Biochemical Tests

A-Level Biology requires you to know and apply a range of biochemical tests to identify the major classes of biological molecules: carbohydrates (reducing sugars, non-reducing sugars, starch), proteins, and lipids. You must also understand the difference between qualitative tests (which simply identify whether a substance is present) and quantitative tests (which measure the amount of substance present), as well as the use of colorimetry and calibration curves for quantitative analysis.

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Benedict's Test — Reducing Sugars

Principle

Reducing sugars (all monosaccharides and most disaccharides, except sucrose) have a free aldehyde or ketone group that can reduce Cu²⁺ ions in Benedict's reagent (an alkaline solution of copper(II) sulphate) to Cu⁺ ions, forming a coloured precipitate of copper(I) oxide (Cu₂O).

Method

1. Add approximately 2 cm³ of the sample solution to a test tube.
2. Add an equal volume of Benedict's reagent (a blue solution).
3. Heat the mixture in a water bath at approximately 80 °C for 5 minutes. (A water bath provides an even, controlled temperature; a Bunsen burner should not be used as the temperature would be uncontrolled.)
4. Observe the colour change.

Results

Colour	Interpretation
Blue (no change)	No reducing sugar present
Green	Trace amount of reducing sugar
Yellow	Small amount
Orange	Moderate amount
Brick-red precipitate	Large amount of reducing sugar

Exam Tip: The colour sequence is blue → green → yellow → orange → brick-red. The test is semi-quantitative in its basic form — the colour gives a rough indication of concentration. For a truly quantitative result, colorimetry is required (see below).

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Benedict's Test — Non-Reducing Sugars

Sucrose is the main non-reducing sugar you need to know. It does not react with Benedict's reagent because both anomeric carbons are involved in the glycosidic bond.

Method

1. First, test the sample with Benedict's reagent as above. If it remains blue, this suggests the sugar may be non-reducing.
2. Take a fresh sample. Add dilute hydrochloric acid (HCl) and heat in a water bath to hydrolyse any glycosidic bonds, releasing the component monosaccharides.
3. Neutralise the acid by adding sodium hydrogencarbonate (NaHCO₃) solution. (This is essential because Benedict's reagent is alkaline and will not work in acidic conditions.) Test with pH paper to confirm the solution is neutral or slightly alkaline.
4. Now repeat the Benedict's test (add Benedict's reagent and heat in a water bath).
5. If the solution changes colour (green → brick-red), a non-reducing sugar was present in the original sample.

Exam Tip: Always state that a control must be performed: test the original (unhydrolysed) sample with Benedict's reagent first to confirm it is negative. If the original sample gives a positive result, the sugar is a reducing sugar, not a non-reducing sugar. Also, you must state the neutralisation step — many students lose marks by omitting this.

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Iodine Test — Starch

Method

1. Add a few drops of iodine solution (iodine dissolved in potassium iodide, I₂/KI) to the sample.
2. Positive result: the solution turns blue-black (or dark blue). The iodine molecules become trapped inside the helical structure of amylose, producing the characteristic colour.
3. Negative result: the solution remains brown/yellow (the colour of iodine solution).

Exam Tip: The iodine test is specific for starch — it does not detect other polysaccharides such as glycogen (which gives a red-brown colour with iodine) or cellulose (no colour change). The test works because amylose forms a helix that traps iodine molecules.

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Biuret Test — Proteins

Principle

The Biuret test detects the presence of peptide bonds. Cu²⁺ ions form coordinate (dative) bonds with the nitrogen atoms in peptide bonds, producing a purple/violet colour.

Method

1. Add approximately 2 cm³ of the sample to a test tube.
2. Add an equal volume of sodium hydroxide (NaOH) solution to make the solution alkaline.
3. Add a few drops of dilute copper(II) sulphate (CuSO₄) solution. Mix gently by swirling (do not shake vigorously).
4. Positive result: the solution turns from blue to purple/violet.
5. Negative result: the solution remains blue.

Exam Tip: Free amino acids and dipeptides do not give a positive Biuret test under standard conditions because at least two peptide bonds are generally required for the characteristic purple colour. The test confirms the presence of a polypeptide or protein.

