The human digestive system: To include: Benedict’s test for sugars; iodine test for starch; and Biuret reagent for protein.

The idea

Food samples are tested chemically to identify reducing sugars, starch and protein. Benedict’s test: add Benedict’s solution to a food solution and heat in a water bath; a blue-to-brick-red colour change shows reducing sugar, with green, yellow or orange indicating intermediate amounts. Iodine test: add iodine solution to the sample; a yellow-brown to blue-black change shows starch. Biuret test: add Biuret reagent; a blue to purple or lilac change shows protein. Each test detects a different nutrient, so a full analysis uses all three. Controls, such as water, show the negative colour. These qualitative tests are assessed in written papers and may be performed in practical work.

What earns the marks

• Benedict’s test detects reducing sugars; the sample is heated with Benedict’s solution in a water bath and a positive result is a colour change from blue to brick-red.
• The iodine test detects starch; iodine solution added to the sample turns from yellow-brown to blue-black when starch is present.
• The Biuret test detects protein; Biuret reagent added to the sample turns from blue to purple or lilac when protein is present.
• A negative result is the reagent’s original colour, so a water control can confirm that the reagent has not changed colour.
• The tests are qualitative: they show whether a nutrient is present, not its exact concentration, although Benedict’s colour can indicate relative amounts of reducing sugar.
• Each test is specific to one nutrient, so identifying an unknown food requires separate tests for sugars, starch and protein.

Common mistakes and how to put them right

• Using iodine solution to test for sugar.

  Put it right: Iodine detects starch, so the correction is to use Benedict’s solution and heat the sample.

• Expecting Biuret reagent to turn purple without protein.

  Put it right: Blue is the negative result and purple or lilac indicates protein.

• Heating Benedict’s test with a Bunsen flame directly.

  Put it right: Use a water bath, which gives safer, more even heating.

Exam tips

• State the reagent, the starting colour and the positive colour for each test, because marks are often awarded for the full colour change.
• Use the phrase reducing sugar rather than just sugar when describing Benedict’s test, and link the result to the nutrient detected.
• When comparing foods, record results in a table with columns for food, Benedict’s, iodine and Biuret, so each conclusion is supported by evidence.