Amount of Substance


Balanced equations and related calculations


Equations represent what happens when chamical reactions take place.

Reactants - the materials you start with

Products - the materials you end up with


The ratio in which the reactants react and the products are produced, in simple whole numbers.

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Balanced equations and related calculations

Balanced symbol equations

Balanced symbol equations use the formulae of reactants and products. there are the same number of atoms of each element on both sides of the arrow. Balanced equations show the amounts of substances that react together and are produced.

State symbols

These are added to equations to show whether a substance is a gas, liquid, solid, or aqueous solution.

(s) --> solid                                     (l) --> liquid

(g) --> gas                                      (aq) --> aqueous solution (dissolved in water)

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Balanced equations and related calculations

Writing balanced equations

In order to write a balanced equation, you need to have the reactants and products.


Oxygen + Hydrogen -----> Water

As a symbol equation:                          O2 + H2 ----> H2O

But this isn't balanaced, as there are 2 oxygen atoms going in to the reaction but only 1 comes out as a product.

O2 + H2 ---> 2H2O

But now there are 2 hydrogen atoms going in and 4 coming out.

O2 + 2H2 ---> 2H2O

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Balanced equations and related calculations

Rules for balancing equations

You cannot change the formulae to make the equation balance. E.g. you cannot use O as oxygen always comes in pairs (O2)

You can only change the numbers of atoms by putting a number (a coefficient) in front of formulae

The coefficient in front of the symbol states how many moles of that substance are reacting

When dealing with ionic equations, the total of the charges must be the same

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