Physics Online
Latent Heat
Introduction
The amount of heat transfer per unit mass required to produce a phase change (with no temperature
change) in a substance is called the latent heat. If the phase change is from solid to liquid (or vice
versa), then it is called the latent heat of fusion. If the phase change is from liquid to gas (or vice versa),
then it is called the latent heat of vaporization. This relation is given by the following equation:
[1] Q = ±mL
Q = heat (J)
m = mass of the substance (kg)
L = latent heat (J/kg)
The sign in the equation must be put in manually to indicate the direction of the phase change. The
purpose of this lab is to test the theoretical value of heat of fusion for water.
In this experiment, we will throw ice cubes into hot water. We can use the law of conservation of
energy along with the accepted value of the specific heat capacity of water and experimental values of
temperature and mass to calculate the experimental value of heat of fusion for water. A conservation
of energy statement for this experiment can be written as follows:
[2] ΣQ = 0
In this experiment, the ice will melt and then change its temperature to the equilibrium value. The hot
water will simply change its temperature to the equilibrium value. These three processes need to be
taken into account in equation [2]. Also, recall that the relationship between heat and temperature
change is Q = mcΔT. Equation [2] can be written more specifically as follows:
[3] miLf + micw(Tf – Ti) + mhcw(Tf – Th) = 0
mi = mass of ice
Lf = heat of fusion for water
cw = specific heat capacity of water
mh = mass of hot water
Tf = final (equilibrium) temperature
Ti = initial temperature of ice
Th = initial temperature of hot water
Equation [3] can be solved for Lf in terms of known or measurable quantities. This is left as an exercise
for the student. Exercise is fun!
Equipment You Procure
 digital camera
 hot tap water
 Ice Cubes
 paper towels
 thermos (optional)
Equipment from Kits
 100 mL graduated cylinder
 scale
 cups and lids from thermal
conductivity kit
 digital thermometer
Experimental Procedures
1) Use the graduated cylinder to add hot tap water to an insulated cup from the thermal conductivity
kit (or your thermos). The cup should be about half full. Place the lid on the cup. Record the
volume of hot tap water and calculate the mass assuming that the density, ρ = 1.00 g/mL. Note: if
your scale can handle it, then you can measure the masses of the cup and water directly.
2) Measure and record the initial temperature of the water once the temperature is fairly steady. The
manufacturer claims that the thermometer has an error of ± 1°C.
3) Measure the mass of a paper towel. Use the paper towel to pull a few Ice Cubes directly from the
freezer. Quickly measure the combined mass. Quickly place the Ice Cubes in the hot water and
place the lid on the cup.
4) Gently shake or stir the mixture. As soon as all of the Ice Cubes have melted, record the final
equilibrium temperature.
5) Use equation [3] solved for Lf to compute the experimental heat of fusion of water. Compare your
experimental result to the theoretical value of (3.34 ± 0.01) E5 J/kg. The specific heat capacity of
H2O is (4190 ± 15) J/(C° kg). You may assume that the initial temperature of the Ice Cubes is 0°C.
Be careful with units since your mass measurements will be in grams. I suggest you convert all mass
measurements to kg.
6) Repeat this experiment two more times with a different initial temperature of water, different
amount of hot water, or different number of Ice Cubes.