The questions are on the solubility test. Take it under real conditions before reading the mark scheme.
Quick answer: Solubility is the maximum mass of a solute that dissolves in 100 g of water at a given temperature. This mark scheme gives the model answers and the marks for the 8 questions of the grade 7 science test on solubility, from key words to spot the mistake, with the equivalent answers to accept.
The model answers show the full reasoning and how you earn each mark. Also, give credit for correct science even when the wording differs. Finally, add the marks to check the total of 20.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS1-2
Question 2 Concentration (3 marks)
- Firstly, concentration (g/L) = mass of solute (g) ÷ volume of solution (L).
- Then 18 g ÷ 0.45 L = 40 g/L.
Marking: 1 mark for the equation; 1 mark for the substitution; 1 mark for 40 g/L.
Question 3 Mass needed (2 marks)
Mass = 25 g/L × 0.60 L = 15 g.
Marking: 1 mark for the method; 1 mark for 15 g with unit.
Question 4 Solubility graph (3 marks)
- Firstly, 55 g per 100 g of water.
- 38 g × 2 = 76 g.
Marking: 1 mark for 55 g; 1 mark for doubling; 1 mark for 76 g.
Question 5 Saturated solution (3 marks)
- At 20 °C only 25 g dissolves, so 40 – 25 = 15 g remains undissolved.
- Therefore the solution is saturated.
Marking: 1 mark for using 25 g; 1 mark for 15 g; 1 mark for saturated.
Question 6 Speeding up (2 marks)
Powder has a much larger surface in contact with the water, so the water pulls away more particles every second.
Marking: 1 mark for larger surface; 1 mark for more particles leaving the solid.
Question 7 Crystals (3 marks)
- Firstly, at 80 °C: 78 g; at 40 °C: 38 g. 78 – 38 = 40 g.
- The cooler water can hold less solute, so the extra comes out as solid.
Marking: 1 mark for the two values; 1 mark for 40 g; 1 mark for the explanation.
Question 8 Spot the mistake (2 marks)
The statement is wrong. In fact, a solute with low solubility gives a saturated solution with a low concentration, because only a small mass can dissolve before the solution becomes saturated.
Marking: 1 mark for saying the statement is wrong; 1 mark for the reason.
