Pearson Edexcel · GCSE · Biology
Course version: 1BI0 · Version from Sept 2016
1.13B Core Practical: Investigate the use of chemical reagents to identify…
Full specification wording
Topic 1 – Key concepts in biology: 1.13B Core Practical: Investigate the use of chemical reagents to identify starch, reducing sugars, proteins and fats
This core practical develops the skill of using chemical reagents as qualitative tests for four food substances.
Checked against the Pearson Edexcel 1BI0 specification (opens in a new tab) on .
- Point 13 of 17
Study notes
From the checked teaching notes for this specification point.
The idea
This core practical develops the skill of using chemical reagents as qualitative tests for four food substances. You prepare a food sample, usually by grinding the food and mixing it with water, then test separate portions. Iodine solution detects starch: a blue-black colour is positive, while iodine's brown-orange colour means starch is absent. Benedict's solution detects reducing sugars such as glucose: heat the sample with Benedict's in a water bath, and a green, yellow, orange or brick-red precipitate indicates a reducing sugar, whereas blue means none. Biuret reagent detects proteins: a colour change from blue to purple (or violet) is positive. For fats, the emulsion test uses ethanol: a cloudy white emulsion forms when fat is present. Each test needs its own portion so reagents do not contaminate one another.
What earns the marks
- Iodine solution is the reagent for starch; a blue-black result is positive and the unchanged brown-orange colour is negative.
- Benedict's solution tests for reducing sugars and requires heating in a water bath; a green/yellow/orange/brick-red precipitate is positive, blue is negative.
- Biuret reagent tests for protein; a colour change from blue to purple/violet indicates protein.
- The emulsion test uses ethanol (added to the sample, then water) and a cloudy white emulsion shows fat is present.
- Separate portions of the sample must be used for each test so that one reagent does not affect another test's result.
Common mistakes and how to put them right
Using iodine to test for sugars.
Put it right: Iodine detects starch, so the correction is to use Benedict's solution for reducing sugars.
Forgetting to heat the sample with Benedict's solution.
Put it right: Benedict's requires heating in a water bath to give the colour change.
Testing all four reagents on one portion.
Put it right: Use a fresh, separate portion for each reagent to avoid contamination.
Exam tips
- Link each reagent to its target substance and its positive colour in a single clear sentence.
- State that Benedict's needs heating, since this is a distinguishing practical detail.
- Describe the emulsion test as producing a cloudy white emulsion, not just 'a change'.
Try the lesson
The start of the Topic 1 – Key concepts in biology: 1.13B Core Practical: Investigate the use of chemical reagents to identify starch, reducing sugars, proteins and fats lesson, made from checked teaching notes for this specification point.
Have a go first
A student adds iodine solution to a food sample and the mixture turns blue-black. Which food substance does this colour change show is present?
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