SPECIFIC LATENT HEAT
Energy is needed to turn a solid into a liquid. The energy is used to increase the potential energy of the molecules, releasing them from their fixed positions, but not freeing them completely. The energy required to turn 1kg of a particular solid to liquid at the same temperature is called its specific latent heat of fusion (L).
To melt a given mass of substance:
energy = mass x specific latent heat of fusion
energy = mL
Therefore : specific latent heat of fusion = energy ÷ mass
To measure the specific latent heat of fusion of water I will need to record the energy used to melt a quantity of ice into liquid. I will need to fill a funnel with ice and place it in a beaker. I will place an electric heater in the funnel which will provide the energy needed to melt the ice. An ammeter and a voltmeter will be placed in the circuit of the electric heater so I can take readings to measure the energy input. The heater will be in the ice for a set amount of time. The time constant will be used to measure the energy also:
Energy = current x voltmeter x time
The time constant I will use will be 300 seconds (5 minutes) which I will measure with a stopwatch. After 300 seconds I will measure the mass of the water produced and put it into the equation to find the specific latent heat of fusion. The experiment will be set out like this:
The mass of the beaker when empty was 95g. After 300 seconds the mass of the beaker was 125g, therefore the mass of liquid water produced was 30g. The voltage of the circuit was 11.23V and the current was measured as 3.83A. I can put these values into the equation for specific latent heat of fusion:
Specific latent heat of fusion = energy ÷ mass
L = (I x V x t) ÷ m
L = (3.83 x 11.23 x 300) ÷ 0.03 (the mass is 0.03kg)
L = 12903.27 ÷ 0.03
L = 430109 J kg-1
The actual specific latent heat of fusion of water is recorded as 0.336MJ/kg-1