Describe how fresh water from a river or reservoir is treated to make it potable.

It is passed through a mesh screen and then filtered through beds of sand and gravel to remove solids, and finally sterilised.

Name three methods used to sterilise water.

Adding chlorine, treating with ozone, or passing the water under ultraviolet light.

How can potable water be obtained from sea water?

By desalination, either by distilling the sea water or by forcing it through a membrane under pressure in reverse osmosis.

OCR GCSE Combined Science Chemistry J250

Chemical Reactions

Topic overview Writing formulae of elements and compounds Balanced equations, and deducing formulae from ions Ionic equations, and state symbols The mole and the Avogadro constant Conservation of mass, and changes of mass in open systems Stoichiometry, and calculating reacting masses Exothermic and endothermic reactions, and reaction profiles Activation energy, and bond energy calculations Oxidation and reduction, by oxygen and by electrons Acids, alkalis and neutralisation The ionic equation for neutralisation, and reactions of acids Dilute and concentrated, weak and strong The pH scale, and hydrogen ion concentration pH as a factor of ten, and measuring pH Products at the electrodes, and molten ionic compounds Electrolysis of aqueous solutions Inert and non-inert electrodes

Elements, Compounds and Mixtures

Topic overview Purity, and melting point data Relative formula mass and empirical formulae Formulations, and separating mixtures Paper and thin layer chromatography Interpreting chromatograms and Rf values Choosing a purification method Metals and non-metals in the Periodic Table Electron arrangement and position in the Periodic Table Ionic, covalent and metallic bonding compared Dot and cross diagrams, and the limits of models Reactions, electron arrangement and Mendeleev's table Carbon's four bonds, and the variety of organic compounds Diamond, graphite, fullerenes and graphene Bond strength, intermolecular forces and predicting state Bulk properties of ionic, molecular and giant structures

Global Challenges

Topic overview Extracting metals by reduction and by electrolysis Biological methods of metal extraction Life cycle assessment Recycling, and the decisions behind it Fractional distillation of crude oil The fractions, and crude oil as a feedstock Dependence on hydrocarbons, and cracking The Earth's early atmosphere, and how oxygen developed The greenhouse effect, and evidence for climate change Effects of carbon dioxide and methane, and air pollutants Producing potable water

Monitoring and Controlling Chemical Reactions

Topic overview Mass of solute, concentration and dilution calculations Measuring the rate of a reaction, and rate graphs Factors affecting rate, and collision theory Surface area and rate Catalysts and how they work Reversible reactions and dynamic equilibrium Changing the conditions of an equilibrium Activation energy, and enzymes as catalysts

Particles

Topic overview The particle model and changes of state Limitations of the particle model How the atomic model changed, and the structure of the atom The size of atoms, and the subatomic particles Calculating protons, neutrons and electrons

Predicting and Identifying Reactions

Topic overview Groups 1, 7 and 0, and why they behave as they do Predicting reactivity from the Periodic Table Reactivity of metals with water and acids Tests for oxygen, hydrogen, carbon dioxide and chlorine

Global Challenges

Producing potable water

3 things to know
Paper 10 (Chemistry)

Overview

What potable water is and how it differs from pure water, treating fresh water by screening, filtering and sterilising with chlorine, ozone or ultraviolet light, desalination by distillation or reverse osmosis and its cost, how hard ground, salt and waste water are to treat, and taking random, representative samples from a reservoir.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is meant by potable water?

Key term

Explain why potable water is not the same as pure water.

Key term

Why is water sterilised before it is supplied to homes?

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by potable water?
Model answer
Water that is safe to drink, with low levels of dissolved salts and no harmful microbes.
2
Explain why potable water is not the same as pure water.
Model answer
Chemically pure water contains only H₂O molecules, whereas potable water still contains dissolved substances at levels that are safe to drink.
3
Describe how fresh water from a river or reservoir is treated to make it potable.
Model answer
It is passed through a mesh screen and then filtered through beds of sand and gravel to remove solids, and finally sterilised.
4
Why is water sterilised before it is supplied to homes?
Model answer
To kill the microbes in it that would otherwise cause disease.
5
Name three methods used to sterilise water.
Model answer
Adding chlorine, treating with ozone, or passing the water under ultraviolet light.
6
How can potable water be obtained from sea water?
Model answer
By desalination, either by distilling the sea water or by forcing it through a membrane under pressure in reverse osmosis.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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