AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2024: Question 2

✅ Correct answer

Concentration of salt solution

💡 Key knowledge

• The independent variable is the one the student changes.
• Here, each tube was placed in a different concentration of salt solution.
• The dependent variable was the change in mass of the tube.

🧠 Exam technique

Look for what is deliberately changed between trials. If the question says “placed each tube into a different concentration”, that is your clue.

❌ Common errors

• Choosing change in mass – that is the result measured, so it is the dependent variable.
• Choosing time in salt solution – this stayed the same at 2 hours.
• Choosing volume of solution Z – the same volume was put in each tube.

✅ Correct answer

Any water / salt solution on the outside of the tube would affect or increase the mass.

A clear alternative: so the mass measured is only the mass of the tube and its contents.

💡 Key knowledge

• You must remove liquid from the outside before weighing.
• Otherwise, the balance measures the tube plus extra liquid stuck on the surface.
• That makes the results not valid.

🧠 Exam technique

For a 1-mark practical question, make the point directly: liquid on the outside would change the mass reading.

❌ Common errors

• Saying only “to make it fair” without explaining how.
• Talking about stopping osmosis – drying the outside does not affect osmosis inside the tube.
• Forgetting to mention mass or validity.

📐 Calculations

You need the percentage change in mass for the 0.0 mol dm⁻³ row.

1. Find the change in mass: 0.96 g
2. Find the starting mass: 15.54 g
3. Use the formula:
   percentage change = (change ÷ starting mass) × 100
4. Substitute the values:
   (0.96 ÷ 15.54) × 100
5. Calculate:
   6.1776...
6. Round to 1 decimal place: 6.2%

✅ Correct answer

Value X = 6.2%

🧠 Exam technique

• Use the starting mass, not the mass after 2 hours.
• Always read the instruction about decimal places.
• Include the % symbol in your final answer.

Examiner insight: many students lost marks by dividing by the wrong mass or forgetting to round to 1 decimal place.

❌ Common errors

• Using 16.50 instead of 15.54 in the denominator.
• Doing 15.54 ÷ 0.96 the wrong way round.
• Writing 6.17% when the question asks for 1 decimal place.
• Forgetting that this is percentage change, so you must multiply by 100.

✅ Correct plotting points

The four points to plot were:

One result at 0.8, −1.3 had already been plotted for you.

📐 How to score full marks on the graph

1. Read each coordinate carefully from the table.
2. Plot all 4 missing points accurately.
3. Use small neat crosses.
4. Draw a line of best fit through the trend of the points.
5. Do not join dot-to-dot with straight lines.

🧠 Exam technique

Top answers were accurate and tidy. The best students:

• checked each x-value and y-value separately
• noticed that 0.0 mol dm⁻³ should not be plotted because the question said only 0.2 to 1.0
• drew a smooth best-fit line showing the overall downward trend

❌ Common errors

• Plotting the 0.0 mol dm⁻³ value when the question said not to.
• Using the wrong sign and plotting −1.3 or −2.5 above zero.
• Joining points dot-to-dot instead of drawing a line of best fit.
• Misreading 0.5 as 5.

✅ Correct answer

About 0.65 mol dm⁻³

Acceptable answers depend on the student’s own line of best fit. The mark scheme allows about 0.64 to 0.66 mol dm⁻³ if no line of best fit was drawn.

💡 Key knowledge

• The concentration of solution Z is where there is 0% change in mass.
• At this point, there is no net movement of water by osmosis.
• This means solution Z and the surrounding solution have the same concentration.

📐 How to read it from the graph

1. Find 0 on the y-axis (percentage change in mass).
2. Draw or imagine a horizontal line to your line of best fit.
3. Drop down to the x-axis.
4. Read the concentration value, which is about 0.65 mol dm⁻³.

🧠 Exam technique

This is a classic graph-interpretation question. You are not taking a value directly from a table; you are estimating from the line of best fit. Read carefully and give a sensible value from your graph.

❌ Common errors

• Choosing the nearest plotted point instead of the x-value where the line crosses 0%.
• Reading from the wrong axis.
• Giving a value with unreasonable precision, such as lots of decimal places from a graph.

💡 What separated stronger answers

• Knowing exactly what an independent variable is.
• Explaining validity in practical work clearly.
• Using the correct formula for percentage change.
• Plotting graphs accurately and drawing a sensible line of best fit.
• Understanding that the isotonic point is where percentage change = 0.

❌ Where students lost marks

• Confusing independent and dependent variables.
• Giving vague practical answers like “for accuracy” without saying why.
• Making calculation errors or not rounding correctly.
• Poor graph skills: missing points, wrong signs, or no line of best fit.
• Not using the graph properly to estimate the concentration.

🧠 Fast revision takeaway

• Osmosis = movement of water through a partially permeable membrane from dilute to more concentrated solution.
• Independent variable = what is changed.
• Percentage change = (change ÷ original) × 100.
• Line of best fit = smooth trend line, not dot-to-dot.
• 0% change in mass tells you the concentration inside solution Z.