How do we represent chemical reactions, and why is mass always conserved?
Represent reactions by word and balanced symbol equations, apply the law of conservation of mass, and classify reactions as exothermic or endothermic.
A focused answer to WJEC GCSE Chemistry topic 1.1, covering word and balanced symbol equations, the law of conservation of mass, state symbols, and identifying exothermic and endothermic reactions from energy transfer.
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What this topic is asking
WJEC topic 1.1 wants you to represent reactions with word equations and balanced symbol equations, to apply the law of conservation of mass, and to recognise exothermic and endothermic reactions from the energy transferred. Balancing equations is a core skill that appears throughout both written papers.
Word equations
A word equation summarises a reaction using the names of the substances. The reactants go on the left, an arrow shows the direction of change, and the products go on the right. For example:
Word equations show what reacts and what forms but give no information about amounts.
Balanced symbol equations
A symbol equation uses chemical formulae. Because atoms are conserved, the equation must be balanced: the number of atoms of each element must be the same on both sides.
To balance :
- There are 2 oxygen atoms on the left but only 1 on the right, so put a 2 in front of water: .
- Now there are 4 hydrogen atoms on the right but only 2 on the left, so put a 2 in front of hydrogen: .
The equation is now balanced: 4 H and 2 O on each side.
State symbols
State symbols can be added in brackets after each formula to show the physical state: solid, liquid, gas and aqueous (dissolved in water). For example:
The law of conservation of mass
This is why a balanced equation must have equal atoms on each side. It also explains two common observations:
- In a sealed container, the mass never changes during a reaction.
- In an open container, the mass may appear to increase (if a gas from the air, such as oxygen, is taken in) or decrease (if a gas is given off and escapes). The mass only seems to change because a gas has entered or left, not because mass was created or destroyed.
Exothermic and endothermic reactions
You can identify these in the lab by measuring the temperature of the mixture: a rise means exothermic, a fall means endothermic.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC sample3 marksBalance the symbol equation for the combustion of methane: CH4 + O2 to CO2 + H2O. Show the balanced equation.Show worked answer →
A Unit 1.1 balancing question. Reward: balance the hydrogen by placing a 2 in front of water, giving . This now needs 4 oxygen atoms on the right (2 in and 2 in ), so place a 2 in front of : . Markers credit the correct balanced equation, awarding marks for getting the hydrogen and oxygen balanced. A common error is to change a subscript (such as writing ) instead of adding a balancing number in front.
WJEC sample4 marksIn a sealed flask, 4.0 g of copper reacts with sulfur to make copper sulfide and the total mass does not change. Explain this using the law of conservation of mass, and state what mass of products forms if 2.0 g of sulfur reacts fully.Show worked answer →
A Unit 1.1 conservation question. Reward: the law of conservation of mass states that mass is not created or destroyed in a chemical reaction, because the atoms are only rearranged - the same atoms are present before and after. In a sealed flask no substance can enter or leave, so the total mass stays the same. The mass of copper sulfide formed equals the total mass of reactants: . Markers credit the statement that atoms are rearranged and conserved, the link to the sealed system, and the answer 6.0 g. A common slip is to think mass is "used up".
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Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) — WJEC (2016)