This outline relates measured energy to the mass undergoing a specified phase change. It supports development of a locally approved method and thermal/electrical risk assessment; it is not a complete validated procedure. Melting and boiling need different mass collection and background-transfer checks. Check current Pearson documents for course-specific requirements.
Aim and scientific principle
Determine the specific latent heat for a stated substance, phase change and pressure by relating net phase-change energy to the mass that actually changes phase. A nearly constant temperature supports the chosen transition model but does not prove that all supplied electrical energy causes that transition.
Begin by translating the title into a testable question. Identify what is deliberately changed, what is measured directly, what is calculated and what must remain controlled. Predict a relationship only after naming the mechanism. A prediction without a reason is weak; a reason without measurable variables cannot guide a practical design.
Apparatus and setup
Use a suitable low-voltage heater, phase-change sample, appropriate insulation, balance, voltmeter and ammeter or electrical energy meter, timer and temperature probe. For melting ice, collect and weigh drained meltwater rather than interpreting the mass of an entire closed ice/water container as melted mass. For boiling, use an appropriate vented arrangement and establish how vessel mass loss represents escaping vapour; splashes and condensate return can invalidate that identification. Consider measuring range, resolution, uncertainty, calibration and suitability for the sample, not the instrument name alone. Draw a labelled scientific setup that shows the working geometry and exactly where readings are taken. Include a control or blank when it distinguishes the target effect from drift, contamination or background response.
Before collecting results, check zero readings, calibration, leaks, alignment or sterility as relevant. Record the instrument resolution beside the first raw-data table. If a digital device displays more digits than its underlying accuracy supports, do not treat every digit as equally certain.
Method
Choose melting or boiling and document the phase-changing mass measurement. Establish the relevant transition condition at the stated pressure before the measurement interval. Do not count initial warming of a cold sample or apparatus as latent heat; remove or quantify pre-existing liquid in a meltwater collection method.
Measure voltage across and current through the heater over the same timed interval as the mass measurement. Record temperature and electrical variation. Ensure safe heater coverage and heat transfer under the approved setup; the electrical meter reading alone does not establish energy delivered to the changing phase.
Check this topic from memory
Attempt the matching topic bank before reopening the notes. Use each missed idea to decide what to review next.
For melting, compare heater-on and heater-off collection rates over matched conditions where feasible, allowing for drainage delay and retained liquid. For boiling, do not assume a heater-off interval has the same temperature and heat loss: the sample may cool. An alternative is comparing steady mass-loss rates at two measured powers, only if pressure and other heat-loss conditions are sufficiently matched.
Apply the appropriate energy balance, state its assumptions and uncertainty, and check repeats. Investigate temperature drift, splashing, condensate return and changing heat transfer before treating the measured mass difference as phase-change mass. A background subtraction is valid only to the extent that its conditions represent the measurement interval.
Plan the analysis before starting. This reveals whether an additional measurement is needed for a derived quantity and prevents a completed experiment from lacking a mass, length, concentration, background or calibration value. A method should be detailed enough for another trained student to reproduce, while avoiding decorative steps that do not affect validity.
Variables and controls
Independent variable: heating time or supplied energy.
Dependent variable: mass undergoing phase change.
Important controls: substance and phase condition, pressure, heater geometry, insulation, ambient airflow, measurement interval and balance/collection method. For matched comparisons, justify how environmental transfer, drainage or vapour-loss conditions remain comparable; changing power need not preserve every heat flow.
For each control, state how it is maintained or monitored. Simply writing "keep temperature constant" is incomplete if the method has no bath, probe, equilibration time or other means of doing so. Choose a range wide enough to reveal the relationship but within safe and scientifically valid limits. Choose the number, range and spacing of values for the expected response and analysis. Concentrating values near an uncertain threshold or optimum can be more useful than uniform spacing; no fixed count establishes an adequate design.
What is distinctive about this practical
For a specified transition at approximately constant pressure and transition temperature, the phase-change energy is Q=mL, where m is the mass changing phase and L is the specific latent heat. Keep warming of sample/apparatus and net environmental heat transfer separate. Electrical energy is E=VIt only for effectively constant measured voltage and current over that interval; otherwise use an energy meter or a justified time sum E=∑ViIiΔti.
Melting ice may receive heat from its warmer surroundings. In an idealised matched collection model with equal durations, negligible storage changes and the same environmental contribution, the heater-on mass exceeds the heater-off mass by the heater-induced melt, giving L=E/(mon−moff). Subtract the appropriate mass/rate contribution, not an arbitrary quantity labelled heat loss. Unremoved surface water, unequal drainage and changing environmental heat input can defeat this correction.
For steady boiling with negligible sensible heating/storage and a constant outward background heat-loss rate H, P=Lr+H, where r is the positive vapour mass-loss rate. Two powers at sufficiently matched conditions give L=(P2−P1)/(r2−r1). This is an energy-balance deduction, not evidence that H actually stayed constant. A heater-off boiling control may cool or stop boiling and therefore need not represent the heated interval. Do not apply the melting subtraction mechanically to boiling.
