Benedict's and Iodine Tests Explained
How to test for reducing sugars, non-reducing sugars and starch with Benedict's reagent and iodine, including the acid hydrolysis route for non-reducing sugars. Includes the 4-mark exam question, a worked model answer and the examiner trap on a blue result.
Chapters (7)
Exam question [4 marks]
A student adds Benedict's reagent to a sample of a sugar solution and heats it. The mixture stays blue. Describe how the student could show that the sample contains a non-reducing sugar.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | The blue result shows there is no reducing sugar | "no reducing sugar present", "negative for reducing sugar", "only non-reducing sugar (or no sugar) present" | "no sugar present"; "no sugar" with no mention of reducing |
| 2 | Heat / boil a sample (fresh or the same sample) with (dilute) hydrochloric acid, to hydrolyse the non-reducing sugar into reducing sugars (monosaccharides) | "break the glycosidic bond", "hydrolyse sucrose to glucose and fructose", "acid hydrolysis" | "heat with acid" with no purpose and no hydrolysis; "dissolve the sugar" |
| 3 | Neutralise the acid, e.g. with sodium hydrogencarbonate (Benedict's needs alkaline conditions) | "add alkali", "add sodium hydroxide until neutral/alkaline" | "add water to dilute the acid"; "cool the mixture" |
| 4 | Heat with Benedict's reagent; a brick-red precipitate shows a non-reducing sugar was present | "blue to brick-red / orange-red precipitate" | "turns red" with no mention of precipitate or the starting blue; any colour change credited to the first Benedict's test |
Mark point 1 is the logic that a blue first result does not mean no sugar. Point 2 needs both the acid and its purpose (hydrolysis), point 3 needs neutralisation, and point 4 needs the colour result. The iodine test for starch (orange-brown to blue-black) is taught in the video but is not credited in this question. Order matters: neutralise before the second Benedict's test.
Model answer (4/4)
The blue result shows there is no reducing sugar in the sample. Boil a sample with dilute hydrochloric acid, which hydrolyses the non-reducing sugar into reducing sugars such as glucose and fructose. Neutralise the acid with sodium hydrogencarbonate, because Benedict's reagent needs alkaline conditions. Then heat with Benedict's reagent. A brick-red precipitate shows that a non-reducing sugar was present.
Examiner trap
- Saying a negative Benedict's test means no sugar present (non-reducing sugar may be present)
- Forgetting to boil with acid then neutralise before re-testing for non-reducing sugar
- Saying the colour is 'red' when the mark scheme wants a colour change from blue to brick-red precipitate
Key terms
- Benedict's reagent
- reducing sugar
- non-reducing sugar
- hydrolyse with acid
- neutralise
- iodine/potassium iodide
- blue-black
- brick red
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:21Reducing and non-reducing sugars
Sugars fall into two groups. Reducing sugars, and non-reducing sugars. A reducing sugar reduces copper two plus ions to copper one plus, which is what the Benedict's test detects. All monosaccharides are reducing sugars, like glucose and fructose. So are some disaccharides, such as maltose and lactose. Sucrose is the one you must remember. It is a non-reducing sugar.
It cannot reduce copper, so on its own it gives no colour change with Benedict's. That is exactly why our student saw blue.
0:56The Benedict's test colour scale
Here is the Benedict's test. Add Benedict's reagent to the sample, and heat it in a water bath. Benedict's starts blue, because it contains copper two plus ions, written Cu 2 plus. If a reducing sugar is present, those ions are reduced to copper one, and a coloured precipitate forms. The colour goes from blue, to green, to yellow, to orange, and finally to brick red.
The more reducing sugar, the further along the colour goes. So the test is semi-quantitative. In the exam, write blue to brick red precipitate. Not just, it turns red.
1:35Testing for non-reducing sugars
Now the clever part. A blue result means no reducing sugar. It does not mean no sugar at all. A non-reducing sugar cannot react until we break it down. So, take a fresh sample of the solution, add dilute hydrochloric acid, and boil it. The acid hydrolyses the glycosidic bond. Sucrose splits into glucose and fructose, and both are reducing sugars.
Next, neutralise the acid by adding sodium hydrogencarbonate. Benedict's only works in alkaline conditions, so skip this step and the test fails. Finally, add Benedict's reagent and heat again, in the water bath. The sucrose is now glucose and fructose, so they can reduce the copper two plus ions. If a brick red precipitate forms, a non-reducing sugar was in the original sample.
2:28The iodine test for starch
One more test. Starch. Add a few drops of iodine dissolved in potassium iodide solution. No heating needed. If starch is present, the colour changes from orange-brown to blue-black. If there is no starch, it stays orange-brown.
Notice that this test shows starch only. It says nothing about sugars.
3:26Examiner trap and recap
The trap. Blue does not mean no sugar. It means no reducing sugar. Recap. Benedict's gives blue to brick red for reducing sugars. For non-reducing sugars, boil with acid, neutralise, then test again. Iodine turns blue-black for starch.
Next time, we move from carbohydrates to lipids, and see how triglycerides are built.
Watch first
This lesson builds on:
- Maltose, Sucrose, Lactose: Glycosidic Bonds · AQA 7402 · 3.1.2
Keep going
More from 3.1 Biological molecules
- Maltose, Sucrose, Lactose: Glycosidic Bonds · AQA 7402 · 3.1.2
- Alpha vs Beta Glucose: Starch vs Cellulose · AQA 7402 · 3.1.2
- Condensation and Hydrolysis Reactions · AQA 7402 · 3.1.1
- Triglycerides: Saturated vs Unsaturated Fats · AQA 7402 · 3.1.3
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Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-004.