Required Practical 7: Food Tests (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Required practical 7: Using qualitative reagents to test for carbohydrates, lipids, and proteins

• Aim: To use qualitative reagents to test for a range of carbohydrates, lipids and proteins. To include: Benedict’s test for sugars, Iodine test for starch, the emulsion test for lipids and the Biuret reagent for protein

• You will:

  • Use qualitative reagents to test for the presence of key biological molecules in a range of foods

  • Safely use appropriate heating devices and techniques including the use of a Bunsen burden and a water bath

• A qualitative food test indicates if a substance is present or absent in a sample (although it doesn’t tell you how much is present)

• Observations are essential in this practical; you are looking for colour changes in particular which can indicate if a substance is present or absent:

Food test colour changes table

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Preparing a sample

• Before you can carry out any of the food tests described below, you may need to prepare a food sample first (especially for solid foods to be tested)

• To do this:

  • Break up the food using a pestle and mortar

  • Transfer to a test tube and add distilled water

  • Mix the food with the water by stirring with a glass rod

  • Filter the mixture using a funnel and filter paper, collecting the solution

  • Proceed with the food tests

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It is important that you carry out the tests methodically, recording your observations carefully

Important hazards

• Whilst carrying out this practical you should try to identify the main hazards and be thinking of ways to reduce harm:

  • Biuret solution contains copper (II) sulfate which is dangerous particularly if it gets in the eyes, so always wear goggles

  • Iodine is also an irritant to eyes (wear goggles)

  • Sodium hydroxide in biuret solution is corrosive, if any chemicals get onto your skin wash hands immediately

  • Ethanol is highly flammable; keep it away from the Bunsen burner used in the Benedict’s test (you should turn the Bunsen off completely)

  • And of course, the Bunsen itself is a hazard!

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Be prepared to explain what molecules are or are not present in a food sample – make sure you know the positive and negative results for each test