2.7 Measuring the energy in food (calorimetry)
6 exam questions worth 12 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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A student burned a piece of bread under 20 g of water. The temperature of the water rose from 19 °C to 31 °C.
Calculate the energy transferred to the water. Use: energy = mass of water × 4.2 × temperature rise.
Mark scheme
- temperature rise = 31 − 19 = 12 (°C) (1)
- 20 × 4.2 × 12 (1)
- = 1008 (J) (1)
Model answer
Temperature rise = 31 − 19 = 12 °C. Energy = 20 × 4.2 × 12 = 1008 J.
The bread had a mass of 0.4 g and released 1008 J.
Calculate the energy released per gram of bread.
Mark scheme
- 1008 ÷ 0.4 (1)
- = 2520 (J/g) (1)
Model answer
Energy per gram = 1008 ÷ 0.4 = 2520 J/g.
The value the student calculated was much lower than the value on the bread packet.
Give two reasons why.
Mark scheme
Any two from:
- heat lost to the surroundings / air (1)
- heat used to warm the boiling tube / clamp (1)
- food not burned completely (1)
- energy given out as light (1)
Model answer
Some heat was lost to the air and used to warm the boiling tube, and some of the bread did not burn completely.
The student compared the energy in different foods.
Give two variables that should be kept the same.
Mark scheme
Any two from:
- volume / mass of water (1)
- mass of food (1)
- distance between food and boiling tube (1)
- starting temperature of water (1)
Model answer
The volume of water and the distance between the burning food and the boiling tube.
Suggest one change to the apparatus that would make the result closer to the true value. Explain your answer.
Mark scheme
- insulate the boiling tube / use a calorimeter / put a draught shield round it (1)
- so less heat is lost to the surroundings (1)
Model answer
Surround the boiling tube with insulation, so less of the heat is lost to the air and more of it warms the water.
Explain why the energy per gram is calculated, not just the total energy released.
Mark scheme
so that foods with different masses can be compared fairly
Model answer
The pieces of food may have different masses, so working out the energy per gram lets them be compared fairly.
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