotton and wool are supplemented or replaced by synthetic fibres such as polyester, and natural rubber by synthetic rubber. Fertilisers help farms to produce more food.
Sustainable development means meeting the needs of people today without stopping future generations from meeting their own needs. Using less, reusing and recycling are part of it.
Potable water
Water that is safe to drink is called potable water. To be potable, drinking water needs low levels of dissolved salts and microbes.
Potable water is not the same as pure water. In chemistry, pure water is only H₂O. Tap water is potable but it contains dissolved substances, so it is a mixture. A chemist would not call it pure.
The way a town makes potable water depends on what water is available. In the UK, rain collects in the ground, lakes and rivers. This is fresh water, with low levels of dissolved salts, so it is the usual source.
Treating fresh water
Water from a river or reservoir is treated in stages. First choose a suitable source of fresh water. Let large particles settle out in a tank. This is sedimentation. Then pass the water through filter beds of sand and gravel to remove solid particles.
Finally sterilise the water to kill harmful microbes. Chlorine, ozone and ultraviolet light can all be used as sterilising agents. Chlorination is the use of chlorine.
Each stage has a reason. Settling and filtering remove solids. Sterilising kills bacteria and viruses, which prevents diseases such as cholera. Fresh water treated like this is the easiest way to make potable water.
A flow of five stages for making potable water from fresh water: choose a fresh water source, sedimentation so solids settle, filter beds to remove solids, sterilise with chlorine, ozone or ultraviolet light, then potable water.
Salty water
If there is not enough fresh water, salty water or sea water can be used. Removing the salt is called desalination. Two ways are distillation and reverse osmosis.
In distillation the sea water is boiled. The water turns to steam, leaving the salts behind. The steam is cooled in a condenser and collected as pure water. In reverse osmosis the sea water is forced under pressure through a membrane that lets water through but not the dissolved salts.
Both ways need large amounts of energy, so they are expensive. This is why most potable water in the UK comes from fresh water instead. Ground water and rivers are the easiest sources to turn into potable water. Sea water and waste water both need much more treatment.
Waste water treatment
Homes, farms and factories produce waste water. It must be treated before it goes back into rivers or the sea. Sewage and farm waste water contain organic matter and harmful microbes. Industrial waste water may also contain harmful chemicals.
Sewage treatment has four main stages. First, screening and grit removal take out large objects and grit. Second, sedimentation lets solids settle to make sewage sludge, leaving a liquid called effluent. Third, anaerobic digestion breaks down the sludge using bacteria without oxygen. Fourth, aerobic biological treatment breaks down the organic matter in the effluent using bacteria and air.
Treated effluent can then be released safely. The gas from anaerobic digestion can be used as a fuel.
Testing and purifying water
Water samples from different sources can be tested. Use a pH probe or universal indicator to find the pH. To find the dissolved solids, weigh an empty evaporating basin, add a measured volume of the water, heat gently until all the water has gone, then weigh the basin again. The extra mass is the dissolved solids. Divide it by the volume to get a concentration.
To purify a sample, use distillation. Heat the sample in a flask with a condenser and collect the distilled water. Distilled water is a good control because it has no dissolved salts. Water used in chemical analysis must not contain dissolved salts, or the salts would affect the results.
Safety matters: wear eye protection, do not heat the basin until it is dry and spitting, and take care with hot glass.
Recap
Finite resources run out. Renewable resources can be replaced. Sustainable development meets today's needs without harming the future.
Potable water is safe to drink. It is not pure. Fresh water is treated by sedimentation, filtration and sterilisation. Salty water needs distillation or reverse osmosis, which use a lot of energy.
Sewage treatment: screening and grit removal, sedimentation, anaerobic digestion of sludge, aerobic treatment of effluent.
Key points
Finite resources run out (crude oil, coal, ores). Renewable resources can be replaced.
Potable water is safe to drink but is not pure. Pure water is only H₂O.
Fresh water: sedimentation, filter beds, then sterilise (chlorine, ozone or UV).
Sea water: distillation or reverse osmosis (large energy use).
Sewage: screening and grit removal, sedimentation, anaerobic digestion of sludge, aerobic treatment of effluent.
Worked examples
Example 1
What is the difference between potable water and pure water?
Potable water is safe to drink.
It contains dissolved salts and other substances, so it is a mixture.
Pure water is only H₂O with nothing else dissolved in it.
Answer: Potable water is safe to drink but contains dissolved substances. Pure water is only H₂O
Common slips
Slip: They are the same thing. Potable water contains dissolved substances. Pure water does not.
Slip: Pure water is always safe to drink. Pure in chemistry means a single substance. It says nothing about safety.
Slip: Potable water has been boiled. Potable water is made by treating the water, for example filtering and sterilising.
Example 2
A sample of tap water has a volume of 100 cm³. It is evaporated and leaves 0.060 g of solid. Calculate the mass of dissolved solids in g per dm³.
