BIOLOGY · Stage 8 · Chapter 6: Nutrition, Health and Movement
Biology 6.3 Measuring the energy in food — Managing Variables in Calorimetry — Alhussan International Academy - Biology (Stages 7-9) Revision Notes & Topical Questions
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Lesson summary
Biology 6.3 Measuring the energy in food — Managing Variables in Calorimetry — Alhussan International Academy — Biology (Stages 7–9) notes: Plan and evaluate an investigation to compare the energy released from burning foods, controlling the variables.Units — energy is measured i…
- Food labels
- Stored energy
- Two temperatures
- Same volume of water
- Sensible volume
- Same mass of food
Section 1 of 6Energy units and food labels
- Units
- energy is measured in kilojoules (kJ), an SI unit, or kilocalories, an old unit still often used.
- (kcal, usually called calories)
- 1 kcal = 4.2 kJ.
- energy is measured in kilojoules (kJ), an SI unit, or kilocalories, an old unit still often used.
- Food labels
- they give the energy in a given mass of food, for example kJ per 100 g or per gram.
Section 2 of 6The idea of calorimetry
- Stored energy
- food contains stored chemical energy that is released as heat when it burns.
- Method
- burn the food under a boiling tube of water, and the more energy released, the more the water temperature rises.
- This lets different foods be compared.
- Apparatus
- a thermometer, boiling tube in a clamp, a mounted needle holding the burning food sample, and a measuring cylinder for the water.
Section 3 of 6Method
- Measure a fixed volume of water in a measuring cylinder and pour it into a boiling tube held in a clamp.
- Measure the starting temperature of the water.
- Find the mass of the food, then set it alight with a Bunsen flame and hold it under the tube until it stops burning.
- Measure the final temperature of the water.
- Find the mass of what is left, to get the mass of food that burned.
- Two temperatures
- measuring before and after gives the temperature rise caused by each food.
Section 4 of 6Controlling the variables
- Same volume of water
- a small volume takes less energy to heat, while a bigger volume does not get as hot.
- Sensible volume
- too much gives a temperature rise too small to measure and too little might boil away, so try about 20 cm3 and adjust.
- Same mass of food
- or find the mass of food burned and calculate the temperature rise per gram.
- Temperature rise per gram = measured temperature rise ÷ mass of food used.
- Other controls
- starting temperature of the water, distance between the flame and the tube, type of tube, and burning each food in the same way.
- Planning
- decide what to compare and measure, identify variables to control, choose a suitable value for each and test that the values give reasonable results.
Section 5 of 6Calculations
- Energy to the water
- energy (J) = mass of water (g) × 4.2 J/g°C × temperature rise (°C).
- Energy value
- energy per gram = energy transferred ÷ mass of food burned.
- Comparing foods
- the food with the larger temperature rise per gram has more energy per gram.
Section 6 of 6Evaluation
- Why results are low
- some heat escapes into the air, the food burns incompletely and some heat warms the tube.
- Improvements
- use a lid or insulation, stir the water, burn the food completely and repeat at least three times with a mean.
- Accuracy
- compare the result with a data book value.
- Energy conservation
- chemical energy in the food becomes thermal energy in the water, the tube and the surroundings.
- Safety
- tie back long hair, wear eye protection, keep flammable materials away and take care with hot tubes.
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