Change of State
7 marks · NormalA student uses an electric heater to warm a metal block from room temperature. The temperature of the block is recorded every minute. After some time, the block begins to melt.
Describe how the arrangement and motion of the particles in the block change as it is heated and melts.
When the temperature is rising, the student records a temperature change of 24 °C for a 0.4 kg block. The heater supplies 8400 J of energy during this time. Calculate the specific heat capacity (c) of the metal.
Explain why the temperature remains constant while the block is melting, even though the heater is still supplying energy.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) During heating, particles vibrate more in the solid. As temperature increases and the solid melts, particles break free from fixed positions and move more freely, becoming a liquid with randomly arranged, close-packed particles moving past each other. · must show greater vibration, breaking structure, random movement in liquid
(b) Specific heat capacity = Q/(mΔT) = 8400 J / (0.4 kg × 24 °C) = 875 J/kg°C · must show substitution, correct calculation, unit
(c) Energy is used to break bonds, not to increase temperature, during melting (change of state). · must show energy breaks bonds, no temperature rise
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Describes increased vibration of solid particles.
M2 (a) · 1 mark · Mentions structure breaking and particles becoming free.
M3 (a) · 1 mark · Describes random movement/arrangement in liquid.
M4 (b) · 1 mark · States or uses correct formula c = Q/(mΔT).
M5 (b) · 1 mark · Substitutes values correctly.
A1 (b) · 1 mark · Arrives at correct final answer with unit: 875 J/kg°C (allow ±5). (follow-through)
M6 (c) · 1 mark · Explains energy is used for breaking bonds not for temperature rise.
Worked solution
a) In a solid, particles are tightly packed and vibrate about fixed positions. As the block is heated, the vibration increases. During melting, particles gain enough energy to break free. In the liquid, particles remain close but move randomly past each other.
b) Specific heat capacity = 875 J/kg°C.
c) When melting occurs, the supplied energy is used to break bonds between particles rather than increase temperature, so temperature remains constant.