Anomalous Potato Osmosis Data in SHS 2
Mr Adjei's SHS 2 biology class (42 students in six groups) has just completed a potato osmosis practical. Each group cut six cylinders from the same potato, blotted them, recorded initial mass on a top-pan balance reading to 0.01 g, and immersed one cylinder in each sucrose solution (0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol dm⁻³) for 30 minutes. Group 2 recorded +2.0% at 0.2 mol dm⁻³; Group 3 recorded −6.2% at 0.4 mol dm⁻³; Group 4 recorded +0.8% at 0.6 mol dm⁻³; Groups 5 and 6 recorded −11.4% and −18.9% at 0.8 and 1.0 mol dm⁻³ respectively. The 0.0 mol dm⁻³ control gained 4.1%. Mr Adjei noticed that Group 4 arrived late from assembly and their cylinders were immersed for only 18 minutes. A student announces that 0.6 mol dm⁻³ is the isotonic point and asks whether the class can stop the analysis.
1. Explain which concentration is the most defensible estimate of the isotonic point, and evaluate the student's claim using the evidence in the data.
Read the worked answer
The isotonic point is the external concentration at which there is no net water movement, so the mass change is zero. In these data the sign changes between 0.2 mol dm⁻³ (+2.0%) and 0.4 mol dm⁻³ (−6.2%), so the best estimate lies between these values, nearer 0.2–0.3 mol dm⁻³. A simple linear interpolation gives approximately 0.25 mol dm⁻³, but this is only an estimate because the intervals are wide and each point is based on limited replication.
The student's claim is not defensible. A value of +0.8% at 0.6 mol dm⁻³ breaks the otherwise monotonic trend: 0.4 mol dm⁻³ is already −6.2%, and 0.8 mol dm⁻³ is −11.4%. An isotonic point cannot sit between two strongly negative values. Group 4's shortened immersion time of 18 minutes would reduce net water movement, and late handling or uneven blotting could add mass. Therefore the 0.6 mol dm⁻³ result is more plausibly procedural error than evidence of isotonicity. The class should not stop; it should treat that point as unreliable and repeat it.
2. Describe two specific checks Mr Adjei should require before accepting or rejecting the 0.6 mol dm⁻³ value, and state the expected result if the anomaly is procedural.
Read the worked answer
First, he should require a timed repeat at 0.6 mol dm⁻³ using fresh cylinders from the same potato and the same stock solution, with standard blotting and a full 30-minute immersion. If the repeat gives a negative mass change consistent with the trend, the original +0.8% is likely a procedural outlier. Second, he should run a paired handling control: two cylinders in 0.6 mol dm⁻³, one removed at 18 minutes and one at 30 minutes, both blotted in the same way. If the 18-minute cylinder is closer to zero or slightly positive while the 30-minute cylinder loses mass, the shortened immersion is a sufficient explanation. He should also check that Group 4 used the labelled 0.6 mol dm⁻³ solution and that the balance was zeroed before weighing. If the anomaly persists across repeats and handling controls, then biological variation or contaminated solution should be considered.
3. How should Mr Adjei frame his feedback so that the student's data-based questioning is corrected without being discouraged?
Read the worked answer
He should first validate the student's move: noticing a single value and turning it into a claim is exactly the kind of reasoning the practical is meant to develop. He can then ask the class to plot percentage mass change against sucrose concentration and to mark the 0.6 mol dm⁻³ point. The plot makes the outlier visible without the teacher simply announcing the answer. A structured prompt such as 'What would the graph look like if 0.6 mol dm⁻³ were truly isotonic?' forces students to compare the claim with the neighbouring points at 0.4 and 0.8 mol dm⁻³.
He should separate reliability from validity: the balance readings may be precise, but one group's shortened immersion makes that condition invalid for estimating the isotonic point. The class should agree a rule: an isotonic estimate requires a zero crossing supported by neighbouring concentrations and repeats. Finally, he can ask the student to design the repeat, giving the student responsibility for correcting the dataset rather than treating the error as a failure.