Atom rearrangement, equations and conservation of mass — AC9S9U07

• Identify reactants and products in word and symbolic equations.
• Model reactions as atoms rearranging rather than appearing, disappearing or changing element identity.
• Balance simple equations by changing coefficients, not subscripts.
• Use a closed-system model to explain conservation of mass.

Recall elements, compounds, atoms, molecules, chemical formulas and evidence that a chemical change forms new substances. Be able to count atoms in a simple formula.

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.

• 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.

1. Identify reactants/products in magnesium + oxygen → magnesium oxide.
2. Count each element on both sides of H₂ + O₂ → H₂O.
3. Explain why changing H₂O to H₂O₂ is invalid.
4. Balance H₂ + Cl₂ → HCl using coefficients.

1. Write a word equation from a described reaction.
2. Count atoms in 2CO₂ and 3H₂O.
3. Balance Mg + O₂ → MgO.
4. Balance N₂ + H₂ → NH₃.
5. Explain conservation of mass using a particle model.
6. Explain why an open-system experiment may appear to lose mass when a gas forms.

A reaction in an open beaker has a final measured mass 1.8 g lower than the starting mass. A student concludes that atoms were destroyed. Evaluate the conclusion, propose a plausible explanation and design a simple change to the experiment that would better test conservation of mass.

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.

1. Balance a simple equation and justify each coefficient using atom counts.
   Make a before/after atom-count table.
2. Use particle reasoning to explain conservation of mass in a closed-system reaction.
   Say atoms rearrange; do not say mass is “made” equal by balancing.
3. An experiment appears to lose mass. Evaluate whether this disproves conservation of mass.
   Define the system boundary and consider escaping gas/measurement uncertainty.

• I can identify reactants and products.
• I can count atoms accurately.
• I can balance using coefficients.
• I can explain conservation in a closed system.

Exit ticket: Why is 2H₂ + O₂ → 2H₂O a better representation than H₂ + O₂ → H₂O?

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.

• AC9S9U06 atomic models
• AC9S9I02 investigations

• Australian Curriculum v9.0
• Victorian Curriculum F–10 Version 2.0
• NSW Science 7–10 Syllabus (2023)

Curriculum equivalents for Model the rearrangement of atoms in chemical reactions using a...

Mapped skill: model the rearrangement of atoms in chemical reactions using a range of representations, including word and simple balanced chemical equations, and use these to demonstrate the law of conservation of mass

These references identify matching or closely related learning. Curriculum sequence, terminology and depth vary, so teachers should use the mapped skill and lesson difficulty to confirm suitability.

Australian Curriculum v9.0 is the canonical source for this SkillrHub lesson. Victoria and NSW entries name the closest published state codes or outcomes; international entries are planning references rather than claims of identical curricula.