Chemical Equation - QS Study
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A chemical equation is a shorthand description of the changes that occur in a chemical reaction such as

Methane + Oxygen → Carbon Dioxide + Water

It allows us to determine the quantitative relationships between reactants and products. To be valid, a chemical equation must be:

  • Consistent with the experimental facts. Just because one can write an equation, it does not mean that it can really happen.
  • Consistent with conservation of mass i.e. the total number of atoms of each element in the reactants must be equal to the total in the products. Atoms cannot disappear!
  • Consistent with conservation of electric charge i.e. the net charge, before and after, must be the same


To construct a chemical equation for a reaction:

  • Write the names of the reactants on the left and products on the right side of the arrow,
  • Replace the names with the correct chemical formulae,
  • Balance the equation, and
  • Write the state symbols after each substance: (s) solid, (g) gas, (I) liquid and (aq) aqueous.