Exam questions · Biology · Enzymes and Digestion
Measuring the Energy in Food
- 6 exam questions
- 20 marks
- 9 quick checks
-
1 State [2 marks]
State the name of the method used to measure the energy in a food by burning it, and the unit used for the energy.
Show answerHide answer
Model answer
The method is calorimetry. The energy is measured in joules (J) or kilojoules (kJ).
Mark scheme
- Calorimetry — 1 mark
- Joules / kilojoules — 1 mark
-
2 Calculate [2 marks]
A student heats 30 g of water. The temperature of the water rises by 12 \(^\circ\)C. Calculate the energy transferred to the water. Use energy (J) = mass of water (g) × 4.2 × temperature change (\(^\circ\)C).
Show answerHide answer
Model answer
Energy = 30 × 4.2 × 12 = 1512 J.
Mark scheme
- Substitutes the values into the formula — 1 mark
- 1512 J — 1 mark
-
3 Calculate [3 marks]
A student burns 0.40 g of pasta under 20 cm³ of water. The temperature of the water rises from 21 \(^\circ\)C to 40 \(^\circ\)C. Calculate the energy released per gram of pasta. Use energy (J) = mass of water (g) × 4.2 × temperature change (\(^\circ\)C).
Show answerHide answer
Model answer
Temperature change = 19 \(^\circ\)C. Energy transferred = 20 × 4.2 × 19 = 1596 J. Energy per gram = 1596 divided by 0.40 = 3990 J per gram.
Mark scheme
- Temperature change of 19 \(^\circ\)C — 1 mark
- 20 × 4.2 × 19 = 1596 J — 1 mark
- 3990 J/g — 1 mark
-
4 Describe [4 marks]
Describe how a student could use a boiling tube, a thermometer and a burning piece of food to compare the energy in two different foods.
Show answerHide answer
Model answer
Measure the same volume of water into a boiling tube and record its starting temperature. Find the mass of the first piece of food and set it alight, holding it under the tube until it stops burning. Record the highest temperature of the water and work out the temperature rise. Repeat with the second food using the same volume of water and the same starting temperature, then compare the energy per gram.
Mark scheme
- Measure a known volume or mass of water and its starting temperature — 1 mark
- Burn a weighed sample of food under the tube — 1 mark
- Record the highest temperature and calculate the rise — 1 mark
- Repeat with the other food, keeping the water volume and distance the same — 1 mark
-
5 Explain [3 marks]
The energy measured for a nut in a school experiment is much lower than the value on the food label. Explain why.
Show answerHide answer
Model answer
Some of the energy from the burning nut heats the surroundings, the air and the apparatus instead of the water. The nut may not burn completely, so not all the energy is released. The result is therefore lower than the true energy content.
Mark scheme
- Heat is lost to the surroundings or apparatus — 1 mark
- Incomplete combustion — 1 mark
- So less energy reaches the water than the food contains — 1 mark
-
6 Evaluate [6 marks]
A student uses an open boiling tube to find the energy in a peanut. Evaluate the method and suggest improvements to make the results more accurate and reliable.
Show answerHide answer
Model answer
The method is simple and lets different foods be compared, but it is not accurate. Much of the energy is lost to the air and the glass, and the peanut may not burn completely, so the value is too low. The student could place a heat shield around the apparatus, bring the nut closer to the tube, and stir the water so the temperature reading is even. Using a copper can or a bomb calorimeter would capture more energy. To make the results more reliable, the experiment should be repeated and a mean calculated, with the same volume of water and the same starting temperature each time.
Mark scheme
- Simple and allows foods to be compared — 1 mark
- Energy is lost to the surroundings, so the value is too low — 1 mark
- Incomplete combustion — 1 mark
- Suggests insulation or a heat shield — 1 mark
- Suggests stirring, a calorimeter or bringing the flame closer — 1 mark
- Repeat and calculate a mean, controlling variables — 1 mark
Quick check
-
1
What is the unit used to measure the energy in food?
Show answerHide answer
C: Joule
Energy is measured in joules (J) or kilojoules (kJ).
-
2
What does a calorimetry experiment measure?
Show answerHide answer
B: The energy released when a food is burned
The energy released by the burning food is found from the temperature rise of the water.
-
3
What is the mass of 50 cm³ of water?
Show answerHide answer
A: 50 g
1 cm³ of water has a mass of 1 g, so 50 cm³ has a mass of 50 g.
-
4
40 g of water warms by 5 \(^\circ\)C. How much energy was transferred? Use energy (J) = mass × 4.2 × temperature change.
Show answerHide answer
D: 840 J
\(40 \times 4.2 \times 5 = 840\) J.
-
5
A piece of food of mass 2.0 g releases 6000 J. What is its energy content per gram?
Show answerHide answer
B: 3000 J per gram
Divide the energy by the mass: \(6000 \div 2.0 = 3000\) J per gram.
-
6
Which is the best reason why the energy measured is lower than the real value?
Show answerHide answer
C: Heat is lost to the surroundings
Heat is lost to the surroundings, so not all of the energy warms the water.
-
7
Which change would make the energy measured more accurate?
Show answerHide answer
A: Putting a heat shield around the apparatus
A heat shield cuts down the heat lost to the air, so more energy reaches the water.
-
8
Which nutrient releases the most energy per gram?
Show answerHide answer
D: Lipid
Lipids release about 37 kJ per gram, about twice as much as carbohydrates or proteins.
-
9
In the energy formula, which mass is used for the calculation of the energy transferred to the water?
Show answerHide answer
B: The mass of the water
The formula uses the mass of the water that is heated. The mass of food is used only at the end, to find the energy per gram.