A student investigates how the temperature of the surroundings affects the rate of cooling of water. (a) (i) The student: • pours 100 cm³ of hot water into a 250 cm³ beaker • uses a thermometer to take the temperature of the water at time t = 0s. The thermometer reading at time t = 0s is shown in Fig. 2.1. [Figure 2.1] Record the temperature of the water at time t = 0s in Table 2.1. Table 2.1 t/s 01°C 0 30 69 60 67 90 66 120 65 150 64 180 63 210 62 240 61

1. (a)(i)Record the temperature of the water at time t = 0s in Table 2.1.[1]
2. (a)(ii)The student then records the temperature of the water every 30s for 240s. The results are recorded in Table 2.1. Before taking each temperature reading, the student carefully stirs the water in the beaker. Explain why.[1]
3. (a)(iii)Calculate the average cooling rate C₁ of the water for the first 90s of the experiment. Use the readings in Table 2.1 and the equation: C₁ = (θ₀ - θ₉₀) / t where θ₀ is the temperature of the water at 0s, θ₉₀ is the temperature at 90s and t is the time of 90 s. Give the unit for C₁. C₁ = .............................................................. unit ....................[2]
4. (a)(iv)Calculate the average cooling rate C₂ of the water for the final 90s of the experiment. Use the equation: C₂ = (θ₁₅₀ - θ₂₄₀) / t where θ₁₅₀ is the temperature of the water at 150s, θ₂₄₀ is the temperature of the water at 240 s and t is the time of 90 s. C₂ = .............................................................. unit ....................[1]
5. (a)(v)Compare your values of C₁ and C₂. Explain any difference in these values.[1]
6. (b)(i)The student repeats the procedure described in (a)(i) but this time he places the 250cm³ beaker inside a larger beaker containing iced water. The arrangement is shown in Fig. 2.2. [Figure 2.2] The student reads the temperature θ of the hot water, records the reading and immediately starts the stop-watch. Table 2.2. shows the temperature at times t = 0s, 30s, 60s, and 90s. Table 2.2 t/s 01°C 0 75 30 68 60 62 90 57 Calculate the average cooling rate of the hot water for the 90s. Use the readings in Table 2.2 and the equation: C₃ = (θ₀ - θ₉₀) / t C₃ = .............................................................. unit ....................[1]
7. (b)(ii)Describe how C₃ differs from C₁. Explain your answer.[1]
8. (b)(iii)The recorded readings show that this experiment is not a valid comparison of C₁ and C₃. By referring to the results recorded in Table 2.1 and Table 2.2, explain why this is not a valid comparison.[1]
9. (b)(iv)State one other variable that should be kept constant to make a valid comparison.[1]

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