Why is it wrong to describe a single copper atom as shiny and malleable?

Because these are bulk properties that belong to a large number of atoms bonded together, not to one atom on its own.

Explain why an ionic compound conducts electricity when molten or dissolved but not when solid.

Because the ions are locked in fixed positions in the solid lattice, but are free to move and carry charge once the lattice is broken up.

Explain why simple molecular substances do not conduct electricity.

Because their molecules carry no overall charge and there are no free electrons or ions to move.

OCR GCSE Combined Science Chemistry J250

Chemical Reactions

Topic overview Writing formulae of elements and compounds Balanced equations, and deducing formulae from ions State symbols in equations Conservation of mass, and changes of mass in open systems Exothermic and endothermic reactions, and reaction profiles Activation energy and why reactions need it Oxidation and reduction in terms of oxygen Acids, alkalis and neutralisation The ionic equation for neutralisation, and reactions of acids The pH scale, acidity and alkalinity Measuring pH with indicators and pH meters 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 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 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 Activation energy, and enzymes as catalysts

Particles

Topic overview The particle model and changes of state 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

Elements, Compounds and Mixtures

Bulk properties of ionic, molecular and giant structures

3 things to know
Paper 3 (Chemistry)

Overview

Why bulk properties belong to many atoms rather than one, why ionic compounds conduct only when molten or dissolved and are brittle, why simple molecular substances are often gases or liquids and do not conduct, why metals are malleable and conduct, why giant covalent structures are insoluble, why polymers soften on heating, and identifying a structure from its properties.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Explain why ionic solids are brittle and shatter when struck.

Key term

Explain why many simple molecular substances are gases or liquids at room temperature.

Key term

Explain why metals can be hammered into shape without breaking.

Questions from this subtopic

Each one with the model answer underneath.

1
Why is it wrong to describe a single copper atom as shiny and malleable?
Model answer
Because these are bulk properties that belong to a large number of atoms bonded together, not to one atom on its own.
2
Explain why an ionic compound conducts electricity when molten or dissolved but not when solid.
Model answer
Because the ions are locked in fixed positions in the solid lattice, but are free to move and carry charge once the lattice is broken up.
3
Explain why ionic solids are brittle and shatter when struck.
Model answer
Because a blow shifts one layer of ions so that like charges line up, and the repulsion between them splits the crystal apart.
4
Explain why many simple molecular substances are gases or liquids at room temperature.
Model answer
Because only weak intermolecular forces hold their molecules together, and these are overcome at low temperatures.
5
Explain why simple molecular substances do not conduct electricity.
Model answer
Because their molecules carry no overall charge and there are no free electrons or ions to move.
6
Explain why metals can be hammered into shape without breaking.
Model answer
Because the layers of positive ions can slide over one another while the delocalised electrons continue to hold the structure together.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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AMetals are on the left and in the centre, and non-metals are on the right-hand side.

BKI, because potassium loses one electron and iodine gains one.

CCaCl₂, because a calcium atom loses two electrons and each chlorine atom gains one.

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Sodium ions give a bright yellow flame colour.

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