A student tested a sample of food, X, with three reagents. The results are shown.
| Test | Colour at end |
|---|---|
| Iodine solution | blue-black |
| Benedict's solution (heated) | blue |
| Biuret reagent | purple |
(a) Describe how the student should carry out the Benedict's test and state what a positive result would look like. [3]
(b) Use the results to name the two types of biological molecule shown to be present in food X. [2]
- (a) add Benedict's solution to the (dissolved / ground-up) food sample; accept add an equal volume of Benedict's(1)
- (a) heat (in a hot / boiling water bath); accept warm / boil / heat to about 80–100 °C; method of heating not required for the mark(1)
- (a) (positive) colour changes from blue to brick-red / orange; accept green / yellow / red as the final colour(1)
- (b) starch; accept carbohydrate / polysaccharide(1)
- (b) protein; reject glucose / reducing sugar (Benedict's stayed blue)(1)
- Answer: (a) add Benedict's, heat in water bath, blue → brick-red (b) starch and protein
- Examiner tip: Benedict's only reacts when heated — 'add Benedict's and leave it' loses the heating mark, so always write 'heat in a hot water bath'.