A colorimeter only gives an absorbance; a calibration curve turns it into a concentration. Each attempt gives you a different investigation (glucose in urine or reducing sugar in fruit juice with Benedict’s solution, starch in bananas with iodine, or protein in drinks with the biuret test) and fresh readings. Work out the volumes for a proportional dilution, choose the filter and set the zero, spot the anomalous reading and calculate the means, then plot every cross yourself and decide between a ruled line and a smooth curve. Read one unknown from your curve, find out why the other cannot be read until it is diluted, multiply back through every dilution and evaluate the method. Allow about 35 minutes.
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Volumes of stock S and water W that add up to the total volume in every row, the missing concentrations in %, the same unit as the stock (9700 March 2023 examiner report), and why evenly spaced standards from a proportional dilution suit a calibration curve better than a serial dilution (November 2024 report).
A filter of the colour the solution absorbs, never the colour of the solution itself, and zeroing with a cuvette of distilled water before the readings (March 2023 report).
The anomalous reading circled and left out of its mean, and every mean to two decimal places like the readings (June 2024 report: follow the decimal places in the table).
Six crosses within half a small square, a ruled line or smooth curve of best fit that suits the points and does not run beyond the standards (March 2024 and November 2024 reports), and range bars from the lowest to the highest reading used.
P read from your own curve to one small square with construction lines, multiplied back through the dilution; Q recognised as outside the standards and found by diluting it, multiplying by both factors; then a real source of error, more replicate tubes for reliability, and extra standards close to P for accuracy.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.