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Emulsion Test — Lipids

Method

1. Add the sample to a clean, dry test tube.
2. Add approximately 5 cm³ of ethanol and shake vigorously to dissolve any lipid present.
3. Pour the ethanol solution into a test tube containing an equal volume of cold distilled water.
4. Positive result: a cloudy white emulsion forms. The lipid comes out of solution as tiny droplets that scatter light, producing the white cloudiness.
5. Negative result: the solution remains clear.

Important notes:

• The test tube must be clean and dry — grease on the glass could give a false positive.
• No colour change is involved — it is the formation of a cloudy white emulsion that indicates lipid.
• The emulsion test does not distinguish between types of lipid (triglycerides, phospholipids, etc.).

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Qualitative vs Quantitative Tests

Aspect	Qualitative Test	Quantitative Test
Purpose	Identifies whether a substance is present or absent	Measures the concentration or amount of a substance
Output	Positive or negative result (often a colour change)	A numerical value (e.g., mg cm⁻³, mol dm⁻³)
Examples	Benedict's test (colour change), iodine test, Biuret test, emulsion test	Colorimetry with calibration curve, titration

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Colorimetry

A colorimeter measures the absorbance (or transmission) of light of a specific wavelength through a coloured solution. The more concentrated the coloured solution, the more light it absorbs and the less it transmits.

How a Colorimeter Works

1. A light source emits white light.
2. A colour filter (or monochromator) selects the appropriate wavelength of light. The filter colour chosen should be the complementary colour of the solution being measured (e.g., for a blue solution, use an orange/red filter; for a red/orange precipitate from Benedict's test, use a blue/green filter).
3. The light passes through the cuvette (a small, transparent container holding the sample).
4. A photoelectric cell on the other side detects the light that passes through.
5. The instrument displays the absorbance value.

Calibration Curves

To determine the concentration of an unknown sample, you must first create a calibration curve (also called a standard curve):

1. Prepare a series of standard solutions of known concentration of the substance being tested.
2. Perform the relevant biochemical test on each standard (e.g., Benedict's test for reducing sugars).
3. Measure the absorbance of each standard using the colorimeter.
4. Plot a graph of absorbance (y-axis) against concentration (x-axis) — this is the calibration curve.
5. Measure the absorbance of the unknown sample (after performing the same test under the same conditions).
6. Use the calibration curve to read off the concentration of the unknown sample.

Exam Tip: When using a colorimeter, always zero the instrument using a blank (a cuvette containing only the reagent without the substance being tested). This accounts for any absorbance by the reagent itself. Also ensure all reactions are left for the same time and at the same temperature to ensure fair comparison.

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Quantitative Benedict's Test

The standard Benedict's test can be made quantitative using a colorimeter:

1. Prepare standard solutions of glucose at known concentrations (e.g., 0, 0.5, 1.0, 1.5, 2.0, 2.5 mmol dm⁻³).
2. Add equal volumes of Benedict's reagent to each standard and to the unknown sample.
3. Heat all tubes in the same water bath at the same temperature for the same time.
4. Allow to cool and filter to remove any precipitate (if measuring the remaining blue colour of unreacted Benedict's reagent).
5. Measure absorbance using a colorimeter with the appropriate filter.
6. Plot a calibration curve and read off the concentration of the unknown.

Alternatively, the mass of precipitate can be measured (filter and weigh after drying), though this is less commonly examined.

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Worked Example — Interpreting a Sequence of Tests on an Unknown

Data-response questions frequently give a table of test results and ask you to deduce the composition of an unknown solution. The skill is to reason from each result in turn, ruling molecules in or out, rather than jumping to a single conclusion.

