Edexcel IGCSE Physics · Spec 5.11P-5.14P
Heating Curves and Specific Heat Capacity Practicals (Paper 2)
Covers interpret a change-of-state plateau, Measure specific heat capacity of a solid and Measure specific heat capacity of water.
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Heating Curves and Specific Heat Capacity Practicals (Paper 2)
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Explained
Flat parts of a heating curve, and measuring c
Heat a pure substance at a steady rate and record its temperature at regular intervals. The graph rises, goes flat, rises again, and goes flat again.
The rising sections are the substance getting hotter as a solid, then as a liquid, then as a gas. The flat sections are the changes of state, melting and then boiling.
Why the temperature stops rising
Energy is still being supplied during a plateau, so something must be absorbing it.
It is being used to overcome the forces of attraction between the particles, separating them so they can move past one another or move apart entirely. It is not being used to make them move faster, and since temperature depends on the average kinetic energy, the temperature does not change.
Once the change of state is complete, all the energy goes back into increasing the kinetic energy, so the temperature rises again.
Reading a plateau tells you a melting or boiling point directly, and a sharp plateau at a fixed temperature is evidence that the substance is pure. A mixture melts over a range instead.
Specific heat capacity of a solid
Use a metal block with two holes, one for an electric heater and one for a thermometer.
Find the mass of the block on a balance, record the starting temperature, switch the heater on for a measured time, and record the highest temperature reached. The energy supplied is the power multiplied by the time, or the voltage times the current times the time.
Then rearrange energy equals mass times specific heat capacity times temperature change to find c. A little oil in the thermometer hole improves the thermal contact, and insulating the block reduces the energy escaping.
Specific heat capacity of water
The same idea with an immersion heater in a known mass of water in an insulated container.
Two extra points matter here. Stir the water so the temperature is the same throughout, otherwise the thermometer reads only its own corner. And keep taking readings for a short while after switching off, because the temperature continues to rise as the heater cools.
Measured values usually come out too high, because some energy warms the container, the heater and the air rather than the water. Less energy than you counted actually went into the water, so the calculated c is larger than the true value.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme asks how a heating curve would differ for a different substance and credits two ideas: that the temperature rises at a different rate, and that the melting point would be different. It accepts an annotation on the graph for either mark, and allows the phrasing that the flat part would be at a different temperature.
It then adds a third option as a higher order idea: that the flat part would be a different length. That is the subtler point, because the length of the plateau depends on how much energy the change of state requires, which is a different property from the temperature at which it happens.
On the calculation, the same mark scheme shows how partial credit works. A candidate who substituted the wrong temperature change, but used the correct equation with the correct specific heat capacity and energy, scored one mark rather than none. It also notes that any power of ten error in an otherwise correct answer scores two of the three marks.
Elsewhere on the same paper, describing particle motion in a gas, it credits the idea of high speed and the idea of randomness, then instructs the examiner to ignore the word freely. Free is not a description of motion; fast and random are.
Reading the graph in an exam
Identify the state in each region before you answer anything. Below the first plateau it is a solid, between the plateaus a liquid, and above the second a gas.
If asked what is happening at a plateau, name the change of state and say that the energy is being used to overcome the forces between particles rather than to raise the temperature. Those two clauses are the answer to almost every version of the question.
Spec 5.11P-5.14P
What you need to know
- Interpret a change-of-state plateau
- Measure specific heat capacity of a solid
- Measure specific heat capacity of water
Active recall
Quick check
Answer each question before opening the answer.
Why is temperature constant during melting?
Energy overcomes attractions instead of increasing kinetic energy
How is electrical energy calculated?
E = IVt
Why stir the water?
To keep the temperature uniform
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