RI 1998 Sec 3 EOY P2 Q7
A substance of mass 0.30 kg initially in the solid state, is heated by a 100 W heater. The figure below shows how the temperature of the substance varies with time.
(a) Determine the melting point of the substance.
(b) Calculate
(i) the specific heat capacity of the substance in the liquid state.
(ii) the specific latent heat of fusion of the substance.
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Answer:
(a) From the figure, the melting point of the substance is 80 °C
(b)
(i) Using Q = mcΔθ,
Q = 100 W * 100 s = 10000 J
m = 0.30 kg
Δθ = 100 - 80 = 20
Hence, specific heat capacity, c = Q / (mΔθ)
= 10000 / (0.30 * 20)
= 1.7 * 103 J kg-1 K-1
(ii) Using Q = ml,
Q = 100 W * (1050 s - 300 s) = 75000 J
m = 0.30 kg
Hence, specific latent heat, l = Q / m
= 75000 / 0.30
= 2.5 * 105 J / kg
