The four food tests at a glance
One reagent and one colour change per nutrient — learn them as a set.
Food tests use a reagent (a test chemical) that changes colour when a particular nutrient is present. You must know four tests for the Double Award.
| Nutrient | Reagent | Heat? | Negative (no change) | Positive result |
|---|---|---|---|---|
| Glucose (reducing sugar) | Benedict's solution | Yes (water bath) | stays blue | brick-red |
| Starch | iodine solution | No | stays orange-brown | blue-black |
| Protein | Biuret solution | No | stays blue | purple/lilac |
| Fat (lipid) | ethanol + water (emulsion test) | No | stays clear | cloudy white emulsion |
Exam tip. To score the result mark you must give the colour before and after — e.g. "blue → brick-red", not just "it goes red". A bare final colour often loses a mark.
Watch out. Only Benedict's needs heating. Students who write "heat the Biuret/iodine" lose method marks.
- Glucose → Benedict's + heat → brick-red.
- Starch → iodine → blue-black.
- Protein → Biuret → purple/lilac.
- Fat → ethanol emulsion test → cloudy white emulsion.