Measuring the energy in foodSpec 1.14B
Food contains stored chemical energy. Energy can be measured using calorimetry: the food is burned and the energy it releases is used to heat a known mass of water.
- Measure a known volume of water into a boiling tube and record its starting temperature. 1 cm³ of water has a mass of 1 g.
- Weigh a small piece of food and hold it on a mounted needle.
- Set fire to the food and hold it under the boiling tube until the food stops burning.
- Record the highest temperature of the water and work out the temperature change.
Calculating the energy in food
Burning 0.5 g of a snack raises the temperature of 20 g of water by 12 °C. Calculate the energy released per gram of the snack.
- energy released = 20 × 4.2 × 12 = 1008 J
- energy per gram = 1008 ÷ 0.5 = 2016 J/g
Answer: 2016 J/g
The result is usually lower than the real value because heat is lost to the surroundings and the food may not burn completely. Surrounding the boiling tube with a draught shield, putting the flame close to the tube and stirring the water improve the results. Wear eye protection, take care with the burning food, and do not use foods such as nuts that people may be allergic to.
Quick check
What are proteins broken down into?
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Amino acids.
What does a catalyst do?
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It speeds up a reaction and is not used up.
Which reagent turns purple when protein is present?
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Biuret reagent (Triple only).
How can the energy in food be measured?
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By burning it to heat water and using the temperature change (calorimetry) (Triple only).