Year 9 Science · AC9S9U07

Atom rearrangement, equations and conservation of mass

Model reactions as rearrangements of existing atoms, move between word, particle and symbolic representations, and use balanced equations to show why mass is conserved in…

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Key conceptTeach from the board

During a chemical reaction, bonds and groupings change, but the atoms themselves are conserved. A word equation names substances; a particle model shows how particles rearrange; a balanced symbolic equation shows equal numbers of each atom type before and after.

To balance an equation, change whole-number coefficients in front of formulas. Changing a subscript changes the substance and is therefore not a valid balancing method. Conservation of mass is easiest to observe in a closed system where gaseous products cannot escape.

Worked examplesWe do

Worked examples

AC9S9U07 - Atom rearrangement, equations and conservation of mass
Example 1

Identify reactants/products in magnesium + oxygen → magnesium oxide.

Example 2

Count each element on both sides of H₂ + O₂ → H₂O.

Example 3

Explain why changing H₂O to H₂O₂ is invalid.

Example 4

Balance H₂ + Cl₂ → HCl using coefficients.

Curriculum examplesCopied content

Australian Curriculum v9.0 — AC9S9U07: atom rearrangement, word/simple balanced equations and conservation of mass.

Victoria Levels 9–10 — VC2S10U08:Exact direct relationship.

NSW Stage 5 — SC5-RXN-01:Partial direct support through reaction types, while the Stage 5 outcome list does not duplicate the AC9S9U07 wording on balancing/conservation.

ComponentAC v9VictoriaNSW
Particle/equation modelDirectDirectStrong partial
Conservation reasoningDirectDirectSupporting within reactions
Questions and answersWith answers
  1. What changes in a chemical reaction? How atoms are bonded/grouped into substances.
  2. What stays conserved? The number of each atom type and therefore total matter/mass in a closed system.
  3. Why balance equations? To represent equal numbers of each atom type before and after.
  4. Why use coefficients? They change how many formula units participate without changing substance identity.
Practice and reviewReady for practice
  • Atoms disappear when substances react. Existing atoms are rearranged.
  • Changing subscripts is a balancing method. It changes chemical identity; use coefficients.
  • If measured mass falls, conservation failed. Check whether matter escaped the chosen system.
  • A balanced equation means equal numbers of molecules. It requires equal counts of each atom type, not necessarily equal particle counts.
Curriculum alignmentStart here
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