Estimating the water potential of plant tissue is a core 9700 Paper 3 investigation. By hand, plan and make a proportional dilution of 1.00 mol dm⁻³ sucrose with two syringes, bore potato cores and trim each cylinder to 2.0 cm against the ruler, blot and weigh it, soak it for 30 timed minutes, then blot and reweigh. Work out the percentage changes, draw your own line of best fit, read off the no-change concentration and turn it into a water potential with the data table, then spot the anomaly and evaluate. Every attempt uses a different potato. Allow about 45 minutes.
This workout needs JavaScript.
A proportional dilution table for six concentrations from 0.00 to 1.00 mol dm⁻³ in 10 cm³, and the six tubes on the bench made up to the volumes you planned.
Four headings with units, initial and final masses exactly as the balance showed them to 0.01 g, signed percentage changes to the same number of decimal places, and the controlled method: equal 2.0 cm cylinders, blotted before every weighing, 30 minutes in every solution.
Concentration on the x-axis and percentage change in mass on the y-axis, a straight line of best fit that ignores the anomaly, the concentration where your line crosses 0 %, and an accuracy mark for a value close to the one careful technique gives.
The water potential of the tissue read from the data table at your concentration, and the results explained in water potential terms.
The anomalous result and what to do about it, standardised variables, uneven blotting as the main source of error, replicates and closer concentrations as the improvement, and a risk assessment for the scalpel.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.