Are covalent bonds broken when a simple molecular substance boils?

No, the molecules stay whole and only the weak forces between them are overcome.

Explain why giant covalent structures have exceptionally high melting points.

Because melting requires many strong covalent bonds running through the whole structure to be broken.

Explain why most metals have high melting points.

Because there is a strong electrostatic attraction between the positive metal ions and the delocalised electrons, which takes a lot of energy to overcome.

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

Elements, Compounds and Mixtures

Bond strength, intermolecular forces and predicting state

3 things to know
Paper 9 (Chemistry)

Overview

Why simple molecular substances melt and boil at low temperatures without breaking covalent bonds, why ionic compounds, giant covalent structures and metals have high melting points, why boiling points rise as molecules get larger, and predicting whether a substance is a solid, liquid or gas at a given temperature from its melting and boiling points.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What has to happen to the forces between particles before a solid will melt?

Key term

Explain why simple molecular substances melt and boil at low temperatures.

Key term

Explain why ionic compounds have high melting points.

Questions from this subtopic

Each one with the model answer underneath.

1
What has to happen to the forces between particles before a solid will melt?
Model answer
Enough energy must be transferred to overcome the forces holding the particles in their fixed positions.
2
Explain why simple molecular substances melt and boil at low temperatures.
Model answer
Because only the weak intermolecular forces between the molecules have to be overcome, and these need very little energy.
3
Are covalent bonds broken when a simple molecular substance boils?
Model answer
No, the molecules stay whole and only the weak forces between them are overcome.
4
Explain why ionic compounds have high melting points.
Model answer
Because strong electrostatic attractions act between the oppositely charged ions throughout the lattice, and a great deal of energy is needed to overcome them.
5
Explain why giant covalent structures have exceptionally high melting points.
Model answer
Because melting requires many strong covalent bonds running through the whole structure to be broken.
6
Explain why most metals have high melting points.
Model answer
Because there is a strong electrostatic attraction between the positive metal ions and the delocalised electrons, which takes a lot of energy to overcome.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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

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