Mass changes when a reactant or product is a gas: 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.

The idea

Mass is conserved in every chemical reaction, so any apparent gain or loss arises because a gas has entered or left the reaction vessel. 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.

What earns the marks

• 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.

Common mistakes and how to put them right

• Claiming that atoms are destroyed or created, which contradicts conservation of mass.

  Put it right 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.

  Put it right by checking whether a gaseous reactant is gained (mass rises) or a gaseous product escapes (mass falls).

• Ignoring the state symbols in the equation.

  Put it right by using (g) to identify the gas whose mass is not measured in an open system.

Exam 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.