Chemistry Key Points

The key points for every single topic in the AQA Additional Science course for Chemistry. Presented in a nice and colourful way!

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Chemistry Key Points
Structure and bonding
Chemical bonding
Elements react to form compounds by gaining or losing electrons, or by sharing
Atoms of metals in Group 1 combine with atoms of non-metals in Group 7 by
transferring electrons to form ions that have the electronic structures of noble
Ionic bonding
Ionic compounds are held together by strong forces between the oppositely charged
ions. This is called ionic bonding.
The ions form a giant structure or lattice. The strong forces of attraction act
throughout the lattice.
We can represent atoms and ions using dot and cross diagrams.
Formulae of ionic compounds
The charges on the ions in an ionic compound always cancel each other out.
The formula of an ionic compound shows the ratio of ions present in the compound.
Sometimes we need brackets to show the ratio of ions in a compound, e.g.
magnesium hydroxide, Mg(OH).
Covalent bonding
A covalent bond is formed when two atoms share a pair of electrons.
The number of covalent bonds an atom forms depends on the number of electrons
it needs to achieve a stable electronic structure.
Many substances containing covalent bonds consist of simple molecules, but some
have giant covalent structures.
The atoms in metals are closely packed together and arranged in regular layers.
The electrons in the highest energy level are delocalised. The strong electrostatic
forces between these electrons and the positively charged metal ions hold the metal

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Structure and properties
Giant ionic structure
Ionic compounds have high melting points and they are all solids at room
Ionic compounds will conduct electricity when we melt them or dissolve them in
water. Their ions can then move freely and can carry charge through the liquid.
Simple molecules
Substances made up of simple molecules have low melting points and boiling points.
Simple molecules have no overall charge, so they cannot carry electrical charge and
do not conduct electricity.…read more

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Developments in nanoscience are exciting but will need more research into possible
issues that might arise from increased use.
How much?
The mass of atoms
The relative mass of protons and neutrons is 1.
The atomic number of an atom is its number of protons (which equals its number of
The mass number of an atom is the total number of protons and neutrons in its
Isotopes are atoms of the same element with different numbers of neutrons.…read more

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Reversible reactions
In a reversible reaction the products of the reaction can react to make the original
We can show a reversible reaction using the (reversible reaction) sign .
Analysing substances
Chemical analysis is used to identify food additives.
Paper chromatography can be used to detect and identify artificial colours.
Instrumental analysis
Modern instrumental techniques provide fast, accurate and sensitive ways of
analysing chemical substances.
Compounds in a mixture can be separated using gas chromatography.…read more

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The effect of concentration or pressure
Increasing the concentration of reactants in solutions increases the frequency of
collisions between particles, and so increases the rate of reaction.
Increasing the pressure of reacting gases also increases the frequency of collisions
and so increases the rate of reaction.
The effect of catalysts
A catalyst speeds up the rate of a chemical reaction.
A catalyst is not used up during a chemical reaction.
Different catalysts are needed for different reactions.…read more

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Salts and electrolysis
Acids and alkalis
When acids are added to water they produce hydrogen ions, H(aq), in the solution.
Bases are substances that will neutralise acids.
Alkalis dissolve in water to give hydroxide ions, OH(aq), in the solution.
The pH scale shows how acidic or alkaline a solution is.
Making salts from metals or bases
When an acid reacts with a base a neutralisation reaction takes place and produces a
salt and water.…read more

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The extraction of aluminium
Aluminium oxide is electrolysed to manufacture aluminium.
The aluminium oxide is mixed with molten cryolite to lower its melting point.
Aluminium forms at the negative electrode and oxygen at the positive electrode.
The positive carbon electrodes are replaced regularly as they gradually burn away.
Electrolysis of brine
When we electrolyse brine we get three products- chlorine gas, hydrogen gas and
sodium hydroxide solution.
The products are important reactants used in industry.…read more


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