Compare percentage mass change of potato cylinders immersed in different sucrose concentrations. Concentration is independent; mass change is the measured response.Labelled apparatus schematic; not to scale. Follow the measurements and connections, not the drawn dimensions.
Osmosis is net water movement through partially permeable membranes from more dilute to more concentrated solution. Do not describe it as sucrose moving into cells.
Use a cork borer, blade, ruler, balance, labelled tubes, measured solution volumes and blotting paper. Control dimensions, potato source, temperature, immersion time and blotting.
Use sufficient solution and keep cylinders fully submerged. Handle cutting tools carefully on a suitable surface, away from fingers; do not eat the samples.
Method
Remove peel if required and cut cylinders of equal diameter and length from the same potato. Blot consistently and record each initial mass.
Prepare a known sucrose-concentration range including distilled water. Use equal measured volumes in labelled tubes.
Immerse every cylinder for the same duration. Record a staggered start/removal sequence if necessary so each receives equal immersion time.
Remove, blot surface solution gently using the same procedure, then record final mass promptly. Do not squeeze water from cells.
Use several independent cylinders at each concentration. Calculate each cylinder's percentage change, then the mean for that concentration.
Percentage change accounts for starting-mass differences; it does not remove the need to control dimensions and surface area to volume ratio.
Analysis and improvement
Percentage mass change = (final − initial) ÷ initial × 100. Positive means gained mass; negative means lost mass. The denominator is initial mass.
A cylinder from 4.00 g to 4.40 g gains 10%; from 4.00 g to 3.60 g gives −10%. Dilute external solutions usually cause net water entry; concentrated ones cause loss.
Plot sucrose concentration on the horizontal axis and mean percentage mass change on the vertical axis. Where the best-fit line crosses zero estimates the concentration with no overall water movement. Read between your measured points (interpolate); this is an estimate, not a direct chemical measurement of cell sap.
Test more concentrations near where the graph crosses zero to improve the estimate. Water molecules still move both ways when there is no net movement (at equilibrium); the movements balance overall.
Zero/check the balance and record resolution. Blot consistently: clinging liquid raises final mass and shifts mass-change values upwards.
Keep temperature, solution volume and immersion time stable. A short duration may not reach equilibrium, so conclusions depend on the stated time.
Measure repeat spread and investigate damaged cylinders or label mistakes. Natural biological variation remains even with a precise balance.
Exam skills: planning, precision and evaluation
State what you change (the independent variable), what you measure (the dependent variable) and what you keep the same (control variables). Explain how you keep each control variable constant, rather than just saying “make it fair”.
Accuracy means how close a result is to the true value. Precision means how close repeated measurements are to each other. Resolution is the smallest change an instrument can show. More digits on a display do not automatically mean a more accurate result.
Repeat measurements for each condition, calculate a mean and describe how spread out the results are. This helps assess and reduce the effect of random errors. Repeating cannot fix an error that pushes results consistently in one direction (a systematic error), such as surface liquid consistently left on potato samples.
Repeatability means getting similar results when the same person repeats the same method with the same equipment. Reproducibility means getting similar results when someone else, or different suitable equipment, repeats the experiment. Results can be consistent but still inaccurate.
Check that instruments read zero correctly and are calibrated where needed. Read scales at eye level: looking from an angle can give a wrong reading (parallax error). Choose suitable ranges, measurement intervals and scale divisions (resolution).
Write down the original readings straight away in a table, with units in the headings. Use decimal places that match the instrument’s resolution. Keep the original data and round only when needed. Do not discard a result just because it differs from your prediction.
An anomalous result does not fit the pattern of the other results. Repeat that measurement and check the method. Only leave it out of a mean if you have a clear reason; state which result you excluded and why.
For continuous variables, plot the independent variable on the horizontal axis and the dependent variable vertically. Use sensible scales, units and a best-fit line or curve; do not automatically join every point or force the graph through zero.
Find the gradient of a straight best-fit line using a large triangle: vertical change ÷ horizontal change. For a curve, draw a tangent to estimate the gradient at one point. Explain what the gradient shows in this experiment, include its units and use measured values to support your conclusion.
Uncertainty describes the possible range around a measurement. For one reading on a scale, half the smallest division is a useful classroom estimate unless the question says otherwise. If you subtract two readings, both have uncertainty. Percentage uncertainty = absolute uncertainty ÷ measured value × 100. Follow the method specified in the question.
Use results as evidence and then explain what they mean. A pattern linking variables (a correlation) does not prove that one causes the other. If the ranges of repeat results overlap, a claimed difference may be less convincing. Keep conclusions within the range tested and suggest an improvement that tackles a specific error.
Mr Kidman's Classroom