A student heats zinc carbonate in a sealed reaction vessel. Zinc oxide and carbon dioxide are formed. After the vessel cools, it is placed on a balance. The student opens a valve and the balance reading decreases.
Why does the balance reading decrease when the valve is opened?
Oxygen from the air entered the open vessel, so its atoms joined the zinc oxide.
Carbon dioxide stayed in the open vessel, but gases do not add to a balance reading.
Both products stayed in the open vessel, but their atoms had less total mass after the reaction.
Carbon dioxide left the open vessel, so its atoms were no longer on the balance.
95 exam-style questions on AQA GCSE Chemistry 3.1 Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations, covering 3.1.1 Conservation of mass and balanced chemical equations, 3.1.2 Relative formula mass, 3.1.3 Mass changes when a reactant or product is a gas, and 3.1.4 Chemical measurements. Each one has a worked solution and a mark scheme showing where the marks go.