The questions are on the balancing equations test. Take it under real conditions before reading the mark scheme.
Quick answer: A chemical equation shows reactants turning into products. This mark scheme gives the model answers and the marks for the 8 questions of the grade 8 science test on balancing equations, from reactants and products to evidence of a reaction, with the equivalent answers to accept.
The model answers show where you earn each mark. Check the working as well as the final result, because the examiner marks balancing and calculation methods separately from the answer.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS1-2, MS-PS1-5
Question 1 Reactants and products (2 marks)
Reactants: hydrogen and chlorine. Then the product: hydrogen chloride. Finally, left: 2 H atoms and 2 Cl atoms.
Marking: 1 mark for reactants and product, 1 mark for the atom count.
Question 2 Balance three equations (3 marks)
(a) 4 K + O₂ → 2 K₂O. (b) Afterwards, 3 Mg + N₂ → Mg₃N₂. (c) Finally, H₂ + Br₂ → 2 HBr.
Marking: 1 mark each; no mark if a subscript changes.
Question 3 Ozone (3 marks)
(a) Firstly, left 3 atoms, right 2 atoms. (b) The numbers of atoms are not equal, but atoms cannot be created or destroyed. (c) Therefore use 6 atoms on each side: 2 O₃ → 3 O₂.
Marking: 1 mark for each part.
Question 4 Zinc sulfide (3 marks)
(a) 13.0 + 6.4 = 19.4 g. (b) Afterwards, conservation of mass: total mass of products equals total mass of reactants. (c) Finally, 26.0 g is twice 13.0 g, so 2 × 19.4 = 38.8 g.
Marking: 1 mark for each part.
Question 5 Conservation of atoms (3 marks)
The bonds in H₂ and Cl₂ break and the atoms rejoin as two HCl molecules. Therefore, no atom is created or destroyed: 2 H and 2 Cl atoms exist before and after. Only the way they connect changes.
Marking: 1 mark for rearrangement, 1 mark for atom count, 1 mark for no creation or destruction.
Question 6 Spot the error (2 marks)
She changed a subscript, so this gives a different substance. Only coefficients can change: 2 Mg + O₂ → 2 MgO.
Marking: 1 mark for the mistake, 1 mark for the equation.
Question 7 State symbols (2 marks)
Firstly, (s) means solid, (aq) means dissolved in water and (g) means gas. Then the gas is hydrogen, H₂.
Marking: 1 mark for three meanings, 1 mark for the gas.
Question 8 Evidence of a reaction (2 marks)
For example: a gas forms (bubbles), a colour change, a temperature change, a solid forms in a liquid, or the reaction gives out light.
Marking: 1 mark for any two correct observations, 2 marks for four.
