Table 1 shows results from a student cooling 0.50 kg of melted chocolate in a mould. The student records the temperature every 2 minutes. The chocolate is s...
Question 1 Report
Table 1 shows results from a student cooling 0.50 kg of melted chocolate in a mould. The student records the temperature every 2 minutes. The chocolate is stirred gently before each reading so that its temperature is uniform. The nearly constant section of the data is caused by a change of state.
| time / min | temperature / °C |
|---|---|
| 0 | 42 |
| 2 | 37 |
| 4 | 32 |
| 6 | 28 |
| 8 | 28 |
| 10 | 28 |
| 12 | 24 |
(a) State the temperature at which the chocolate changes state. [1]
(b) Complete this sentence: during the constant-temperature section, the chocolate changes from a ______ to a ______. [2]
(c) Calculate the average rate of temperature decrease between 0 min and 4 min, in °C/min. [2]
(d) Explain why energy is still transferred from the chocolate between 6 min and 10 min. [2]
(e) Describe one change to the student's method that would make the results more reliable. [2]
Answer Details
(a) The constant-temperature section is at 28 °C. [1 mark]
(b) During this section the chocolate changes from a liquid to a solid. [2 marks]
(c)
\[\text{temperature decrease}=42-32=10\,°\text{C}\]
\[\text{rate}=\frac{10}{4}=2.5\,°\text{C/min}\]
2.5 °C/min. [2 marks]
(d) Energy is still transferred from the warmer chocolate to the cooler surroundings. During solidification, it is released as latent heat as particles form stronger attractions, even though the temperature is constant. [2 marks]
(e) Repeat the investigation and calculate a mean temperature for each time. Repeats reduce the effect of random variation, making the results more reliable. A data logger taking readings at fixed times would also be creditworthy because it reduces reading uncertainty. [2 marks]
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