A student measured the energy stored in a cashew nut. She held the food on a mounted needle and set it alight, then held the burning food under a boiling tu...
Question 1 Report
A student measured the energy stored in a cashew nut. She held the food on a mounted needle and set it alight, then held the burning food under a boiling tube containing 25 cm3 of water, as shown in Fig. 6.1. She measured the temperature of the water before burning and the highest temperature reached, using the thermometer, and measured the volume of water with a measuring cylinder. Her results are shown in Table 6.1. The cashew nut was weighed on a balance before being fixed to the mounted needle and set alight. It was held as close as possible under the boiling tube so that most of the heat passed into the water, and the water was stirred gently before the highest temperature was read from the thermometer.
| Mass of food / g | Volume of water / cm3 | Start temp / °C | Highest temp / °C | Temp rise / °C |
|---|---|---|---|---|
| 0.40 | 25 | 21 | 46 |
(a) Measure, using the thermometer in Fig. 6.1, the highest temperature reached by the water. [1]
(b) Measure, using the measuring cylinder in Fig. 6.1, the volume of water in the boiling tube. [1]
(c) Complete Table 6.1 by calculating the temperature rise of the water. [1]
(d) Record the piece of apparatus used to hold the burning food. [1]
(e) Calculate the energy released by the burning food using: energy (J) = volume of water (cm3) × 4.2 × temperature rise (°C). Show your working. [2]
(f) Calculate the energy released per gram of food. Show your working. [2]
(g) The value obtained is much lower than the value printed on the food packet. Explain why. [3]
(h) State two variables that should be kept the same to make a comparison of foods fair. [2]
(i) Describe two changes to the method that would give more accurate results. [2]
(j) Suggest one safety precaution during this investigation. [1]
(k) Explain how repeating the test improves the investigation. [2]
Answer Details
This is the food-energy (calorimetry) experiment: burning food heats water, and the temperature rise lets you calculate the energy released.
(a) Highest temperature [1] Reading the thermometer in Fig. 6.1 gives \(46\,^{\circ}\text{C}\). [1]
(b) Volume of water [1] Reading the measuring cylinder gives \(25\ \text{cm}^3\). [1]
(c) Temperature rise [1] \[46-21=25\,^{\circ}\text{C}\] [1]
| Mass of food / g | Volume of water / cm3 | Start temp / °C | Highest temp / °C | Temp rise / °C |
|---|---|---|---|---|
| 0.40 | 25 | 21 | 46 | 25 |
(d) Apparatus holding the food [1] The mounted needle. [1]
(e) Energy released [2] \[E=\text{volume}\times 4.2\times \text{temp rise}=25\times 4.2\times 25=2625\ \text{J}\] working [1], answer \(2625\ \text{J}\) [1].
(f) Energy per gram [2] \[\frac{2625}{0.40}=6562.5\ \text{J g}^{-1}\] (accept about \(6560\ \text{J/g}\) or \(6.56\ \text{kJ/g}\)) working [1], answer [1].
(g) Why the value is much lower than the packet [3] Any three of: much heat is lost to the surrounding air, the tube and the clamp [1]; not all the heat from the flame passes into the water [1]; the food may not burn completely, so some energy stays in the unburnt food [1].
(h) Two variables for a fair comparison [2] Any two of: volume of water; distance from the food to the tube; starting temperature; the same thermometer. [2]
(i) Two changes to improve accuracy [2] Any two of: shield the apparatus from draughts or insulate the tube; stir the water before reading; hold the flame closer to the tube; use a lid to trap heat. [2]
(j) One safety precaution [1] Any one of: wear eye protection; tie back long hair; keep the flame away from flammable materials; take care with the hot tube. [1]
(k) Why repeat the test [2] Repeating lets you calculate a mean and reduces the effect of anomalies [1], making the result more reliable [1].
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