Conservation Of Mass (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

The Law of Conservation of Mass

• The Law of Conservation of Mass states that no matter is lost or gained during a chemical reaction.

• Mass is always conserved

  • The total mass of the reactants equals the total mass of the products

  • This is why all chemical equations must be balanced

Closed systems

• A closed system is a reaction where nothing can enter or leave

• If a reaction takes place in a closed system, the total mass remains constant throughout

• For example, when calcium chloride solution reacts with sodium sulfate solution:

  • A precipitate of calcium sulfate forms

  • No gas is produced

  • So, no mass is lost

CaCl₂ (aq) + Na₂SO₄ (aq) ⟶ CaSO₄ (s) + 2NaCl (aq)

Diagram showing the conservation of mass in a precipitation reaction

Open systems

• An open system is a reaction where:

  • Reactants can enter, typically gases from the air e.g. oxygen in combustion reactions

  • Products can leave, typically gases such as carbon dioxide

Mass decreasing

• If a reaction takes place in an open system and a gaseous product escapes, the mass of the flask and its contents will decrease

• For example, when hydrochloric acid reacts with calcium carbonate:

  • Carbon dioxide gas is produced

  • This escapes into the air

2HCl (aq) + CaCO₃ (s) ⟶ CaCl₂ (aq) + H₂O (l) + CO₂ (g)

• The mass recorded on a balance will decrease as CO₂ leaves the flask

Mass increasing

• If a reaction takes place in an open system and a gaseous reactant is absorbed from the air, the mass of the flask and its contents will increase

• For example, when magnesium burns in air

  • The magnesium reacts with oxygen

  • The magnesium oxide produced has a greater mass than the magnesium alone

2Mg (s) + O₂ (g) ⟶ 2MgO (s)

• The oxygen absorbed from the air adds to the mass of the solid product

Conservation of mass calculations

• Since the total mass of reactants equals the total mass of products, you can calculate the mass of an unknown reactant or product