Reducing and non-reducing sugar tests for Cambridge A-Level Biology
The Benedict's test is the standard A-Level Biology test for reducing sugars. Reducing sugars (such as glucose, fructose and maltose) reduce copper(II) ions in Benedict's solution to copper(I) oxide, giving a colour change from blue to brick-red. Non-reducing sugars (such as sucrose) give a negative result until they are hydrolysed with dilute acid into their reducing monomers, then tested again.
This guide covers both procedures, the expected colour changes, the semi-quantitative version of the test, and the precise mark-scheme wording examiners reward in the Cambridge International A-Level Biology (9700) specification.
Benedict's solution contains Cu(II)
Benedict's solution is blue because of copper(II) sulfate. Reducing sugars reduce Cu(II) to Cu(I), forming insoluble red copper(I) oxide.
Non-reducing sugars need hydrolysis
Sucrose gives a negative result. Boil with dilute HCl to hydrolyse it into glucose and fructose, neutralise with sodium hydrogencarbonate, then retest with Benedict's.
Colour shows concentration
Green to yellow to orange to brick-red shows increasing concentration of reducing sugar. The test is semi-quantitative, not exact.
What is a reducing sugar?
A reducing sugar is one that can donate electrons to another molecule, reducing it. In the Benedict's test, the reducing sugar reduces copper(II) ions (Cu2+) to copper(I) ions (Cu+), which form insoluble brick-red copper(I) oxide.
Reducing sugars have a free aldehyde or ketone group that is not locked up in a glycosidic bond. All monosaccharides (glucose, fructose, galactose) are reducing sugars. The disaccharides maltose and lactose are also reducing sugars. Sucrose is the main non-reducing sugar you need to know.
Procedure: Benedict's test for reducing sugars
The standard Benedict's test takes about five minutes and gives a colour change from blue to brick-red if reducing sugar is present. Read the colour after heating, not before.
| Step | What to do | Why |
|---|---|---|
| 1 | Add 2 cm³ of the sample solution to a test tube | Standard volume so results are comparable |
| 2 | Add an equal volume of Benedict's solution | Excess Benedict's so the test is not limited by the reagent |
| 3 | Place the tube in a boiling water bath (around 95–100°C) for 3–5 minutes | Heat is needed for the redox reaction to proceed |
| 4 | Observe the final colour | Colour indicates whether reducing sugar is present and roughly how much |
Reading the colour change
Benedict's is semi-quantitative: The colour after heating gives a rough indication of how much reducing sugar is present. You should be able to name each colour and what it indicates in the exam.
| Final colour | Reducing sugar concentration | Interpretation |
|---|---|---|
| Blue (no change) | None | Negative result |
| Green | Very low | Trace amount of reducing sugar |
| Yellow | Low | Small amount of reducing sugar |
| Orange | Moderate | Moderate amount of reducing sugar |
| Brick-red | High | Large amount of reducing sugar |
Why the test is semi-quantitative Benedict's gives a relative measure, not an exact one. For a quantitative reducing sugar test, filter and dry the copper(I) oxide precipitate and weigh it, or use a colorimeter to measure the absorbance of the remaining blue Benedict's solution. The more reducing sugar in the original sample, the more Cu(II) is reduced, so less blue colour remains and the absorbance reading is lower.
Procedure: Testing for non-reducing sugars
Non-reducing sugars such as sucrose do not react with Benedict's because the reducing group is locked up in the glycosidic bond. To test for a non-reducing sugar, you must first break that bond by hydrolysis, then run the Benedict's test on the resulting monosaccharides.
| Step | What to do | Why |
|---|---|---|
| 1 | Run a standard Benedict's test on a fresh sample. Result: Negative (blue) | Confirms no reducing sugar is present in the original sample |
| 2 | Take a new sample and add 2 cm³ of dilute hydrochloric acid | Acid catalyses hydrolysis of the glycosidic bond |
| 3 | Heat in a boiling water bath (around 95–100°C) for 5 minutes | Hydrolyses sucrose into glucose and fructose |
| 4 | Cool, then neutralise with sodium hydrogencarbonate until fizzing stops | Benedict's only works in alkaline conditions; acid must be neutralised |
| 5 | Add Benedict's solution and heat again in a boiling water bath for 3–5 minutes | Tests for the reducing monosaccharides produced by hydrolysis |
| 6 | Observe colour. Brick-red (or shade thereof) confirms non-reducing sugar in the original sample | A positive result after hydrolysis only is the diagnostic combination |
Why neutralisation matters Benedict's solution is alkaline. If you add it to an acidic sample, the acid reacts with the alkali and the test will not work properly. Use sodium hydrogencarbonate (NaHCO3) and add until fizzing stops, then add a little extra. Universal indicator or pH paper can confirm the pH is around 8.
Where students lose marks
Cambridge International examiner reports flag the same procedural errors every year. Most are about controls and the logic of the non-reducing test, not the chemistry itself.
Common mistakes that cost easy marks Forgetting to test the original sample with Benedict's first to confirm it is not already a reducing sugar. Forgetting to neutralise after acid hydrolysis. Saying the colour change is 'red' rather than the specific 'brick-red'. Writing that Benedict's 'reduces' the sugar (it is the sugar that reduces Cu(II)). Heating with a Bunsen flame rather than a water bath. Not stating that a boiling water bath (around 95–100°C) is used, or the heating time (3–5 min).
Worked example: Identifying an unknown sugar
A student is given an unknown sugar solution and asked to identify whether it is a reducing sugar, a non-reducing sugar or neither.
Step 1: Run a Benedict's test on the original sample. If the result is brick-red, the sugar is a reducing sugar. Stop.
Step 2: If the result is blue (negative), boil a fresh sample with dilute HCl for 5 minutes to hydrolyse it.
Step 3: Cool the sample and neutralise with sodium hydrogencarbonate.
Step 4: Run a second Benedict's test on the hydrolysed, neutralised sample.
Step 5: A positive result (orange or brick-red) confirms a non-reducing sugar in the original. A second negative result means no sugar was present, or the sample contains something other than a sugar.
Full-mark answer should reference the precise volumes, the use of a boiling water bath (around 95–100°C), heating time (3–5 min) and the need for both tests as controls for each other.
Key facts to memorise
- Benedict's solution contains copper(II) sulfate and is alkaline blue
- Reducing sugars include glucose, fructose, galactose, maltose, lactose
- Sucrose is the main non-reducing sugar at A-Level
- Heat in a boiling water bath (around 95–100°C) for 3–5 minutes
- Colour scale: Blue → green → yellow → orange → brick-red
- Non-reducing test: Negative Benedict's, then HCl + heat, neutralise, repeat
- Neutralise with sodium hydrogencarbonate before the second Benedict's test
- For quantitative measurement, use a colorimeter or weigh the precipitate
