Figure: Graph of balance reading against time for magnesium reacting with acid

Why does the graph eventually become horizontal?
The escaped gas is being replaced by air
The reaction has reached equilibrium
A reactant is used up, so no gas is made
The reaction continues at a constant rate
55 exam-style questions on Edexcel GCSE Chemistry 2.6 Calculations involving masses, covering 2.6.1 Relative formula mass and percentage composition, 2.6.2 Empirical and molecular formulae, 2.6.3 Conservation of mass, 2.6.4 Reacting masses and concentration in g dm⁻³, 2.6.5 The mole and the Avogadro constant, and 2.6.6 Limiting reactants and stoichiometry. Each one has a worked solution and a mark scheme showing where the marks go.