Compare temperature rise of a known water mass heated by burning ethanol, propanol, butanol and pentanol. These four fuels are required for the separate-chemistry core comparison.Apparatus and method schematic; not to scale. Use the stated controls and measurements.
Use spirit burners, a metal calorimeter or approved vessel, measured water mass, thermometer, balance, stirrer and a secured support with a fixed flame-to-vessel gap.
Independent variable: alcohol identity. Record water temperature change and fuel mass burned; comparing temperature rise alone requires matching both water mass and fuel amount.
Alcohols are flammable: cap burners when not in use, use a snuffer to extinguish, keep spare fuel away from flames and never refill a lit/hot burner. Use eye protection and handle hot apparatus carefully.
Method and calculations
Weigh the capped burner before burning. Put the same known water mass in the vessel, measure initial temperature and arrange a consistent wick length and gap.
Light and stir water gently during heating. Stop after a defined measured fuel amount or temperature/time condition appropriate to the comparison, extinguish and record final water temperature.
Allow safe handling and weigh the capped burner again. Fuel mass burned = starting mass − final mass; evaporation also contributes to apparent mass loss if the burner remains uncapped.
Repeat independently for all four fuels with matching starting conditions. Use fresh water at similar initial temperature, a clean vessel and the same geometry.
Water energy gain E = mcΔT, with water c about 4,200 J kg⁻¹ °C⁻¹ or 4.2 J g⁻¹ °C⁻¹. Choose units consistently.
Measured energy per gram = water energy gain/fuel mass used. (Higher tier extension) For energy per mole, use fuel moles = mass/molar mass; distinguish this from the per-gram comparison.
Losses, precision and evaluation
Some energy heats the container and surroundings, so the water gains less energy than combustion releases. The calculated energy released is usually too small. If you write combustion energy change as a negative value (negative enthalpy), your measured value is less negative than the true value.
Soot suggests incomplete combustion, which also lowers energy recovered. Draughts, flame geometry, wick length and burner gap change transfer efficiency.
Use appropriate insulation/lid and a draught shield without restricting oxygen or enclosing a flame dangerously. These reduce losses but do not prove all heat reached the water.
Stir so the temperature is even throughout the water. Choose a thermometer with small enough scale divisions to measure the rise. Keep its probe away from the hot base so it measures the water, rather than a local hot spot.
Cap promptly to reduce evaporation before/after weighing. Fuel evaporated but not burned increases apparent mass used and lowers calculated energy per gram.
Larger temperature rises reduce relative reading uncertainty but may increase heat loss; choose a suitable range, and state the trade-off. Constant heat-transfer conditions are essential across fuels.
Support the conclusion with measured comparable energy/mass values, repeat spread and limitations. Avoid claiming a classroom result is an exact standard energy change for combustion (standard enthalpy of combustion).
Exam skills: planning, precision and evaluation
State what you change (the independent variable), what you measure (the dependent variable) and what you keep the same (control variables). Explain how you keep each control variable constant, rather than just saying “make it fair”.
Accuracy means how close a result is to the true value. Precision means how close repeated measurements are to each other. Resolution is the smallest change an instrument can show. More digits on a display do not automatically mean a more accurate result.
Repeat measurements for each condition, calculate a mean and describe how spread out the results are. This helps assess and reduce the effect of random errors. Repeating cannot fix an error that pushes results consistently in one direction (a systematic error), such as fuel evaporation counted as combustion mass loss.
Repeatability means getting similar results when the same person repeats the same method with the same equipment. Reproducibility means getting similar results when someone else, or different suitable equipment, repeats the experiment. Results can be consistent but still inaccurate.
Check that instruments read zero correctly and are calibrated where needed. Read scales at eye level: looking from an angle can give a wrong reading (parallax error). Choose suitable ranges, measurement intervals and scale divisions (resolution).
Write down the original readings straight away in a table, with units in the headings. Use decimal places that match the instrument’s resolution. Keep the original data and round only when needed. Do not discard a result just because it differs from your prediction.
An anomalous result does not fit the pattern of the other results. Repeat that measurement and check the method. Only leave it out of a mean if you have a clear reason; state which result you excluded and why.
For continuous variables, plot the independent variable on the horizontal axis and the dependent variable vertically. Use sensible scales, units and a best-fit line or curve; do not automatically join every point or force the graph through zero.
Find the gradient of a straight best-fit line using a large triangle: vertical change ÷ horizontal change. For a curve, draw a tangent to estimate the gradient at one point. Explain what the gradient shows in this experiment, include its units and use measured values to support your conclusion.
Uncertainty describes the possible range around a measurement. For one reading on a scale, half the smallest division is a useful classroom estimate unless the question says otherwise. If you subtract two readings, both have uncertainty. Percentage uncertainty = absolute uncertainty ÷ measured value × 100. Follow the method specified in the question.
Use results as evidence and then explain what they mean. A pattern linking variables (a correlation) does not prove that one causes the other. If the ranges of repeat results overlap, a claimed difference may be less convincing. Keep conclusions within the range tested and suggest an improvement that tackles a specific error.
Mr Kidman's Classroom