The diagram shows the cooling record for molten magnesium chloride before it is fed into an electrolysis cell with electrodes. A technician needs the substa...
Question 1 Report
The diagram shows the cooling record for molten magnesium chloride before it is fed into an electrolysis cell with electrodes. A technician needs the substance to be liquid before electrolysis can take place. The sample is allowed to cool in a dry container, and its temperature is measured every few minutes. Section QR is horizontal because a solid-liquid change is taking place.
(a) State the state of magnesium chloride before point Q and name its state after 12 min. [2]
(b) Identify the section where the substance changes state, give its temperature, and name the change. [3]
(c) Explain why the temperature stays constant between Q and R even though energy is being removed. [4]
(d) Calculate the mean rate of temperature decrease between 8 min and 12 min. State two differences between the particles in a solid and those in a liquid, and state which state can flow. [5]
Answer Details
(a) Before point Q, magnesium chloride is a liquid. After 12 min, it is a solid. [2]
(b) The change of state occurs on section QR, at 714 degrees C. Since the sample is cooling from liquid to solid, the change is freezing or solidification. [3]
(c) Energy is still removed between Q and R. It is used to form stronger attractions between particles as they become arranged in a solid. Therefore the temperature does not decrease until the change of state is complete. [4]
(d) Between 8 min and 12 min:
\[\text{temperature decrease}=714-650=64\text{ degrees C}\]
\[\text{time}=12-8=4\text{ min}\]
\[\text{mean rate}=\frac{64}{4}=16\text{ degrees C min}^{-1}\]
Solid particles are in fixed positions in a regular arrangement. Liquid particles are still close together but have an irregular arrangement and can move past each other. The liquid state can flow. [5]
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