2.5.12Structure & Functions in Living Organisms
Practical: Energy Content of a Food Sample — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions
5 min read 5 parts Lesson 15 of 21 Unit quiz
Part 1 of 5Method
The chemical energy stored in food is released as heat when the food burns, and this heats a known mass of water.
1
Measure a fixed volume of water, such as 25 cm³, into a boiling tube and clamp the tube in a holder.
2
Record the starting temperature of the water with a thermometer, and the mass of the dry food sample on a balance.
3
Mount the food on a needle, set it alight in a Bunsen flame, then hold it directly beneath the boiling tube.
4
Keep the burning food close to the tube until it has burned completely, then record the highest temperature of the water.
5
Repeat with other foods such as nuts or crisps, using fresh water and a similar mass of food each time.
Part 2 of 5Calculating the Energy
A bigger rise in temperature means more energy was transferred to the water.
Calculating the Energy
Equation
- Energy per gram in J/g equals mass of water in g times temperature rise in °C times 4.2, divided by the mass of food in g.
Why 4.2
- It takes 4.2 J of energy to raise the temperature of 1 g of water by 1 °C, and 1 cm³ of water has a mass of 1 g.
Units
- Divide by 1000 to convert joules to kilojoules, since food energy is often quoted in kJ per 100 g.
Comparing foods
- Foods rich in fat usually release more energy per gram than foods rich in carbohydrate or protein.
Part 3 of 5Limitations and Improvements
Heat loss
- Much of the heat warms the air and the tube rather than the water, so calculated values are lower than the true energy content.
Better insulation
- A draught shield or a metal can around the flame, and keeping the flame close to the tube, reduces heat lost to the surroundings.
Incomplete burning
- Some food may leave charred residue, so relighting it until no more burns gives a fairer result.
Reliability
- Repeating each food several times and calculating a mean reduces the effect of random error.
Fair test
- Use the same volume of water, same starting temperature and similar mass of food for every sample.
Part 4 of 5Worked Example
Question
- A 0.80 g sample of food heats 25 g of water from 20.0 °C to 32.0 °C.
- Calculate the energy released per gram of food.
- Temperature rise = 32.0 - 20.0 = 12.0 °C.
- Energy transferred to water = 25 × 12.0 × 4.2 = 1260 J.
- Energy per gram = 1260 ÷ 0.80 = 1575 J/g.
Answer
- 1575 J/g (about 1.6 kJ/g);
- the true value is higher because heat was lost to the surroundings.
Part 5 of 5Exam Checklist & Tips
2.5.12 – Key Points for the Exam
- Calorimetry Measuring the heat released by a process, here from burning food, by the temperature rise of water.
- Specific heat capacity of water The energy needed to raise 1 g of water by 1 °C, which is 4.2 J.
- Energy content The amount of chemical energy stored in a mass of food.
- Reliability How consistent repeated results are.
- Insulation A material that reduces the transfer of heat to the surroundings.
Common Mistakes
- Using the mass of the food after burning instead of its mass before burning.
- Forgetting to divide by the mass of food, which gives total energy rather than energy per gram.
- Claiming the calculated value is exact; heat loss makes it an underestimate.
- Changing the volume of water between foods, which makes the comparison unfair.
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