Suppose a student is given an unknown solution X and performs the following tests:

Test	Result on X
Benedict's (heated, unhydrolysed)	Remains blue
Iodine solution	Blue-black
Benedict's after HCl hydrolysis and NaHCO₃ neutralisation	Brick-red precipitate
Biuret	Remains blue

Step-by-step deduction:

1. The unhydrolysed Benedict's stays blue, so there is no free reducing sugar — this rules out glucose, maltose, and lactose as significant free components.
2. Iodine gives blue-black, so starch is present. Starch is a polysaccharide of α-glucose and would not itself give a positive Benedict's result, which is consistent with test 1.
3. After acid hydrolysis and neutralisation, Benedict's now gives a brick-red precipitate. Hydrolysis has broken glycosidic bonds to release reducing monosaccharides. This is consistent with the starch (test 2) being hydrolysed to glucose, but it could also indicate that a non-reducing sugar such as sucrose was present and has now been hydrolysed. The test cannot, on its own, distinguish these two sources of the positive result.
4. Biuret stays blue, so there is no protein present.

Conclusion: X contains starch and no protein; a non-reducing sugar may or may not be present, because the post-hydrolysis positive could arise from the starch alone. To resolve the ambiguity, the student would need to hydrolyse a starch-free control or use chromatography to identify the specific monosaccharides released.

Exam Tip: The single most common mark-scheme trap on these questions is claiming the post-hydrolysis positive Benedict's "proves sucrose is present" when starch is also present. The rigorous answer states that a non-reducing sugar may be present but that the result is not conclusive because starch also yields reducing sugars on hydrolysis. Awarding this cautious, evidence-limited reasoning is exactly what the AO3 (analysis and evaluation) mark rewards.

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Summary

• Benedict's test detects reducing sugars (blue → green → yellow → orange → brick-red upon heating).
• Non-reducing sugars are detected by first hydrolysing with HCl, neutralising with NaHCO₃, then retesting with Benedict's reagent.
• The iodine test detects starch (brown → blue-black).
• The Biuret test detects proteins/peptide bonds (blue → purple/violet).
• The emulsion test detects lipids (cloudy white emulsion forms when ethanol solution is added to water).
• Qualitative tests identify presence/absence; quantitative tests measure concentration.
• Colorimetry involves measuring absorbance of coloured solutions and using calibration curves of known standards to determine the concentration of unknowns.

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A-Level Deep Dive

Spec mapping

This lesson is mapped to AQA 7402 Section 3.1.4.2 (biuret test) and to the qualitative-test components of Sections 3.1.2 (Benedict's, iodine) and 3.1.3 (emulsion); it also supports Section 3.1.4.2 colorimetric work where Benedict's is rendered quantitative (refer to the official AQA specification document for exact wording). The tests are foundational AO1 content but the more rigorous AO2 / AO3 material lies in colorimetric quantitation, calibration-curve interpretation, and method evaluation.

Historical context: Hermann Christian von Fehling's copper-reduction test (1849) was the qualitative parent of what AQA examines as Benedict's. Stanley Benedict (1909) refined the reagent with citrate to stabilise Cu²⁺ at alkaline pH. The biuret reaction (Cu²⁺ coordinating with peptide-bond nitrogens to give a violet colour) was characterised in the nineteenth century. Sudan stains (e.g. Sudan III) provide an alternative lipid-detection method by dissolving in lipid droplets — referenced for completeness but not the primary AQA test. AQA paraphrases the chemistry — never invent verbatim attribution.

The five canonical tests at a glance

Test	Detects	Reagent	Positive result	Negative result	Key control
Benedict's	Reducing sugars (monosaccharides, maltose, lactose)	Alkaline Cu²⁺ citrate	Blue → green → yellow → orange → brick-red precipitate (heated 80 °C, 5 min)	Stays blue	Use water bath, not Bunsen; same heating time for all samples
Modified Benedict's	Non-reducing sugars (sucrose)	HCl hydrolysis → NaHCO₃ neutralisation → Benedict's	Negative on original sample; positive after hydrolysis	Negative on both	Neutralise! Benedict's fails in acid
Iodine	Starch	I₂ in KI (yellow-brown)	Blue-black colour change	Stays yellow-brown	Glycogen gives red-brown (subtler)
Biuret	Proteins / peptide bonds	NaOH + dilute CuSO₄	Purple / violet	Stays blue	Mix gently — vigorous shaking can precipitate
Emulsion	Lipids	Ethanol + water	Cloudy white emulsion	Stays clear	Clean dry tube — grease gives false positive

Why Benedict's needs alkaline conditions