Core Practicals 13–14: Latent Heat and Boyle’s Law

For a suitable electrical phase-change experiment, measure heater current, voltage, operating time and the mass that changes phase. With steady input and negligible losses, VIt=mL. During a pure substance’s phase change at fixed pressure, temperature is approximately constant while energy changes molecular potential energy.

For melting ice, distinguish heater-induced melting from melting caused by the surroundings, using an appropriate control run where the method permits. For steady vaporisation, measure mass-loss rate and account for heat loss. Two steady heater powers can estimate L from the power difference divided by the difference in mass-loss rates, if background loss is approximately unchanged.

Trap a fixed amount of gas in the approved syringe or Boyle’s-law apparatus. Vary its volume and measure pressure after allowing it to reach room temperature again. Rapid compression heats the gas, so immediate readings do not test an isothermal relationship.

Use absolute pressure. If a gauge measures pressure relative to the atmosphere, add the appropriate atmospheric pressure. Include significant connecting-tube or sensor volume in the trapped gas volume. Avoid leaks and stay within the apparatus pressure range. Plot p against 1/V; ideal isothermal behaviour gives a straight line with gradient nRT or NkT.

Record repeated readings and cover a useful range. A p against V curve and a p against 1/V straight line describe the same model in different forms, but their gradients have different meanings. A fixed omitted dead volume distorts the inverse-volume analysis; more repetitions do not repair it.

Uncorrected heat loss makes VIt/m overestimate latent heat, whereas extra unaccounted ambient melting can increase measured m and bias the value low. Name the actual energy or mass error before predicting its direction. Use the centre’s hot-liquid, heater and pressure-system procedures.

Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.