Lesson focus: Food and fuelA substance that is burned to release energy, such as petrol or wood. contain chemical energy storeEnergy stored in substances such as food, fuel and batteries, released by reactions. that can decrease when reactions transfer energy to the surroundings.
Compare fuels fairly using temperature-change evidence and explain why simple calorimetryMeasuring the energy in something, such as food, by seeing how much it heats up water. underestimates stored energy.
Build on earlier learning
Burning fuelsCombustion established how fuels react with oxygen; this lesson measures part of the energy transferred.
Practical
Energy in food
- Measure the temperature of 20 cm³ of water in a boiling tube.
- Set fire to a small piece of food and hold it under the tube until it burns out.
- Record the temperature rise and calculate the energy transferred.
Record: The mass of food, temperature rise and energy per gram.
Safety: Check for nut allergies before using nuts; wear eye protection.
Energy in food
0.5 g of food burns under a tube of 20 cm³ (0.020 kg) of water. Choose a food and calculate the energy from the temperature rise.
Controls
Illustrative values. Most of the energy heats the air and the tube, so the measured value is much lower than the label.
You will be successful when you can…
- Identify the chemical store of a fuel.
- Calculate temperature change.
- Compare energy transferred per gram.
- Evaluate heat-loss and combustion limitations.
Words that carry the science
- fuel
- A substance used to transfer energy by a reaction such as combustion.
- chemical energy store
- Energy associated with the chemical composition of a system.
- calorimetry
- Measurement of energy transfer using a temperature change.
- temperature change
- Final temperature minus initial temperature.
- specific energy
- Energy transferred per unit mass, often expressed in joules per gram.
Food and fuels provide chemical stores
01Food and fuels provide chemical stores
Food and fuelA substance that is burned to release energy, such as petrol or wood. have chemical energy storeEnergy stored in substances such as food, fuel and batteries, released by reactions.. During combustion, this store decreases while energy transfers by heating and radiation to the surroundings.
Food labels usually state energy in kilojoules or kilocalories. One kilocalorie is about 4.184 kJ; a food calorie written with a capital C usually means one kilocalorie.
02A fair comparison needs energy per mass
Heating equal masses of water provides comparable evidence only if distance, container, starting temperature and airflow are controlled.
A larger sample may cause a larger temperature rise simply because more fuel burned. Compare energy transferred per gram of fuel, not temperature rise alone.
03Simple calorimetry loses energy
The water receives only part of the transferred energy. The container and air warm, radiation escapes, and incomplete combustion may leave soot or unburned fuel.
A simple classroom result therefore estimates the energy reaching the water, not the full chemical-store decrease. A draught shield and lid can reduce, but not eliminate, losses.
Compare two food samples
A 0.50 g nut transfers 4.2 kJ to water. A 0.80 g crisp transfers 4.8 kJ.
- Use energy per gram for a fair comparison.
- Nut: 4.2 ÷ 0.50 = 8.4 kJ/g.
- Crisp: 4.8 ÷ 0.80 = 6.0 kJ/g.
- Compare the calculated values and keep the same units.
How to make the reasoning visible: The nut transferred more measured energy per gram, although both values may be underestimates.
Calorimetry
Use these illustrative results to compare the energy released by each food.
| Food | Mass burned / g | Water rise / °C | Measured energy / kJ g⁻¹ |
|---|---|---|---|
| peanut | 0.50 | 24 | 8.4 |
| crisp | 0.80 | 27 | 6.0 |
| cracker | 0.60 | 15 | 4.4 |
| marshmallow | 1.20 | 18 | 2.6 |
Rank the foods using the fairest evidence.
Why might the peanut value still be below its label value?
Check your understanding
Answer from memory, then check against the suggested answer.
Which store decreases when food burns?
How is temperature change calculated?
Why compare per gram?
A larger food sample causes a bigger temperature rise. Does it have a higher specific energy?
Why does a lid improve validity but not make the result exact?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student compares the energy in foods by burning each food under 20 cm³ of water and measuring the temperature rise.
What does the temperature rise show, and why is the comparison unfair if different masses are burned?
(2)Give two reasons the measured energy is lower than the real value, and one improvement.
(3)Total for this question: 5 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., identify what the method measures, then balance its usefulness against specific limitations.