Specification Point 1.25: 
Write word equations and balanced chemical equations (including state symbols):
• for reactions studied in this specification
• for unfamiliar reactions where suitable information is provided

Writing Balanced Chemical Equations

When balancing equations, there needs to be the same number of atoms of each element on either side of the equation:

  • Work across the equation from left to right, checking one element after another
  • If there is a group of atoms (such as nitrate group, NO3), which has not changed from one side to the other, then count the whole groups, rather than counting the individual atoms

Using state symbols:

State symbols are written after formulae in chemical equations to show which physical state each substance is in:

Solid Liquid Gas Aqueous
(s) (l) (g) (aq)

Example 1:

Aluminium (s)  +   Copper (II) Oxide (s)  →   Aluminium Oxide (s)  +   Copper (s)  

Unbalanced symbol equation:    Al     +     CuO     →     Al2O3     +     Cu

ALUMINIUM: There is 1 Aluminium atom on the left and 2 on the right so if you end up with 2, you must start with 2. To achieve this, it must be 2Al

          2Al     +     CuO     →    Al2O3     +     Cu

OXYGEN: There is 1 Oxygen Atom on the left and 3 on the right so if you end up with 3, you must start with 3. To achieve this, it must be 3CuO

          2Al     +     3CuO     →    Al2O3     +     Cu

COPPER: There is 3 Copper Atoms on the left and 1 on the right. The only way of achieving 3 on the right is to have 3Cu

          2Al     +     3CuO     →    Al2O3     +     3Cu

         The Equation is Now Balanced

Example 2:

Magnesium Oxide (s)  +  Nitric Acid (aq)  →   Magnesium Nitrate (aq)  +   Water (l)  

Unbalanced symbol equation:  MgO   +   HNO3  →   Mg(NO3)2  +  H2O

  • MAGNESIUM: There is 1 Magnesium Atom on the left and 1 on the right so there are equal number of Magnesium Atoms on both sides so is kept the same

          MgO   + HNO3  →   Mg(NO3)2  + H2O

  • OXYGEN: There is 1 Oxygen Atom on the left and 1 on the right so there are equal number of Oxygen Atoms on both sides so is kept the same (remember that you are counting the Nitrate Group as separate group, so do not count the Oxygen Atoms in this Group)

          MgO   + HNO3  → Mg(NO3)2  + H2O

  • HYDROGEN: There is 1 Hydrogen Atoms on the left and 2 on the right. Therefore you must change HNO3 to 2HNO3

          MgO   + 2HNO3  → Mg(NO3)2  + H2O

         The Equation is Now Balanced

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.