Mass changes when a reactant or product is a gas — AQA GCSE Chemistry
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Mass changes when a reactant or product is a gas explained
Mass changes when a reactant or product is a gas covers 2 specification points, including When a metal reacts with oxygen the mass of… and Students should be able to explain any….
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Open a point below for its explanation and revision guidance.
Some reactions may appear to involve a change in mass but this can usually be explained because a reactant or product is a gas and its mass has not been taken into account. For example: when a metal reacts with oxygen the mass of the oxide produced is greater than the mass of the metal or in thermal decompositions of metal carbonates carbon dioxide is produced and escapes into the atmosphere leaving the metal oxide as the only solid product.
Mass appears to change in some reactions because a gaseous reactant or product is not included in the measurement. When a metal such as magnesium reacts with oxygen, the oxygen from the air joins the metal, so the white magnesium oxide product has a greater mass than the original metal. In the thermal decomposition of a metal carbonate, for example calcium carbonate → calcium oxide + carbon dioxide, the carbon dioxide escapes as a gas, leaving only the solid metal oxide; the remaining solid therefore has less mass than the original carbonate. In both cases, total mass is conserved if the gas is accounted for. Reactions in closed systems show no overall mass change.
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, matter can leave or enter as a gas, so the mass reading may change even though atoms are conserved. Use the balanced symbol equation to identify gaseous reactants or products. If a gas escapes, the final mass is lower; if a gas from the air is absorbed, the final mass is higher. For example, magnesium burning in air, 2Mg + O₂ → 2MgO, gains mass because oxygen particles from the air join the solid. In contrast, marble chips reacting with acid, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂, lose mass because carbon dioxide particles escape. Explain using the particle model: atoms are rearranged, not destroyed, but gas particles can move into or out of the container.
Your focus
- Explain apparent mass changes in terms of gaseous reactants or products.
- Describe examples such as metal oxidation and carbonate decomposition.
- Apply conservation of mass to reactions in open and closed systems.
Show all 6 objectives
- Identify gaseous reactants and products from a balanced symbol equation.
- Describe whether gas particles leave or enter a non-enclosed reaction system.
- Explain an observed mass change using the particle model and conservation of atoms.
Mass changes when a reactant or product is a gas exam tips
Marking Points
- Apparent mass changes are usually due to a gaseous reactant or product not being measured.
- Metal + oxygen → metal oxide: oxygen from the air adds to the metal, so the oxide has greater mass than the metal.
- Thermal decomposition of a metal carbonate: carbon dioxide gas escapes, so the remaining solid oxide has less mass than the carbonate.
- Total mass is conserved when all substances, including gases, are taken into account.
- In a closed system, the total mass before and after a reaction is unchanged.
- Example equation: CaCO₃ → CaO + CO₂ shows the gaseous product leaving the solid.
- Identify from the balanced symbol equation which reactants or products are gases.
- State whether gas particles leave the reaction mixture or enter it from the surroundings.
- Compare the total mass of gaseous material lost or gained with the mass of the contents remaining.
- Explain the mass change by referring to gas particles moving into or out of the non-enclosed system.
- Use the balanced equation to show that atoms are conserved even when the measured mass changes.
Examiner Tips
- 💡Identify whether a gas is a reactant or a product and state whether it is entering or leaving the reaction vessel.
- 💡Use a closed system example to explain why mass does not change when gases are contained.
- 💡Write the balanced symbol equation to show which substance is gaseous, for example CO₂ in a decomposition.
- 💡Link the observation to conservation of mass rather than saying mass is lost or gained overall.
- 💡Underline the gaseous species in the equation before writing your explanation.
- 💡Use the phrase non-enclosed system to show that gas can escape or enter.
- 💡Link each mass change to named particles, such as oxygen molecules entering or carbon dioxide molecules leaving.
Common Mistakes
- Thinking that mass is destroyed or created in these reactions; the correction is that gases are involved and must be included in the mass balance.
- Assuming the mass loss on heating a carbonate is due to the metal oxide escaping; the correction is that carbon dioxide gas escapes while the metal oxide remains as the solid.
- Ignoring the role of oxygen from the air when a metal gains mass; the correction is that oxygen is a reactant and its mass is added to the metal.
- Thinking that mass is destroyed or created in the reaction; correct this by stating that atoms are conserved and gas particles move into or out of the non-enclosed system.
- Ignoring the state symbols in the equation; correct this by using (g) to identify which substances are gases.
- Assuming all reactions in open containers lose mass; correct this by checking whether a gaseous reactant is taken in from the air, which can increase the measured mass.