Fig. 6.1 shows a cooling curve for a hot liquid that solidifies as it cools. (a) State the freezing point of the substance. [1] (b) State the state of the s...

(a) Freezing point [1]

The freezing point is 63 °C. This is read from the flat section of the cooling curve, where the temperature remains constant while the substance changes from liquid to solid.

(b) State at 90 °C [1]

At 90 °C the substance is a liquid. The curve shows the substance starting at 90 °C and cooling, reaching the freezing point at 63 °C, so at 90 °C it is above its freezing point and entirely liquid.

(c) Why temperature stays constant during solidification [2]

The thermal energy being lost is the latent heat of fusion being released as the liquid changes to a solid [1]. This energy comes from forming bonds between particles rather than from changing their kinetic energy, so the temperature stays constant [1]. The energy released compensates for the heat lost to the surroundings, maintaining a steady temperature until all the liquid has solidified.

(d) Name of thermal energy released [1]

The energy released is the (specific) latent heat of fusion. This is the same amount of energy per kilogram that would need to be supplied to melt the solid back into a liquid.

(e) Duration of solidification [1]

\[ \text{duration} = 9 - 3 = 6 \text{ minutes [1]} \]

(f) Particle motion at 25 °C [1]

Once fully solidified and cooled to 25 °C, the particles vibrate about fixed positions in a regular arrangement. They have kinetic energy (vibration) but do not move from place to place. The lower the temperature, the less vigorous the vibration.