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Chemistry Paper 1 / Quantitative chemistry
Lesson 3.6 · AQA Combined Science

Mass changes involving gases

A gas can change the measured mass

Mass is always conserved overall. An open system lets matter enter or leave, so the mass you measure can change. Explain which gas moves and in which direction.

Metal gains oxygen from the air

2Mg + O₂ → 2MgO
Magnesium joins with oxygen from the air. The oxide contains the original magnesium plus the added oxygen, so it has a greater mass than the metal alone.
2.4 g Mg makes 4.0 g MgO: oxygen gained = 4.0 − 2.4 = 1.6 g.

A carbonate loses carbon dioxide

CaCO₃ → CaO + CO₂
On thermal decomposition, carbon dioxide leaves an open system. The solid residue has less mass than the starting carbonate.
10.0 g carbonate gives 5.6 g oxide: carbon dioxide released = 10.0 − 5.6 = 4.4 g.

Explain the apparently missing mass

A good answer is: “Carbon dioxide gas escapes into the surroundings, so less material remains on the balance. Total mass is conserved if the escaped gas is included.”
“The mass disappears” is not an explanation. If gas is retained within the weighed system, the total stays the same.

Remember

Mass gain: gas enters and reacts. Mass loss: gas escapes. Include the gas to show conservation of mass.