Learning outcomes
- Measure temperature change systematically.
- Plan fair comparisons of insulation.
- Control thermal variables.
- Interpret cooling curves.
- Evaluate heat-loss and thermometer limitations.
15.1 Temperature measurement
Place the thermometer bulb fully in the substance but not touching the container walls or base. Stir gently when appropriate and wait for the reading to stabilise.
Read at regular time intervals using the same thermometer depth. Record initial temperature before the first interval and keep timing continuous.
15.2 Cooling investigations
Use equal volumes, identical containers and the same starting temperature when comparing insulation. Fit lids with equivalent holes around thermometers and keep the surroundings similar.
The rate of cooling usually decreases as the object approaches room temperature. A cooling graph is therefore commonly curved rather than a straight line.
15.3 Heating and energy input
When using an electrical heater, measure potential difference, current and heating time if energy is calculated. Ensure the heater is immersed consistently and the liquid is stirred.
Not all electrical energy heats the target substance; some warms the container and escapes. This often makes a calculated specific heat capacity too large if losses are ignored.
15.4 Comparing insulation
A fair comparison may use the temperature decrease over a fixed time or the time taken for the same temperature decrease. State which measure will be used before collecting data.
Thickness, surface coverage and lid design must be controlled unless one is the independent variable. Repeat with the starting conditions restored.
15.5 Thermal evaluation
Main limitations include heat loss, evaporation, uneven temperature, thermometer lag and inconsistent starting conditions. Improvements include insulation, lids, stirring, rapid readings, data logging and correcting for container heat capacity when appropriate.
Do not simply say ‘stop heat loss completely’. Explain how the improvement reduces it; some loss remains unavoidable.
Worked examples
Temperature fall
Water cools from 82.0 °C to 65.5 °C in 300 s. Temperature fall = 16.5 °C; mean cooling rate over this interval = 16.5/300 = 0.055 °C/s.
Fair comparison
Material A gives a 12 °C fall and B gives an 8 °C fall under the same conditions. B is the better insulator because less thermal energy is transferred from the water in the fixed time.
Practical focus
Investigation or training activity
Compare two insulating materials around identical containers. Plan controls, collect temperature–time data and plot cooling curves on the same axes.
Examination guidance
- Keep initial temperature and volume equal.
- Place the thermometer consistently.
- Stir before reading when safe.
- Do not force a straight line through a cooling curve.
- Link insulation improvement to reduced thermal transfer.
Check your understanding
- Why should the thermometer not touch the beaker?
- Why are cooling curves usually curved?
- Name two controlled variables in an insulation test.
- Why may calculated specific heat capacity be too high?
Answers
- It may read the container temperature rather than the liquid.
- Cooling rate decreases as temperature approaches the surroundings.
- Any two of volume, initial temperature, container, lid, surface area or room conditions.
- Some input energy heats the container or is lost, so too much energy is attributed to the substance.
