Why would a carton labelled 'pure orange juice' not count as pure to a chemist?

Because orange juice is a mixture of water, sugars, acids and many other compounds rather than one single compound.

Is bottled spring water a pure substance in the scientific sense?

No, because it is a mixture of water with dissolved minerals and gases.

Can a mixture of two elements ever be described as pure in the scientific sense?

No, because a pure substance contains only one element or only one compound.

A sample contains nothing but copper atoms. Is it an element, a compound or a mixture, and is it pure?

It is an element, and it is pure because only one kind of atom is present.

OCR GCSE Chemistry A J248

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 Nanoparticles: size and surface area to volume Uses and risks of nanoparticulate materials

Global Challenges

Topic overview Extracting metals by reduction and by electrolysis Biological methods of metal extraction Rate against yield in industry, and reaction conditions The Haber process and fertiliser production Nitrogen, phosphorus and potassium in agriculture Life cycle assessment Recycling, and the decisions behind it Alloys, and the corrosion of metals Comparing glass, ceramics, polymers, composites and metals Functional groups and homologous series Predicting the products of organic reactions Addition polymerisation Condensation polymerisation, and making a polymer DNA and other natural polymers Fractional distillation of crude oil The fractions, and crude oil as a feedstock Dependence on hydrocarbons, and cracking Chemical cells and fuel cells 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 Concentration in mol/dm³, and titration Titration calculations, and the mass of a solute Molar gas volume and the volumes of gases Theoretical mass and percentage yield Atom economy, and choosing a reaction pathway 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 Transition metals Predicting reactivity from the Periodic Table Reactivity of metals with water and acids Tests for oxygen, hydrogen, carbon dioxide and chlorine Tests for aqueous cations and anions Flame tests, and identifying metal ions Identifying a species from test results Instrumental methods of analysis

Elements, Compounds and Mixtures

Purity, and melting point data

3 things to know
Paper 3

Overview

What pure means to a chemist and in everyday speech, orange juice and spring water as mixtures, why chemists need the strict definition, and how a sharp melting point shows purity.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What does pure mean to a chemist and in everyday speech, and give an example where the two meanings disagree.

Key term

In science, what does it mean to say that a substance is pure?

Key term

What does the word 'pure' usually mean when it is used about food or drink in everyday language?

Questions from this subtopic

Each one with the model answer underneath.

1
In science, what does it mean to say that a substance is pure?
Model answer
A pure substance is a single element or a single compound, with nothing else mixed into it.
2
What does the word 'pure' usually mean when it is used about food or drink in everyday language?
Model answer
In everyday use it means nothing has been added, so the product is natural or unadulterated.
3
Why would a carton labelled 'pure orange juice' not count as pure to a chemist?
Model answer
Because orange juice is a mixture of water, sugars, acids and many other compounds rather than one single compound.
4
Is bottled spring water a pure substance in the scientific sense?
Model answer
No, because it is a mixture of water with dissolved minerals and gases.
5
Can a mixture of two elements ever be described as pure in the scientific sense?
Model answer
No, because a pure substance contains only one element or only one compound.
6
A sample contains nothing but copper atoms. Is it an element, a compound or a mixture, and is it pure?
Model answer
It is an element, and it is pure because only one kind of atom is present.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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