GCSE Combined Science (AQA)

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Changes of Mass in Reactions

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Quantitative chemistry Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations

Changes of Mass in Reactions

6:34 Conservation and Changes of Mass in Chemical Reactions
Spec 4.3.1.3
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  • In a chemical reaction, the mass of the products must equal the mass of the reactants, according to the law of conservation of mass.
  • Example: Iron reacts with sulfur in a one-to-one ratio to produce iron sulfate, maintaining mass balance.
  • The particle model shows no atoms are lost or gained during reactions, maintaining constant mass regardless of state changes.
  • Example: Water breaks down into hydrogen and oxygen gases, maintaining mass balance.
  • Apparent changes in mass can occur if gas reactants or products are not accounted for.
  • Example: Magnesium reacting with hydrochloric acid produces hydrogen gas, which if allowed to escape, results in a perceived mass loss.
  • Burning metal in air increases mass due to oxygen combining with the metal.
  • Changes in mass are observed in non-enclosed systems where substances can enter or leave freely.
  • Closed systems prevent mass changes as no substances can enter or leave.
  • Example: Copper reacting with oxygen shows increased mass due to oxygen's mass being added.
  • Thermal decomposition of metal carbonates releases carbon dioxide, causing a perceived mass decrease in non-enclosed systems.
  • Understanding mass changes in reactions involves accounting for all reactants and products, including gases.
  • For exams, explain observed mass changes in non-enclosed systems using balanced equations and particle models.

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