The semi-quantitative Benedict's test is a 9700 Paper 3 favourite. By hand, switch on the water bath, plan your volumes and make a halving series from a 1.0 % glucose solution so that every tube ends with the same volume, add the unknown U and the same volume of Benedict's solution to each tube, then heat the whole rack in boiling water for exactly 5 minutes on the stopwatch. Compare the colours and the precipitate against a white tile, place U between two standards, and say how a colorimeter or the mass of precipitate would make the test quantitative. Every attempt uses a different unknown and a different batch of reagent. Allow about 40 minutes.
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The concentration heading with its unit, five concentrations that halve each time, volumes that make each tube from the one before and leave every tube with the same volume (tube 1 holds twice the transfer volume, and one transfer volume is discarded from tube 5), and tubes on the bench that really hold those concentrations.
Headings for every column (a colour or an amount described in words has no unit), colours and amounts of precipitate that match your own tubes after heating, and the standardised method checked on the bench: the same volume of solution and of Benedict's solution in every tube, heated together in boiling water for 5 minutes.
U given as a range between the two standards whose colours bracket it, why comparing colours by eye is only semi-quantitative, and how extra standards between the bracketing pair would narrow the range.
A colorimeter (precipitate removed, absorbance of the remaining blue solution with a red filter, a calibration curve from the standards), or the dry mass of the filtered precipitate plotted against concentration.
The variables kept constant (both volumes, the bath temperature and the heating time), the subjective colour match as the main source of error, and a risk assessment that names scalding and the irritant reagent with matching precautions.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.