AQA GCSE Biology Biology Paper 1 (Foundation), June 2024: Question 6

✅ Correct Answers (Choose Any Two)

• Size (or length/surface area) of potato pieces
• The type/variety or source of potato used
• Volume of salt solution (e.g. 100 cm³)
• Time the potato is left in solution (e.g. 20 minutes)
• The potato was uncooked / temperature

❌ Common Errors & Mark Scheme Guidance

• Do NOT write just "size of potato": You must say the size of the cut piece of potato, not the whole potato vegetable!
• Do NOT write just "time": "Time" alone is too vague; specify "time potato pieces were kept in the solution".
• "Amount of solution" is allowed, but volume is the scientifically precise term.

✅ 06.2 What to do before weighing

Blot or dry the surface of each potato piece using a paper towel.

💡 Why Blotting Matters

Liquid clinging to the outside of the potato piece contributes to the mass reading. Excess water on the surface would cause an inaccurate, falsely high mass measurement that does not reflect actual water absorbed or lost by osmosis.

✅ 06.3 Apparatus to measure mass

Balance (or weighing scale / digital top-pan balance).

🧠 Top Tip

Never write "weighing machine". Always use the correct scientific term: electronic balance or balance.

✅ 06.4 Resolution of Apparatus

Correct box to tick: 0.1 g

✅ 06.5 Greatest Change in Mass

Correct box to tick: D

1. Identify the figures for Potato D from Table 4:
   Mass at start = 6.0 g
   Mass after 20 minutes = 4.9 g
   Change in mass = 4.9 - 6.0 = -1.1 g
2. Substitute into formula [Mark 1]:
   (-1.1 ÷ 6.0) × 100 (Examiners ignore minus sign during substitution)
3. Calculate raw answer [Mark 2]:
   18.3333... %
4. Round to 1 decimal place as requested [Mark 3]:
   -18.3 % (or 18.3 %)

❌ Calculation Traps to Avoid

• Dividing by the final mass: A classic error is dividing by the end mass ( 4.9 g ) instead of the initial mass ( 6.0 g ). Always divide by the start mass.
• Incorrect rounding: 18.333... rounds to 18.3 , NOT 18 or 18.33. Follow the instruction in the question prompt!

🧠 Why calculate percentage change?

Potato cylinders never start with identical masses. Calculating percentage change allows a valid comparison of mass changes between pieces with different initial masses.

✅ Best Presentation Method

Tick: Line graph

💡 Line Graph vs. Bar Chart

Both variables (concentration of solution and percentage change in mass) are continuous quantitative data. Continuous data should always be plotted on a line graph (or scatter graph with a line of best fit), whereas bar charts are used for categoric/discrete data.

💡 Key Knowledge: Definition of Osmosis

Osmosis is the net movement of water molecules from a dilute solution (high water concentration) to a more concentrated solution (low water concentration) across a partially permeable membrane.

❌ Common Misconceptions

• Never say "salt moved into/out of the potato". Salt ions are solutes and do not move freely in this osmosis experiment. Only water moves.
• Don't write "semi-permeable" if you can avoid it; the spec specifically requires partially permeable.

✅ Correct Answer Range

Any concentration in the range:
0.15 to 0.25 mol/dm³

🧠 How to Deduce This from the Table

• At 0.1 mol/dm³: mass increased by +4.8% (water entered).
• At 0.3 mol/dm³: mass decreased by -4.4% (water left).
• No change in mass ( 0% ) must occur between 0.1 and 0.3 mol/dm³. Since +4.8 and -4.4 have roughly equal magnitudes, zero change is roughly halfway: 0.2 mol/dm³.