AQA GCSE Combined Science: Trilogy Biology Paper 1 (Foundation), 2024: Question 7

✅ Correct answer

Concentration of salt solution

This is the variable the student changes between samples.

💡 Key knowledge

• The independent variable is the one you deliberately change.
• Here, the student placed the tubes into different concentrations of salt solution.
• The mass change is the dependent variable because it is measured.

🧠 Exam technique

• Look for the variable that changes in the method.
• In AQA practical questions, “what was changed?” usually means the independent variable.

❌ Common errors

• Choosing change in mass because it is what they measure.
• Choosing time in salt solution because it is mentioned in the method, even though it is kept the same.
• Choosing volume of solution Z because it is controlled, not changed.

✅ Correct answer

So water/solution on the outside of the tube would not affect the mass and make the result invalid.

💡 Key knowledge

• Only the mass of the tubing and the solution inside should be measured.
• If liquid remains on the outside, the tube appears heavier than it really is.
• This would reduce the reliability of the mass change result.

🧠 Exam technique

For one-mark “why” questions, keep the reason short and direct: extra water would change the mass.

❌ Common errors

• Saying only “to make it dry” without explaining the effect on mass.
• Writing “to prevent evaporation” — that is not the key point here.
• Vague answers like “to make it fair” may not gain the mark unless explained.

📐 Calculations — step by step

1. Find the change in mass: 16.50 − 15.54 = 0.96 g
2. Use the percentage change formula:
   percentage change = change ÷ starting mass × 100
3. Substitute the values:
   0.96 ÷ 15.54 × 100
4. Calculate:
   = 6.17...%
5. Round to 1 decimal place:
   6.2%

✅ Correct answer

X = 6.2%

Marks are awarded for using the correct method and giving the rounded answer.

💡 Key knowledge

• Always use the starting mass in the denominator.
• Percentage change is based on the original value, not the final value.
• Keep units in grams until the final percentage answer, which has %.

❌ Common calculation traps

• Using 16.50 instead of 15.54 as the starting mass.
• Doing 15.54 ÷ 0.96 × 100 in the wrong order.
• Forgetting to round to 1 decimal place.
• Writing only the change in mass, not the percentage.

✅ Correct answers

• Plot the four points for 0.2 to 1.0 mol/dm³.
• Draw a smooth line of best fit through the points.

The point for 0 mol/dm³ was already plotted, but the mark scheme says you may ignore attempts to plot it again.

💡 Key knowledge

• Use the x-axis for concentration of salt solution in mol/dm³.
• Use the y-axis for percentage change in mass.
• The graph should show the overall trend, not dot-to-dot joins.

🧠 Exam technique

• Read the axis labels carefully before plotting.
• Marks are given for accuracy: the mark scheme allows a small tolerance of ± ½ small square.
• If you miss one point but plot three correctly, you can still gain partial credit.
• For a line of best fit, do not force the line through every point if it creates a poor trend.

❌ Common errors

• Plotting the wrong coordinates, often by mixing up mass change and percentage change.
• Joining points with straight segments instead of a best-fit curve/line.
• Ignoring the axes or using the wrong scale.
• Forgetting that the graph is used later to estimate the concentration of solution Z.

✅ Correct answer

About 0.65 mol/dm³

Acceptable range if no line is drawn: 0.64 to 0.66 mol/dm³.

💡 Key knowledge

• Solution Z is found by using the point where the mass change would be 0%.
• That is where the line of best fit crosses the x-axis.
• This means the solution has the same concentration as the contents of the tubing, so there is no net movement of water.

🧠 Exam technique

• Use the graph, not the table, to estimate the value.
• Draw a line across from 0% change in mass to the line of best fit, then drop down to the x-axis.
• Give a sensible value to 2 decimal places if needed from the graph, but 0.65 mol/dm³ is the key estimate.

❌ Common errors

• Choosing the concentration where the mass change is closest to zero from the table only, without using the graph.
• Reading the wrong axis.
• Giving a value far from the intersection point because the line of best fit was not drawn carefully.

💡 Key knowledge

• Osmosis is the net movement of water through a partially permeable membrane.
• Water moves from a dilute solution to a more concentrated solution.
• If the outside salt solution is very dilute, water enters the tubing and mass increases.
• If the outside salt solution is more concentrated, water leaves the tubing and mass decreases.

🧠 How to turn this into marks

• Use the word osmosis when explaining mass change.
• Include the idea of a partially permeable membrane.
• Link the direction of water movement to the concentration difference.

✅ Answers

• 07.1: concentration of salt solution
• 07.2: water/solution on the tube would affect the mass
• 07.3: 6.2%
• 07.5: about 0.65 mol/dm³

💡 What gained top marks

• Correctly identifying variables.
• Clear percentage change calculation with rounding.
• Accurate plotting and a sensible best-fit line.
• Reading concentration from the graph where the line crosses 0% change.