Enter the water temperature at time $t = 0\,\text{s}$ into Table 2.1.
Before each temperature reading is taken, the student stirs the water in the beaker carefully. Explain why.
Calculate the average cooling rate $C_1$ of the water during the first $90\,\text{s}$ of the experiment. Use the values in Table 2.1 and the equation $C_1 = \frac{\theta_0 - \theta_{90}}{t}$, where $\theta_0$ is the temperature at $0\,\text{s}$, $\theta_{90}$ is the temperature at $90\,\text{s}$ and $t$ is $90\,\text{s}$. Give the unit for $C_1$.
Calculate the average cooling rate $C_2$ of the water during the last $90\,\text{s}$ of the experiment. Use the equation $C_2 = \frac{\theta_{150} - \theta_{240}}{t}$, where $t$ is $90\,\text{s}$.
Compare your values of $C_1$ and $C_2$. Explain any difference between these values.
Calculate the average cooling rate $C_3$ of the hot water for the $90\,\text{s}$. Use the readings in Table 2.2 and the equation $C_3 = \frac{\theta_0 - \theta_{90}}{t}$.
Describe how $C_3$ differs from $C_1$. Explain your answer.
The recorded readings show that this experiment is not a valid comparison of $C_1$ and $C_3$. Using the results shown in Table 2.1 and Table 2.2, explain why this comparison is not valid.
State one other variable that ought to be kept constant to make a valid comparison.