Rate from a graph tangent
rate at a time = gradient of tangent = change in y / change in x
An instantaneous rate is the gradient of a tangent, not the gradient between arbitrary curve points.
Work it, then mark it
Do each calculation on paper first, then reveal the mark scheme and tick the marks you actually earned — the same way you should mark past papers.
A tangent to a mass-loss curve passes through (40 s, 1.6 g) and (100 s, 0.7 g). Calculate the magnitude of the rate of mass loss.
Do the calculation on paper first — then mark it.
Where the marks get lost
- Using points on the curve instead of points on the tangent.
- Forgetting to use differences in both coordinates.
Exam tip: Choose two well-separated, easy-to-read points on the tangent line to reduce reading uncertainty.
Calculations are the most trainable marks in chemistry
They come up every paper and reward a clean method. Send me one you keep dropping marks on — your first lesson is free.