Students should be able to explain any observed changes in mass in…

The idea

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.

What earns the marks

• 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.

Common mistakes and how to put them right

• Treating the balance reading as the total mass of all substances.

  Put it right by stating that escaped gas particles are no longer measured.

• Saying gas particles 'disappear' or 'vanish'.

  Put it right by explaining that they move into the surroundings and are not destroyed.

• Describing only the solid substances and omitting the gas.

  Put it right by naming the gaseous species from the equation and explaining its movement.

Exam tips

• 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.