Formulae, Equations and Conservation of Mass
Chemical symbols, ionic and empirical formulae, balanced symbol, ionic and half equations, state symbols and mass changes in open systems.
How to study for GCSE Chemistry
Move between particle models, equations, calculations, observations and data; practise the specific required activities and paper structure named by your exam board.
Core concepts
Concept 1
A chemical formula represents the fixed ratio of atoms or ions, while a balanced equation represents conservation of each element during reaction.
Exam cue: Derive the formula first, then balance coefficients without changing subscripts.
Concept 2
State symbols distinguish solids, liquids, gases and aqueous species; ionic and half equations focus on the particles or electron changes that matter.
Exam cue: Remove unchanged spectator ions only when constructing an ionic equation.
Concept 3
Mass is conserved in a closed system, while an apparent mass change in an open system occurs when gas enters or leaves.
Exam cue: Use the particle model to explain the direction of an observed open-system mass change.
Risk pitfalls and guardrails
Changing a compound's subscripts to balance an equation.
Guardrail: Do not rely on a memorised keyword alone: preserve charges, ratios, units and conditions, and keep single Chemistry separate from Combined Science.
Assuming a balanced equation must have the same number of molecules on both sides.
Guardrail: Do not rely on a memorised keyword alone: preserve charges, ratios, units and conditions, and keep single Chemistry separate from Combined Science.
Claiming conservation of mass fails whenever a gas is produced.
Guardrail: Do not rely on a memorised keyword alone: preserve charges, ratios, units and conditions, and keep single Chemistry separate from Combined Science.
Memory anchors
Formula
The types and simplest fixed ratio of atoms or ions in a substance.
Coefficient
A number before a formula that changes the amount, not the substance.
State symbols
(s), (l), (g) and (aq) identify physical state or aqueous solution.
Spectator ion
An ion unchanged on both sides of a full ionic equation.
Conservation of mass
Atoms are rearranged, so total mass remains constant in a closed system.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
What is the relative formula mass of CO₂? Use C = 12 and O = 16.
Using Ar values Ca 40, O 16 and H 1, calculate the formula mass of calcium hydroxide.
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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