AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Required practical activity 4: use qualitative reagents to test for a…
Full specification wording
The human digestive system: Required practical activity 4: use qualitative reagents to test for a range of carbohydrates, lipids and proteins.
This practical develops the skill of identifying biological molecules using qualitative reagents.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 18 of 25
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This practical develops the skill of identifying biological molecules using qualitative reagents. You prepare a food sample, often by grinding with water and filtering for most tests. Benedict’s solution is added and heated in a water bath; a colour change from blue to brick-red indicates reducing sugar. Iodine solution indicates starch with a colour change from orange-brown to blue-black. For lipids, add ethanol to an unfiltered sample, shake, then pour into water; a cloudy white emulsion indicates lipid. Biuret reagent indicates protein with a colour change from blue to purple. You must use clean apparatus and suitable controls to make comparisons valid.
What earns the marks
- The lipid emulsion test requires an unfiltered food sample extracted with ethanol, poured into water to observe a cloudy white emulsion.
- Aqueous samples for Benedict's, iodine and Biuret tests are prepared by crushing or grinding the food with water and filtering.
- States that Benedict’s solution is added to the sample and heated in a water bath, with a colour change towards brick-red indicating a reducing sugar.
- States that iodine solution is added at room temperature and a blue-black colour indicates starch.
- States that Biuret reagent is added and a colour change from blue to purple indicates protein.
Common mistakes and how to put them right
Using a filtered aqueous sample for the lipid test.
Put it right by using an unfiltered sample extracted directly with ethanol.
Heating Benedict’s test tubes directly over a Bunsen flame rather than in a water bath.
Put it right: Use a water bath at around 75 °C to heat gently and safely.
Adding iodine solution and then heating the sample.
Put it right: The iodine test for starch is carried out at room temperature without heating.
Exam tips
- Learn the reagent, the conditions and the positive result for each food test as a three-part sequence so you can recall them quickly.
- When describing a method, include practical details such as volumes, water bath temperature and the need for a control, because these show understanding of valid testing.
Try the lesson
The start of the The human digestive system: Required practical activity 4: use qualitative reagents to test for a range of carbohydrates, lipids and proteins. lesson, made from checked teaching notes for this specification point.
Have a go first
In Required practical activity 4, which reagent is used to test a food sample for the presence of starch?
Listen
This practical develops the skill of identifying biological molecules using qualitative reagents. You prepare a food sample, often by grinding with water and filtering for most tests. Benedict’s solution is added and heated in a water bath; a colour change from blue to brick-red indicates reducing sugar. Iodine solution indicates starch with a colour change from orange-brown to blue-black. For lipids, add ethanol to an unfiltered sample, shake, then pour into water; a cloudy white emulsion indicates lipid. Biuret reagent indicates protein with a colour change from blue to purple. You must use clean apparatus and suitable controls to make comparisons valid. A strong answer earns marks for these points. The lipid emulsion test requires an unfiltered food sample extracted with ethanol, poured into water to observe a cloudy white emulsion. Aqueous samples for Benedict's, iodine and Biuret tests are prepared by crushing or grinding the food with water and filtering. States that Benedict’s solution is added to the sample and heated in a water bath, with a colour change towards brick-red indicating a reducing sugar. States that iodine solution is added at room temperature and a blue-black colour indicates starch. A common slip: Using a filtered aqueous sample for the lipid test. Instead, using an unfiltered sample extracted directly with ethanol. Exam tip: Learn the reagent, the conditions and the positive result for each food test as a three-part sequence so you can recall them quickly.
Key words
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Benedict's solution
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Iodine solution
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Biuret reagent
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Lipid emulsion test
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See every definition
- Benedict's solution
- Added to the sample and heated in a water bath, a colour change towards brick-red indicates a reducing sugar.
- Iodine solution
- Added at room temperature, a blue-black colour indicates starch.
- Biuret reagent
- Added to the sample, a colour change from blue to purple indicates protein.
- Lipid emulsion test
- Ethanol is added to an unfiltered sample, shaken, then poured into water, and a cloudy white emulsion indicates lipid.
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