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How can chemical reagents identify the food groups, and how is the energy in food measured?

Use chemical reagents to identify starch, reducing sugars, proteins and fats in food samples, and explain how the energy contained in food can be measured using calorimetry.

A focused answer to Edexcel GCSE Biology 1.13B and 1.14B, covering the iodine, Benedict's, biuret and emulsion (or Sudan III) food tests and how calorimetry measures the energy in food.

Generated by Claude Opus 4.89 min answer

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  1. What this dot point is asking
  2. The four food tests
  3. Carrying out the tests safely and fairly
  4. Calorimetry: measuring the energy in food
  5. Try this

What this dot point is asking

Edexcel statements 1.13B and 1.14B are Biology only. Statement 1.13B is the core practical: using chemical reagents to identify starch, reducing sugars, proteins and fats. Statement 1.14B asks you to explain how calorimetry measures the energy in food. You need the reagent, the method and the positive colour change for each test, plus the method and limitations of calorimetry.

The four food tests

The Benedict's test is semi-quantitative: the final colour tells you roughly how much reducing sugar is present, from green (a little) to brick-red (a lot). This makes it useful for comparing the sugar content of different drinks or foods.

Carrying out the tests safely and fairly

To get reliable results: grind the food and mix it with distilled water to make a solution or suspension first, use the same volume of reagent each time, and keep the water-bath temperature and timing the same when comparing samples. Wear eye protection, because Benedict's and biuret reagents are irritants and ethanol is flammable, so keep it away from flames.

Calorimetry: measuring the energy in food

Food stores chemical energy that is released when it burns. In a simple school calorimetry experiment you burn a known mass of food (such as a peanut or a crisp) directly under a boiling tube holding a measured volume of water, and record the temperature rise of the water.

The number 4.24.2 is the energy in joules needed to raise 1 g1\ g of water by 1 ∘C1\,^{\circ}\text{C}.

Try this

Q1. State the reagent and the positive result for the test for starch. [2 marks]

  • Cue. Iodine solution; the orange-brown iodine turns blue-black.

Q2. Give two ways to reduce heat loss and make a calorimetry result more accurate. [2 marks]

  • Cue. Any two of: insulate or shield the apparatus from draughts, hold the food closer to the tube, use a lid or screen, or relight the food if the flame goes out.

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 20184 marksDescribe how you would test a food sample to find out whether it contains reducing sugar and protein. Give the reagent and the positive result for each test.
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A 4-mark describe question rewards the reagent, the method detail and the colour change for each test.

For reducing sugar: add Benedict's solution to the food sample and heat in a water bath at about 80 ∘C80\,^{\circ}\text{C}. A positive result changes the blue solution to green, then yellow, orange or brick-red, depending on how much sugar is present.

For protein: add biuret solution (or sodium hydroxide then copper sulfate) at room temperature. A positive result changes the blue solution to purple (lilac).

Markers reward both reagents, the heating step for Benedict's, and both correct colour changes. Mixing up the colours or forgetting that Benedict's must be heated loses marks.

Edexcel 20213 marksA student burns 1.0 g of a crisp under a boiling tube of water to measure its energy content. Explain why the energy value they calculate is lower than the true value.
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A 3-mark explain question rewards reasons linked to heat loss.

Not all the energy released by the burning crisp is transferred to the water: heat is lost to the surrounding air, to the boiling tube and to the apparatus, and some of the crisp may not burn completely.

Because less energy reaches the water than the crisp actually contains, the measured temperature rise is too small, so the calculated energy value is lower than the true value.

Markers reward at least two sources of heat loss (or incomplete combustion) and the link to a smaller temperature rise. Vague answers such as some energy is lost without saying where score less.

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