Changes of State

Set in 2 of the 24 papers. The definition has three parts and the marks follow them: the energy required, per kilogram (per unit mass), to change the temperature by 1 °C (1 K is accepted). When it is worth 2 marks, the energy and the temperature change are one mark and the mass is the other, which is given only if you have earned the first. An equation on its own is ignored, and so is "heat" in place of "energy", so write the word energy. "Raise" or "increase" is accepted in place of "change".

Set in 2 of the 24 papers. Argue from the equation. The energy transferred is the same for both substances (what one loses the other gains, apart from some lost to the surroundings), so for equal masses the substance with the smaller specific heat capacity has the larger temperature change. Saying which value is smaller earns a point of its own on top of saying they differ, and a short algebraic comparison with ΔQ = m × c × ΔT can earn another. For a store such as concrete or water, the credited ideas are that it absorbs and releases a lot of energy per kilogram, so it keeps supplying heat, or keeps its temperature, for longer.

Set in 8 of the 24 papers, always for 3 marks and always on Paper 2. When the energy is the unknown the marks are: the temperature change, the substitution, the evaluation; using the final temperature in place of the change loses the first mark only, as the later marks follow through. When c or the mass is the unknown they are substitution, rearrangement and evaluation, so write the rearranged line even when you could do it in your head. Mass goes in kilograms when c is in J/kg °C; a mass left in grams is a power-of-ten error and costs one mark.

When the heater's power, or its voltage and current, is given, first find the energy supplied (energy = power × time, or E = I × V × t) with the time in seconds; leaving minutes unconverted costs a mark. Evaluate a "show that" answer to at least one more significant figure than the printed value.