Food tests
Food tests
- You can test a food to find which substances it contains.
- Add the test chemical and watch the colour change.
- Learn each test and its positive result.
The five tests
| Substance | Test | Positive result |
|---|---|---|
| starch | iodine solution | orange-brown → blue-black |
| reducing sugar | Benedict's + heat | blue → brick-red |
| protein | biuret solution | blue → purple |
| fats/oils | ethanol emulsion test | a cloudy white layer |
| vitamin C | blue DCPIP | blue → colourless |
Practice
Iodine solution turns blue-black with:
Orange-brown iodine turns blue-black when starch is present.
Practice
The test for reducing sugar is:
Benedict's solution turns from blue to brick-red on heating with a reducing sugar.
Practice
Biuret solution turns purple with:
Biuret goes from blue to purple when protein is present (no heating needed).
Two things to remember
- Only the Benedict's test needs heating.
- DCPIP loses its colour (goes colourless); the others gain a new colour.
Practice
Which statement is correct?
Only Benedict's needs heat; DCPIP decolourises while the other tests gain a colour.
You've got it
Key idea
- starch + iodine → blue-black; reducing sugar + Benedict's (heat) → brick-red
- protein + biuret → purple; fat → cloudy (ethanol emulsion); vitamin C → DCPIP colourless
- only Benedict's needs heat; DCPIP loses colour, the others gain one