Mass changes when a reactant or product is a gas — AQA GCSE Combined Science
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Mass changes when a reactant or product is a gas explained
Mass is conserved in every chemical reaction, so any apparent gain or loss arises because a gas has entered or left the reaction vessel.
Read the full explanation
In an open system, a gaseous reactant taken from the air adds mass that was never measured at the start. For example, heated magnesium combines with oxygen: 2Mg + O₂ → 2MgO, so the white powder weighs more than the original ribbon because oxygen atoms from the air have joined the solid. Conversely, heating copper carbonate gives CuCO₃ → CuO + CO₂; the carbon dioxide escapes, so the remaining black copper oxide weighs less than the green solid. In both cases the total mass of all atoms is unchanged; only the mass inside the container changes.
Students should be able to explain any observed changes in mass in non-enclosed systems during a chemical reaction given the balanced symbol equation for the reaction and explain these changes in terms of the particle model.
In a non-enclosed system, gases can enter or leave, so the mass measured on a balance may change even though atoms are conserved. To explain this, read the balanced symbol equation and identify every substance with state symbol (g). If a gaseous reactant is used, its particles are added to the reaction mixture and the measured mass rises; if a gaseous product forms, its particles escape into the air and the measured mass falls. For example, in 2Mg + O₂ → 2MgO oxygen particles from the air join the magnesium, so the oxide has greater mass. In CaCO₃ → CaO + CO₂, carbon dioxide particles leave, so the solid residue has less mass. The particle model shows the same atoms rearranged, with gas particles moving freely and no longer on the balance.
Your focus
- Describe how the measured mass can increase when a metal reacts with oxygen in an open system.
- Describe how the measured mass can decrease when a metal carbonate thermally decomposes in an open system.
- Use a balanced symbol equation to identify the gas responsible for an observed mass change.
Show all 6 objectives
- Interpret a balanced symbol equation to identify gaseous reactants and products.
- Explain an observed mass change in a non-enclosed system using the particle model.
- Predict whether the measured mass will increase or decrease for a given reaction and justify the prediction.
Mass changes when a reactant or product is a gas exam tips
Marking Points
- State that mass is conserved overall and that no atoms are created or destroyed in a chemical reaction.
- Identify the gas that is gained or lost: oxygen from the air in a metal oxidation, or carbon dioxide released in a carbonate decomposition.
- Explain a mass increase by naming the gaseous reactant that has been added to the solid, for example oxygen joining magnesium to form magnesium oxide.
- Explain a mass decrease by naming the gaseous product that has escaped, for example carbon dioxide leaving during the thermal decomposition of a metal carbonate.
- Use the balanced symbol equation to show which species is gaseous and therefore not weighed in an open system.
- Link the explanation to an open (non-enclosed) system rather than claiming that mass has genuinely been created or destroyed.
- Identify from the balanced symbol equation which reactant or product is gaseous, using state symbols where given.
- Explain a mass increase by stating that gas particles from outside enter the reaction mixture and are then included in the measured mass.
- Explain a mass decrease by stating that gas particles are produced and escape from the container into the surroundings.
- Describe the rearrangement of atoms in terms of the particle model, showing that the same atoms are present before and after the reaction.
- Link the observed balance reading to whether gaseous particles are gained by or lost from the system.
- Apply the explanation to a named reaction, such as a metal reacting with oxygen or a metal carbonate decomposing.
Examiner Tips
- 💡Name the specific gas and say whether it is gained from the air or lost to the atmosphere, rather than writing only that 'a gas is involved'.
- 💡Quote the relevant part of the balanced equation, such as 2Mg + O₂ → 2MgO, to justify your explanation.
- 💡Use the phrase 'in an open system' to show you understand why the measured mass changes while total mass stays constant.
- 💡Underline the (g) species in the equation before writing your explanation so you do not miss the gas.
- 💡State clearly whether the gas is a reactant entering or a product leaving, then link that to the direction of the mass change.
- 💡Use particle language such as 'atoms rearrange', 'gas particles escape' and 'particles from the air join the mixture' to address the particle model requirement.
Common Mistakes
- Claiming that atoms are destroyed or created, which contradicts conservation of mass; correct this by stating that the missing or extra mass belongs to a gas that entered or left the vessel.
- Assuming the solid product always weighs less; correct this by checking whether a gaseous reactant is gained (mass rises) or a gaseous product escapes (mass falls).
- Ignoring the state symbols in the equation; correct this by using (g) to identify the gas whose mass is not measured in an open system.
- Treating the balance reading as the total mass of all substances; correct this by stating that escaped gas particles are no longer measured.
- Saying gas particles 'disappear' or 'vanish'; correct this by explaining that they move into the surroundings and are not destroyed.
- Describing only the solid substances and omitting the gas; correct this by naming the gaseous species from the equation and explaining its movement.