When a material changes phase (e.g. solid to liquid, liquid to solid or vice-ver

Question

When a material changes phase (e.g. solid to liquid, liquid to solid or vice-versa) a certain
amount of energy is required to do this. This energy is called the Latent Heat, L, and is related
to the thermal energy required for the phase transition, Q, and the mass of the material, m,
by,
Q = mL
1.0 kg of solid mercury (Hg) at its melting point of ?39C is placed in a 0.5 kg aluminium
calorimeter containing 1.2 kg of water at 20C.
(i) Calculate the latent heat of fusion for mercury if the equilibrium temperature of the
system is found to be 16.5C.
(ii) What mass of mercury would be needed to turn the water into ice?
[cHg = 140 J kg?1 C?1; Lf,H2O = 3.33

Explanation / Answer

Cwater=1000cal/kg, CMercury=33Cal/kg --------specific heats

1.2*1000*(20-16.5)=1*s+1*33*(55.5)

s=2368.5 Calories/kg =9.947 kJ/kg



water into ice, final temp=0degrees
s_fusion_water=.08cal/kg
1.2*1000*(20-0) + 1.2*0.08=m*2368.5 + m*33*39

m=6.56 kg

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