How do we write chemical formulae and balance equations, and what does conservation of mass mean?
Writing chemical formulae by balancing ion charges, turning reactions into word equations and balanced symbol equations with state symbols, and the conservation of mass in a chemical reaction.
A focused CCEA GCSE Single Award Science answer on chemical formulae and equations, covering writing formulae by balancing ion charges, word and balanced symbol equations with state symbols, how to balance, and the conservation of mass.
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What this dot point is asking
CCEA wants you to write chemical formulae by balancing ion charges, turn reactions into word equations and then balanced symbol equations with state symbols, and explain the conservation of mass.
Writing formulae from ions
A quick method is to use each ion's charge as the subscript of the other ion, then simplify. So and give . Brackets are used for a compound ion present more than once, such as .
Word and symbol equations
A word equation lists reactants and products by name, for example magnesium + oxygen gives magnesium oxide. A symbol equation replaces the names with formulae and must be balanced.
To balance, change only the big numbers (coefficients) in front of formulae, never the small subscripts inside a formula, because changing a subscript changes the substance.
Conservation of mass
This is exactly why a symbol equation must balance: the same atoms appear on both sides. If a reaction in an open container seems to lose or gain mass, it is because a gas has escaped or been taken in.
At Higher tier you may also meet ionic equations, which show only the ions that actually change and leave out the spectator ions that are unchanged. For example, every acid-alkali neutralisation is really just , no matter which acid and alkali are used. Like all equations, an ionic equation must balance both the atoms and the total charge on each side.
Examples in context
Example 1. Why a rusting nail gains mass. When iron rusts it combines with oxygen from the air to form iron oxide, so the nail gets heavier. Mass is still conserved: the increase equals the mass of oxygen that has joined the iron. Weighing in a sealed container would show no change at all, because nothing can enter or leave.
Example 2. Why a burning candle seems to lose mass. A candle burns and seems to disappear, losing mass, because the products carbon dioxide and water vapour escape into the air as gases. If you could trap every product, the total mass would equal the mass of the wax plus the oxygen used. This is the classic CCEA example of conservation of mass with an escaping gas.
Try this
Q1. Write the formula of the compound made from and ions. [1 mark]
- Cue. .
Q2. In a sealed flask, 10 g of a reactant fully reacts with 6 g of another. What is the total mass of products? [1 mark]
- Cue. 16 g, because mass is conserved.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA SAS 20213 marksBalance this equation for the combustion of methane: CH4 + O2 gives CO2 + H2O.Show worked answer →
Three marks for a correctly balanced equation.
Start with the carbon, which is already balanced (one each side).
Balance the hydrogen: four H on the left in CH4, so put a 2 in front of H2O to give four H on the right.
Now balance the oxygen: the right has two O in CO2 and two O in 2H2O, four in total, so put a 2 in front of O2.
The balanced equation is CH4 + 2O2 gives CO2 + 2H2O. Markers reward only changing the big numbers, never the small subscripts, and a final check that each atom balances.
CCEA SAS 20194 marksIn a sealed flask, 8 g of methane reacts completely with 32 g of oxygen. Use conservation of mass to find the total mass of products, and explain your reasoning.Show worked answer →
Four marks for the principle, the calculation and the explanation.
The law of conservation of mass states that no atoms are created or destroyed in a chemical reaction, so the total mass stays the same.
Total mass of reactants equals 8 plus 32, which is 40 g.
So the total mass of products (carbon dioxide and water) is also 40 g.
This works because the same atoms are simply rearranged into new substances. Markers reward stating conservation of mass, the addition, and the reason that atoms are only rearranged.
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Sources & how we know this
- CCEA GCSE Science: Single Award specification — CCEA (2017)