To include: Benedict’s test for sugars; iodine test for starch; and Biuret…

The idea

This statement specifies the three named qualitative tests you must know: Benedict’s test for sugars, the iodine test for starch and the Biuret test for protein. Benedict’s solution is blue and, when heated with a reducing sugar such as glucose, changes through green and yellow to brick-red; the colour reflects how much reducing sugar is present. Iodine solution is yellow-brown and turns blue-black in the presence of starch. Biuret reagent is blue and turns purple when protein is present. You should be able to state the reagent, the conditions and the positive result for each test, and use these tests to identify unknown food samples. The tests are qualitative, so they show whether a substance is present rather than giving an exact concentration.

What earns the marks

• Names Benedict’s solution as the reagent for reducing sugars and states that the sample must be heated in a water bath.
• Describes the positive Benedict’s result as a colour change from blue through green and yellow to brick-red.
• Names iodine solution as the reagent for starch and states that a blue-black colour is a positive result.
• Names Biuret reagent as the reagent for protein and states that a colour change from blue to purple is a positive result.
• Applies the tests to identify the biological molecules present in an unknown food sample from the observed colours.
• Recognises that these tests are qualitative and indicate presence rather than exact concentration.

Common mistakes and how to put them right

• Stating that Benedict’s test detects all sugars.

  Put it right: Benedict’s test detects reducing sugars such as glucose, while sucrose is a non-reducing sugar and gives a negative result unless it is first hydrolysed.

• Describing the iodine positive result as brown or orange.

  Put it right: Iodine solution is yellow-brown and turns blue-black with starch.

• Saying Biuret reagent turns purple immediately without noting the starting colour.

  Put it right: Biuret reagent starts blue and changes to purple when protein is present.

Exam tips

• Make a three-column summary table of reagent, conditions and positive result for the three tests and use it for quick revision.
• When a question gives a colour result, name the molecule detected and the test used, rather than only naming the colour.
• Use the term reducing sugar accurately when writing about Benedict’s test, and remember that starch and protein tests do not require heating.