Flashcards · Biology · Enzymes and Digestion
Measuring the Energy in Food
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What is calorimetry?
A method of finding the energy in a substance by burning it and measuring how much it heats a known mass of water.
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What is the formula for the energy transferred to the water?
Mass of water (g) × 4.2 × temperature change (\(^\circ\)C), giving the energy in joules.
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What does the number 4.2 stand for?
The energy in joules needed to raise 1 g of water by 1 \(^\circ\)C.
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What is the mass of 1 cm³ of water?
1 g.
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How do you change joules to kilojoules?
Divide by 1000.
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How do you find the energy per gram of food?
Divide the energy transferred to the water by the mass of food burned.
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25 g of water warms by 10 \(^\circ\)C. How much energy was transferred?
\(25 \times 4.2 \times 10 = 1050\) J.
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Why must the temperature change be calculated rather than just the final temperature?
The formula needs the rise in temperature (final minus starting temperature), not the temperature itself.
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Why is a measured energy value lower than the true value?
Some energy is lost to the surroundings and the food may not burn completely.
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Name one way to reduce heat loss in a calorimetry experiment.
Use a draught excluder or heat shield, or keep the food close to the tube.
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Why should the water be stirred?
To spread the heat evenly so the thermometer shows the true temperature of all the water.
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Why repeat the experiment and take a mean?
It reduces the effect of random errors and makes the result more reliable.
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Which nutrient releases the most energy per gram?
Lipids, at about 37 kJ per gram.
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Name two variables to keep the same in the experiment.
The volume of water, and the distance between the flame and the tube.
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Which type of calorimeter loses the least energy to the surroundings?
A sealed bomb calorimeter, where the food burns in oxygen surrounded by water.
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What is the usual mistake when the question gives mass of food and mass of water?
Using the mass of food in the heating formula. The formula uses the mass of water.