AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Biochemical tests using Benedict's solution for reducing sugars and…
Full specification wording
Carbohydrates: Biochemical tests using Benedict's solution for reducing sugars and non-reducing sugars and iodine/potassium iodide for starch.
To test for reducing sugars (e.g. glucose), add Benedict's solution and heat in a water bath.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 8 of 8
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
To test for reducing sugars (e.g. glucose), add Benedict's solution and heat in a water bath. A positive result forms a green, yellow, orange, or brick-red precipitate, as Cu²⁺ reduces to Cu⁺. For non-reducing sugars (e.g. sucrose), first confirm a negative Benedict's result. Then, boil a fresh sample with dilute hydrochloric acid to hydrolyse the glycosidic bonds. Neutralise with sodium hydrogencarbonate, then repeat the Benedict's test. For starch, add iodine in potassium iodide solution; a positive result turns blue-black. Benedict's is qualitative but becomes quantitative by filtering and weighing the precipitate, or by using a colorimeter with a calibration curve.
What earns the marks
- Describe the reducing sugar test: add Benedict's solution and heat in a water bath to observe a green, yellow, orange, or brick-red precipitate.
- Describe the non-reducing sugar test: boil the sample with dilute hydrochloric acid to hydrolyse it, neutralise (e.g., with sodium hydrogencarbonate), and retest with Benedict's solution and heat.
- Describe the starch test: add iodine in potassium iodide solution to the sample, looking for a colour change to blue-black.
- Explain how to make the Benedict's test quantitative: use a colorimeter to measure absorbance and compare against a calibration curve of known glucose concentrations.
Common mistakes and how to put them right
Omitting the heating step during acid hydrolysis for non-reducing sugars.
Put it right: Specify boiling or heating the sample with dilute hydrochloric acid to ensure hydrolysis occurs.
Forgetting to neutralise the sample after acid hydrolysis.
Put it right: State that the solution must be neutralised because Benedict's reagent requires alkaline conditions to function.
Describing the Benedict's positive result as a solution colour change rather than a precipitate.
Put it right: Explicitly state that a coloured precipitate (e.g., brick-red) forms.
Exam tips
- While 'iodine solution' is often accepted in exams, it is best practice to write 'iodine in potassium iodide solution' to demonstrate precise biochemical knowledge.
- When describing the non-reducing sugar test, ensure you sequence the steps logically: initial negative Benedict's test, boil with dilute HCl, neutralise, and finally retest with Benedict's and heat.
Try the lesson
The start of the Carbohydrates: Biochemical tests using Benedict's solution for reducing sugars and non-reducing sugars and iodine/potassium iodide for starch. lesson, made from checked teaching notes for this specification point.
Have a go first
Which colour change shows a positive result when iodine/potassium iodide solution is added to a food sample containing starch?
Listen
To test for reducing sugars (e.g. glucose), add Benedict's solution and heat in a water bath. A positive result forms a green, yellow, orange, or brick-red precipitate, as Cu²⁺ reduces to Cu⁺. For non-reducing sugars (e.g. sucrose), first confirm a negative Benedict's result. Then, boil a fresh sample with dilute hydrochloric acid to hydrolyse the glycosidic bonds. Neutralise with sodium hydrogencarbonate, then repeat the Benedict's test. For starch, add iodine in potassium iodide solution; a positive result turns blue-black. Benedict's is qualitative but becomes quantitative by filtering and weighing the precipitate, or by using a colorimeter with a calibration curve. A strong answer earns marks for these points. Describe the reducing sugar test: add Benedict's solution and heat in a water bath to observe a green, yellow, orange, or brick-red precipitate. Describe the non-reducing sugar test: boil the sample with dilute hydrochloric acid to hydrolyse it, neutralise (e.g., with sodium hydrogencarbonate), and retest with Benedict's solution and heat. Describe the starch test: add iodine in potassium iodide solution to the sample, looking for a colour change to blue-black. Explain how to make the Benedict's test quantitative: use a colorimeter to measure absorbance and compare against a calibration curve of known glucose concentrations. Exam tip: While 'iodine solution' is often accepted in exams, it is best practice to write 'iodine in potassium iodide solution' to demonstrate precise biochemical knowledge.
See it
Key words
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Biochemical tests for carbohydrates
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Acid hydrolysis of non-reducing sugars
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Qualitative vs quantitative analysis
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See every definition
- Biochemical tests for carbohydrates
- Tests using Benedict's solution for reducing sugars and non-reducing sugars, and iodine/potassium iodide for starch.
- Acid hydrolysis of non-reducing sugars
- Boiling a fresh sample with dilute hydrochloric acid to hydrolyse the glycosidic bonds, then neutralising with sodium hydrogencarbonate before repeating the Benedict's test.
- Qualitative vs quantitative analysis
- Benedict's is qualitative but becomes quantitative by filtering and weighing the precipitate, or by using a colorimeter with a calibration curve.
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