Measuring Specific Latent Heat

These experiments measure the energy needed to change the state of 1 kg of a substance without changing its temperature.

Measuring latent heat of fusion of ice

1. Set up two identical funnels with filter paper, each containing the same mass of ice at 0 $^\circ$C.
2. Place a beaker under each funnel to collect meltwater.
3. In one funnel, insert an electric heater with known power $P$. Leave the other as a control.
4. Run the heater for time $t$.
5. Measure the mass of water collected from each funnel.
6. The extra mass of water in the heated funnel ($\Delta m$) is due to the heater.
7. Calculate: $L_f = \frac{Pt}{\Delta m}$

The control funnel accounts for ice that melts naturally due to room temperature.

Measuring latent heat of vaporisation of water

1. Boil water in an insulated container using an electric heater.
2. Once boiling steadily, note the mass of the container + water.
3. Run the heater at known power $P$ for time $t$.
4. Weigh the container + water again to find the mass of water that boiled away ($\Delta m$).
5. Calculate: $L_v = \frac{Pt}{\Delta m}$

Sources of error

• Heat loss to surroundings (gives a higher calculated $L$ than the true value).
• Steam carrying water droplets (gives a higher $\Delta m$, so a lower calculated $L_v$).

Common errors and how to correct them

• Forgetting the control experiment when measuring $L_f$. Without it, ice melting from room temperature is counted incorrectly.
• Not ensuring the water is boiling steadily before starting the timer.

How to apply this in an exam

Describe the key measurements: power, time, mass change. Write $E = Pt = mL$. Identify sources of error and how they affect the result.