Required Practical: Food Tests (Benedict's, Biuret, Iodine)

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Required Practical: Food Tests (Benedict's, Biuret, Iodine)

Painted required practical apparatus showing the three food tests: three pairs of test tubes on a wooden rack, each pair showing the before-and-after colour change. Benedict's test for reducing sugars (blue heated in water bath turns brick red). Biuret test for proteins (pale blue turns purple, no heating). Iodine test for starch (yellow-brown turns dark blue-black, no heating). Parchment notes that Benedict's requires water-bath heating while biuret and iodine do not, and to always test for all three when asked to identify an unknown food.

What each test detects

  • Benedict's reagent — tests for reducing sugars (e.g. glucose). Heat the sample with Benedict's in a water bath. Positive result: blue → brick red. Negative: stays blue.
  • Biuret reagent — tests for proteins. Add biuret to sample, no heating. Positive result: pale blue → purple/lilac. Negative: stays blue.
  • Iodine solution — tests for starch. Add iodine to sample, no heating. Positive result: yellow-brown → blue-black. Negative: stays yellow-brown.

Exam tip

When asked to identify an unknown food, perform all three tests on separate samples. Examiners often give a food that contains MORE than one nutrient — students who only do one test miss marks. Also note the heating step: only Benedict's requires a water bath. Forgetting to heat Benedict's (or wrongly heating biuret/iodine) is a common mark-losing error.

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