AQA GCSE Combined Science: Trilogy Biology Paper 1 (Foundation), 2020: Question 5

💡 Key knowledge

• Osmosis is the movement of water through a partially permeable membrane.
• Water moves from a dilute solution to a more concentrated solution.
• If potato gains mass, water has entered the cells.
• If potato loses mass, water has left the cells.
• The point where change in mass = 0 shows equal concentration inside and outside the cells.

🧠 Exam technique

• Use the wording in the question carefully: “independent variable” means what is changed.
• For 2-mark explanations, include both what happened and why.
• On graphs, plot accurately and draw a smooth curve if the data changes in a pattern.
• In calculations, show substitution, working and final rounded answer.

✅ Correct answer

Concentration of the sugar solution

💡 Key knowledge

The independent variable is the variable the student deliberately changes. In this method, each potato piece was placed in a different concentration of sugar solution.

❌ Common errors

• Choosing change in mass — that is the dependent variable because it is measured.
• Choosing time — the time was kept the same at 24 hours, so it is a control variable.
• Choosing starting mass — that should also be controlled as much as possible, not changed on purpose.

✅ Correct answer

The potato gained water.

Water moved into the potato by osmosis.

Because the concentration of water outside the potato was greater than inside the potato cells.

💡 Key knowledge

• 0.0 mol/dm³ sugar solution is effectively pure water.
• Outside the potato, the solution is more dilute than inside the cells.
• So water moves into the potato cells through their partially permeable membranes.
• This causes the potato mass to increase.

🧠 Exam technique

For full marks, do not just say osmosis . Say clearly: water moved into the potato by osmosis .

Strong answers include the concentration difference: higher water concentration outside than inside .

❌ Common errors

• Saying sugar moved into the potato — the mark scheme wants movement of water.
• Saying simply it absorbed solution — too vague.
• Mixing up the direction of movement and saying water moved out.

✅ Correct answer

• All points correctly plotted.
• A curve drawn as the line of best fit through all the points.

🧠 Exam technique

1. Read each coordinate as (concentration, change in mass) .
2. Plot each point carefully to within half a square.
3. Do not join the dots with straight lines.
4. Draw a smooth curve that follows the overall trend of the data.

Examiner insight: students often lost the second mark by drawing a ruler line or by linking points one-to-one instead of drawing a smooth best-fit curve.

❌ Common errors

• Plotting change in mass on the wrong axis.
• Missing the negative values below zero.
• Using a jagged line through each point instead of a smooth curve.
• Careless plotting more than half a square away.

✅ Correct answer

Concentration ≈ 0.23 to 0.24 mol/dm³

A sensible quoted value is 0.24 mol/dm³.

💡 Key knowledge

The potato does not change mass when the concentration inside the cells equals the concentration outside. This is the point where the graph crosses change in mass = 0 .

🧠 Exam technique

You must use the line of best fit, not just guess from the table. Look for where the curve crosses the x-axis.

Top answers usually read the graph carefully and give a sensible value between the two surrounding data points: 0.2 mol/dm³ → +0.25 g and 0.3 mol/dm³ → −0.40 g .

❌ Common errors

• Writing 0.25 mol/dm³ because of the change in mass value — wrong quantity.
• Choosing exactly 0.2 or 0.3 mol/dm³ from the table rather than interpolating from the graph.
• Reading the y-axis instead of the x-axis.

📐 Calculations

1. Use the value for change in mass at 0.2 mol/dm³: 0.25 g
2. Use the starting mass at 0.2 mol/dm³: 7.96 g
3. Substitute into the equation:
   percentage change in mass = (change in mass ÷ mass at start) × 100
4. Calculation:
   (0.25 ÷ 7.96) × 100 = 3.14070352...
5. Round to 3 significant figures:
   3.14%

✅ Correct answer

Percentage change in mass = 3.14%

🧠 Exam technique

• Use change in mass, not final mass.
• Use the starting mass in the denominator because that is what the formula says.
• Include the percentage sign.
• Round only at the end.

Examiner insight: many students lost marks by using 8.21 g instead of 0.25 g, or by dividing by the wrong mass.

❌ Common calculation traps

• Using 8.21 instead of 0.25 for the change in mass.
• Forgetting to subtract to find change in mass if needed.
• Dividing by final mass instead of starting mass.
• Giving too many decimal places instead of 3 significant figures.

✅ Answers by part

• 05.1: concentration of the sugar solution
• 05.2: potato gained water; water moved in by osmosis because water concentration outside was greater than inside the cells
• 05.3: all points plotted correctly; smooth curved line of best fit
• 05.4: about 0.23–0.24 mol/dm³
• 05.5: 3.14%

💡 Final revision takeaway

This is a classic osmosis practical question. To score highly, you need to know the variable types, explain water movement clearly, plot a graph accurately, read where the graph crosses zero, and complete percentage calculations with the correct formula and rounding.