Signed mass change
Subtract starting mass from final mass, keeping the sign.
AQA 8461 · Osmosis evidence chain
Osmosis: percentage change, and why raw mass will not do
The percentage-change mark. Cylinders start at different masses, so only percentage change is comparable between them — and the sign has to survive the calculation.
Reviewed family · Studara-authored items · No grade-uplift claim
The lost mark
One answer. Several places the method can fail.
Studara does not reduce the response to right or wrong. The reviewed contract checks these states independently.
Percentage of the initial
Divide by the INITIAL mass and multiply by 100.
Explain the direction
Link the sign to the direction of water movement and the concentration gradient.
Why it goes wrong
The diagnosis stays uncertainty-safe.
A wrong response can support a specific measured diagnosis. A blind multiple-choice diagnostic records only a possible cause until later working provides stronger evidence.
01
The error
Reporting the raw change in mass instead of the percentage change relative to the STARTING mass.
02
Why it survives revision
The direction of every change is still correct, so the graph still slopes the right way and still crosses zero in roughly the right place. The conclusion drawn from it can be entirely right while the analysis mark is entirely lost.
The worked repair
Repair the smallest broken step.
Teaching is not evidence. This one move is the repair; the proof checks begin after it disappears.
Divide the change by the INITIAL mass, multiply by 100, and keep the minus sign where there is one.
What happens after the repair
Correction is the start, not the result.
Immediate
Asked again straight away, on a different question
The repair leaves the screen and a new authored item checks the method unassisted. Being taught something is never evidence that it was learned, so this is the first stage that counts at all.
Delayed
Asked again after at least 1 day
A parallel form of the same method, unassisted, with the repair no longer on screen. A retest in the same session does not count.
Transfer
Asked again when the surface changes
The transfer form asks for the concentration at which there is no net movement, which means reading the x-intercept rather than reporting the values.
Exam
Asked again under exam conditions
Timed, carrying the mark tariff, in an exam-like response format and aligned to the specification. Any one of those missing and the evidence is refused.
REVIEWED BOUNDARY
This page covers one reviewed family: Osmosis evidence chain. Calculate signed percentage mass change, then connect water direction, concentration and membrane states. Other families within the same GCSE Biology topic are outside this reviewed journey, and Studara does not apply the proof label to material that has not been reviewed. No grade-uplift claim is made anywhere on this site.
Straight answers
Before you use this guide.
Why use percentage change in mass rather than the raw change?
Because the cylinders do not all start at the same mass. Only percentage change makes the results comparable between them.
What does the point where the line crosses zero mean?
It is the concentration at which there is no net movement of water — the solution is the same concentration as the inside of the cells.
Does Studara guarantee a higher grade?
No. Studara has not run a controlled outcome study and publishes no grade-uplift claim. What has been measured, what is withheld, and the metric that failed its bar are all on the public evidence page.
Turn this method into evidence