Which food test needs the mixture to be heated?

Benedict's test for reducing sugars

Food tests and the energy in food

Core practical 1.13 uses chemical tests to find out what a food contains. You also measure the energy in food by burning it under water (calorimetry).

GCSE · 1BI0 · 1.13B, 1.14B (Biology only)
About 15 minutes

Practise this lesson free (6 questions)

The four food tests

Grind solid food with a little water first.

Test for	Reagent and method	Positive result
Starch	Add iodine solution	Orange-brown to blue-black
Reducing sugars (such as glucose)	Add Benedict's reagent and heat in a hot water bath for a few minutes	Blue to green, yellow or brick-red (more sugar gives a redder colour)
Protein	Add biuret reagent (potassium hydroxide, then copper sulfate solution)	Blue to purple (lilac)
Lipids (fats and oils)	Shake the food with ethanol, then pour into water	A cloudy white emulsion forms

Working safely

Heat Benedict's tests in a water bath, not over a naked flame.
Ethanol is flammable, so keep it away from flames.
Wear eye protection: biuret reagent contains potassium hydroxide, which is corrosive.

Calorimetry: energy in food

Burn a weighed piece of food under a boiling tube holding a known volume of water (1 cm³ of water has a mass of 1 g). Measure the temperature rise.
energy (J) = mass of water (g) × 4.2 × temperature rise (°C)
energy per gram = energy ÷ mass of food (g)
The result is lower than the true value: heat escapes to the air and the tube, and the food may not burn completely.

Worked example

1.0 g of a crisp is burned under 20 g of water. The water warms from 21 °C to 45 °C. Find the energy per gram.

Temperature rise = 45 − 21 = 24 °C
Energy = 20 × 4.2 × 24 = 2016 J
Energy per gram = 2016 ÷ 1.0 = 2016 J/g

Answer: 2016 J/g

Key idea

Iodine: blue-black for starch. Benedict's (heated): brick-red for reducing sugars. Biuret: purple for protein. Ethanol emulsion: cloudy white for lipids. Energy = mass of water × 4.2 × temperature rise.

Quick check

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Extra practice

2 more questions

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0.4 g of food heats 20 g of water by 15 °C. Calculate the energy released per gram of food, in J/g.
Show the answer
Answer: 3150 J/g
Energy = 20 × 4.2 × 15 = 1260 J. Per gram: 1260 ÷ 0.4 = 3150 J/g
What is the positive result of the iodine test for starch?
1. Cloudy white
2. Brick-red
3. Blue-black
4. Purple
Show the answer
Answer: Blue-black
Iodine turns from orange-brown to blue-black.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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