Convert the volume: 100 cm³ = 0.100 dm³.
Concentration = mass ÷ volume.
0.060 ÷ 0.100 = 0.60 g/dm³.
Answer: 0.60 g/dm³
Common slips
Slip: 0.060 × 100 = 6.0 g/dm³ Do not multiply by the cm³. Change the volume to dm³ and divide.
Slip: 0.060 ÷ 100 = 0.00060 g/dm³ Dividing by 100 uses cm³. The unit is g per dm³, so use 0.100 dm³.
Slip: 0.60 g The unit is a concentration, g per dm³.
Example 3
A town has a river near it and a coast 20 km away. Explain why most of its drinking water is made from river water and not from sea water.
River water is fresh water, so it has low levels of dissolved salts.
It only needs sedimentation, filtering and sterilising.
Sea water would need distillation or reverse osmosis to remove the salt.
These methods need large amounts of energy, so they are much more expensive.
Answer: River water is fresh and needs only filtering and sterilising, but sea water needs distillation or reverse osmosis which use a lot of energy
Common slips
Slip: Sea water is too clean. Sea water is salty and not safe to drink.
Slip: River water is already pure. River water has microbes and dissolved substances, so it must still be treated.
Slip: Distillation is cheap. Distillation uses a lot of energy to boil the water, so it is expensive.
Slip: River water needs desalination. Desalination is for sea water. River water is fresh water.
Questions and answers
Earth's resources, potable water and waste water practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
Water that is safe to drink is called pure or potable?
Show answer
Answer: potable
Safe to drink water has a special name.
It is potable.
Question 2 Easy
Name one substance used to sterilise drinking water.
Show answer
Answer: chlorine
Sterilising agents kill microbes.
Chlorine, ozone and ultraviolet light are used.
Question 3 Easy
Is crude oil a finite or renewable resource?
Show answer
Answer: finite
Crude oil takes millions of years to form.
It is finite.
Question 4 Easy
Is wood from a replanted forest a finite or renewable resource?
Show answer
Answer: renewable
Trees can be replanted and regrow.
It is renewable.
Question 5 Easy
What part of water treatment removes solid particles: filter beds or sterilising?
Show answer
Answer: filter beds
Solids are caught by sand and gravel.
This is done by filter beds.
Question 6 Medium
Name one process that removes salt from sea water by boiling and condensing.
Show answer
Answer: distillation
Boil the water and condense the steam.
This is distillation.
Question 7 Medium
Name the process that uses a membrane and pressure to remove salt from sea water.
Show answer
Answer: reverse osmosis
A membrane lets water through but not salts.
This is reverse osmosis.
Question 8 Medium
Does desalination need large or small amounts of energy?
Show answer
Answer: large
Boiling and forcing water through membranes both take energy.
The amounts are large.
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 14 marks
(a) Explain the difference between pure water and potable water. [2 marks] (b) Name two of the stages used to make potable water from fresh water. [2 marks]
Show answer and marking points
Answer: (a) pure water is only H₂O but potable water is safe to drink and contains dissolved substances (b) filtering and sterilising
(a) pure water contains only H₂O (one substance) (1)
(a) potable water is safe to drink but contains dissolved substances (1)
(b) first stage, for example filtration or sedimentation (1)
(b) second stage, for example sterilising with chlorine, ozone or UV (1)
Exam-style question 25 marks
(a) Describe how sea water can be made into potable water by distillation. [3 marks] (b) Give one reason why distillation is not used to make most of the potable water in the UK. [1 mark] (c) Name another method of removing salt from sea water. [1 mark]
Show answer and marking points
Answer: (a) heat the sea water so it boils, the steam passes into a condenser where it cools and condenses, the water is collected and the salts are left behind (b) it uses a large amount of energy so it is expensive (c) reverse osmosis
(a) sea water is boiled or heated (1)
(a) steam is cooled or condensed (1)
(a) pure water collected and salts left behind (1)
(b) it needs large amounts of energy or is expensive (1)
(c) reverse osmosis (1)
Common mistakes
Saying potable water is pure water
Potable water is safe to drink but contains dissolved substances. Pure water is only H₂O.
Saying filtering removes dissolved salts
Filter beds remove solid particles. Dissolved salts need distillation or reverse osmosis.
Mixing up anaerobic and aerobic
Anaerobic means without oxygen (sludge). Aerobic means with oxygen (effluent).
Saying chlorine is added to filter the water
Chlorine is added to sterilise the water, which means killing microbes.
Forgetting why desalination is not used more
It needs large amounts of energy, so it is expensive.
Questions students ask
Is Earth's resources, potable water and waste water on the Foundation or Higher tier?
Earth's resources, potable water and waste water is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 4.
What is a common mistake on Earth's resources, potable water and waste water?
Saying potable water is pure water. Potable water is safe to drink but contains dissolved substances. Pure water is only H₂O.
Where are the answers to the Earth's resources, potable water and waste water questions?
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.