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
In a non-enclosed system, matter can leave or enter as a gas, so the mass reading may change even though atoms are conserved.
Read the full explanation
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
- 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
- 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
- 💡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 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.