1.3.4Cell Level Systems

Tests for Biological Molecules — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 10 of 13

📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision

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Part 1 of 7Why use food tests

Different food molecules react with specific chemicals to give a visible change.

Why use food tests

Food tests allow us to find out which carbohydrates, proteins or lipids are in a sample, for example to analyse the nutrient content of foods such as milk, potato, egg white or margarine.

Remember that these tests are qualitative: they show whether the molecule is present and give a rough idea of the amount from the intensity of the colour,

but they do not measure the exact amount.

Part 2 of 7Preparing a sample

If the food is solid, crush it with a pestle and mortar and add distilled water.

Preparing a sample

Mix to dissolve the molecules, then filter or let it settle to obtain a liquid sample.

Always wear eye protection, especially when heating or using alkaline solutions such as sodium hydroxide, which is corrosive.

When heating a test tube in a water bath, point it away from people and do not heat directly in a flame, so liquid does not spit out.

Part 3 of 7The four tests

Molecule	Reagent and method	Positive result	Negative result
Starch	Add a few drops of iodine solution to the sample	Blue-black	Stays orange-brown
Reducing sugars (such as glucose)	Add Benedict's solution and heat in a water bath at about 80 degrees Celsius for several minutes	Blue turns green, yellow, orange then brick-red	Stays blue
Protein	Add Biuret reagent (sodium hydroxide, then dilute copper sulfate), or Biuret solution	Blue turns purple (lilac)	Stays blue
Lipid	Mix the sample with ethanol, then pour into water (the emulsion test)	Cloudy white emulsion	Stays clear

In the emulsion test the lipid dissolves in ethanol but is insoluble in water, so tiny droplets form a cloudy suspension when water is added. An alternative is the Sudan III test, in which a red layer forms on top of the water.

Part 4 of 7Interpreting colour changes

The Benedict's test is semi-quantitative.

A trace of reducing sugar gives a green colour, a medium amount gives yellow or orange and a high concentration gives a brick-red precipitate.

A table of the colours can be used to compare the sugar content of different foods or solutions.

Starch is not a reducing sugar,

so a sample of potato juice would give a negative Benedict's test and a positive iodine test,
but if starch is first broken down by boiling with acid or digested by amylase,
then it gives a positive Benedict's result because glucose or maltose are produced.

Sucrose, table sugar, is a non-reducing sugar and only gives a positive result after it is boiled with dilute acid and neutralised.

Part 5 of 7Planning and evaluating a food test investigation

To compare foods fairly, use equal masses or volumes of each sample, equal volumes of reagent, and the same heating time and temperature.

Include a negative control (distilled water) to show the reagent does not change colour by itself and a positive control (a known solution of the molecule) to show the test works.

Colour matching is subjective, so use a colorimeter or compare against the same colour chart; repeating gives more reliable results.

Limitations include only being able to detect certain molecules and that the colour may be hard to see in coloured food such as beetroot or blackcurrant juice.

Food tests are used by the food industry for nutritional labelling and by clinics, such as testing urine for glucose in diabetes.

Part 6 of 7Worked example: testing unknown solutions

Four unlabelled solutions are known to be glucose, starch, egg albumen and a vegetable oil emulsion.

A student tests each with iodine, Benedict's solution, Biuret reagent and the emulsion test.

Solution A gives a blue-black colour with iodine only, so it is starch.

Solution B gives a brick-red result with Benedict's only, so it is glucose.

Solution C turns purple with Biuret reagent only, so it is egg albumen.

Solution D gives a cloudy emulsion only, so it is the oil.

Each result is explained by the matching molecule and no other test is positive,

which is the logic examiners expect: state the test, the observation and the conclusion, in that order.

A sample may contain several molecules, such as milk, which gives positive results for reducing sugar (lactose), protein and lipid. A conclusion should therefore only be made after all four tests have been done. If a food gives a blue colour with Biuret reagent, the answer is that no protein is present, and not that the test is wrong.

Part 7 of 7Exam-style questions

A student adds Benedict's solution to a food sample and heats it. The solution turns brick-red. A second sample of the same food is tested with iodine, which stays orange-brown. What do the results show? [3]3 marks
Describe how you would test a sample of milk for protein and state the positive result. [3]3 marks

Key terms

Term	Meaning
Reducing sugar	A sugar, such as glucose, that gives a positive result with Benedict's solution.
Benedict's solution	A blue reagent that forms a brick-red precipitate with reducing sugars when heated.
Iodine solution	An orange-brown reagent that turns blue-black with starch.
Biuret reagent	A blue reagent that turns purple with protein.
Emulsion test	A test for lipids using ethanol and water.
Qualitative	Showing whether something is present, not how much.
Control	A test used for comparison, to make sure the results are valid.

Avoid these mistakes

Saying iodine turns black-blue for sugars; it tests for starch only.

Forgetting to heat the Benedict's mixture in a water bath.

Writing that the emulsion test needs heat; it is shaken with ethanol and then water is added.

Recording that Benedict's goes 'red' straight away instead of the colour sequence green, yellow, orange, brick-red.

Last updated: 20 October 2026