Mass Changes when a Reactant or Product is a Gas (AQA GCSE Chemistry: Combined Science)

• Some chemical reactions may appear to involve a change in mass due to the presence of a gaseous reactant or product
• If the reaction flask is open and a gaseous product is allowed to escape, then the total mass of the reaction flask will decrease as product mass is lost when the gas leaves the system
• For example:
  • When a metal reacts with oxygen, the mass of the oxide produced is greater than the mass of the metal you started with
  • When metal carbonates thermally decompose, carbon dioxide is produced and escapes into the atmosphere leaving the metal oxide as the only solid product

• By analysis of the balanced chemical equations and the corresponding state symbols, you should be able to deduce the changes in mass for non-enclosed reaction systems
• For example, the reaction between hydrochloric acid and calcium carbonate produces carbon dioxide gas:

2HCl (aq) + CaCO₃ (s) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)

• Mass will be lost from the reaction flask as carbon dioxide gas escapes to the atmosphere
  • So, the mass of the reaction mixture will decrease

• If the mass of a reaction flask is found to increase then it is probably due to one of the reactants being a gas found in the air and all of the products are either solids or liquids
• For example, the reaction of magnesium with oxygen produces magnesium oxide:

2Mg (s) + O₂ (g) → 2MgO (s)