Recording and processing results
Retain timed electrical readings, temperature, pressure where material, raw mass readings and the collection/vapour-loss definition. Express phase-changing mass in kilograms and L in joules per kilogram. Plot cumulative mass against time only over justified steady intervals; its gradient is a mass rate, not latent heat. For a constant-loss two-power model, power against mass-loss rate has gradient L and intercept H; reversed axes change the gradient interpretation. Include electrical, timing, mass, background-matching and thermal-storage uncertainty. A small difference between two large masses or rates can have large relative uncertainty. Put units in column headings, record raw readings consistently with instrument resolution and the method and distinguish repeats from calculated means. Do not delete an anomaly merely because it is inconvenient. Recheck the condition, look for a procedural explanation and state any exclusion rule.
Choose a fitted line, curve or ruled joins according to the data, scientific relationship and question; do not connect or smooth points mechanically. A gradient should use a large triangle and values read from the fitted line. When a relationship is linearised, explain the transformation and connect gradient or intercept to the requested physical quantity. When observations are categorical, define the category rule before seeing the result.
Safety, ethics and responsible practice
Protect against hot surfaces, steam or cold burns, keep water from electrics and use low-voltage equipment. A useful safety statement names the hazard, identifies who or what could be harmed and gives a control that reduces likelihood or severity. "Take care" does not specify an action that reduces the identified risk.
Worked application: from raw evidence to a defensible conclusion
This is a constructed melting-model check, not experimental data or an approved heater specification. Suppose matched equal-duration collections give 0.012 kg with the heater on and 0.002 kg with it off, while measured heater energy is 3000 J. Under the stated equal-background and negligible-storage assumptions, the heater-induced melt is 0.010 kg and the inferred specific latent heat is 300000 J/kg. Dividing by the uncorrected 0.012 kg instead gives 250000 J/kg, illustrating a downward bias when environmental melting is wrongly attributed to electrical input.
These numbers do not validate the assumptions or establish a reference value for a named material. If drainage, pre-existing water or background transfer differs, the subtraction can be wrong. Retain raw evidence and assess those effects rather than accepting a plausible-looking result.
This chain is stronger than a memorised recipe because every step answers the practical question. If the graph contradicts the prediction, report the evidence honestly and investigate method, range, controls and model assumptions before claiming a new scientific law.
Evaluation: limitation, effect and improvement
Limitation: sample/apparatus energy storage, environmental heat transfer and a mass reading that includes more than the intended phase change can invalidate the balance.
Likely effect: unaccounted outward heat loss or initial warming makes electrical energy divided by correctly measured phase-change mass too large. Environmental heat input that produces extra melt can instead make that uncorrected ratio too small. Pre-existing liquid in collected meltwater or liquid splashed out during boiling can overstate changed-phase mass and bias the ratio downward. The direction depends on the actual energy and mass balance, not a universal heat-loss rule.
Targeted improvement: establish a steady transition condition, suitable insulation and an appropriate vented cover where compatible with the approved apparatus. Never seal a heated boiling vessel. Verify heater coverage, mass collection and drainage/condensate behaviour. Use matched background or two-power comparisons only after checking their thermal assumptions; repeat across relevant conditions and quantify residual mismatch. Repeats alone do not remove common bias.
Separate random variation, systematic bias and model limitation. Repeats help reveal random variation but do not remove a zero error. Calibration establishes the relation between reference values and indications; appropriate adjustment or correction can address an identified offset. It does not make biased sampling representative. Finer resolution and greater repeatability are distinct properties, and finer resolution may improve an estimate only when the reading is a material source of uncertainty.
Common misconceptions and corrections
Repeats automatically make a result accurate. Repeats estimate random variation; they do not correct a biased method or calibration error.
A control variable is just a list item. Explain the apparatus or procedure that keeps it stable.
The line must pass through every point. Use the overall pattern and a best-fit relationship, then investigate justified anomalies.
Percentage error and percentage uncertainty are interchangeable. Error compares a measured value with a suitable reference. Uncertainty characterises dispersion of values attributed to the measurand; it may be expressed as a standard uncertainty or an interval with stated coverage, and includes more than resolution alone. State the reference, denominator and uncertainty convention used.
A successful endpoint proves the theory. The observation supports a conclusion only within the method, range and assumptions tested.
More sophisticated apparatus is always better. The improvement must address the dominant limitation and be practical in the stated context.
Assessment guidance
Pearson assesses practical understanding throughout the science papers and concentrates it in written Units 3 and 6. Planning answers need an operational independent variable, a measured dependent variable, a justified range, controls, repeats, safety and a stated analysis. Data answers need units, consistent precision, sensible significant figures and transparent calculations. Graph answers need labelled axes, a scale using the grid well and line treatment appropriate to the data and question. Evaluation answers should use a limitation-effect-improvement chain specific to the apparatus. Do not claim that a memorised core-practical recipe is the only acceptable method when the question supplies a different valid context.
Retrieval practice
Extend or adapt this practical to answer a related question. Where the design uses a continuous variable, justify its planned values. List apparatus with range and resolution where relevant, control methods, a raw record layout, the expected analysis and one hazard-control pair. Then identify one random uncertainty, one systematic bias and one model limitation. Explain which can be reduced by repeats, which needs calibration followed by appropriate correction or adjustment, or a redesign, and how your conclusion would change if the anomaly were confirmed.
