SHC: definition, unit, water value
Specific heat capacity () is the energy needed to raise the temperature of 1 kg of a substance by 1 °C (or 1 K) — two elements: per kilogram AND per degree. Unit: J/kg °C (= J kg⁻¹ K⁻¹). Formula : = energy (J), = mass (kg), = SHC, = temperature change (°C or K). Water has a very high J/kg °C, so it is a good coolant and mildens coastal climates.
Rearranging ΔQ = mcΔT (and Q = Pt)
From : ; ; . When a heater supplies the energy, (power × time in seconds), so . The step most often dropped is multiplying power by time — watts are not joules. And is always a change: 20 °C → 55 °C gives °C, not 55 °C.
Electrical method for SHC
Electrical method for SHC: (1) measure mass on a balance; (2) record the initial temperature; (3) switch on the heater and time ; (4) record the final temperature; (5) ; (6) energy (or ); (7) . Mark-earning details: insulate the container (cut heat loss), stir the liquid (even temperature), and wait for the reading to stabilise before recording.
Drawn from real examiner reports.
Using final temperature as ΔT
If a liquid warms from 40 °C to 75 °C, use °C, not 75 °C. Many candidates substitute the final temperature, giving a about twice too small. The formula needs the change (the delta symbol means a difference), never an absolute temperature.
November 2024 Paper 2P Q5(b): approximately half of all candidates stated 75 °C as the temperature change instead of the correct 35 °C, limiting their access to subsequent marks despite correct rearrangement of the formula.
Leaving time in minutes for Q = Pt
Energy from a heater is with in seconds. A heater run for 4 minutes has s, not 4 — using 4 makes 60 times too big. June 2024 Paper 2P Q8(b)(ii) reported this was the only common error on an otherwise well-answered SHC calculation.
June 2024 Paper 2P Q8(b)(ii): the examiner highlighted that omitting the conversion from minutes to seconds was the only common error on this calculation; all candidates who made the conversion scored full marks.
SHC definition omits "per kilogram"
"The energy needed to heat a substance by 1 °C" omits per kilogram and scores zero — that describes heat capacity (J/°C), not specific heat capacity (J/kg °C). A 1 kg and a 10 kg block need very different energy for the same 1 °C rise. Full form: "the energy to raise the temperature of 1 kg of a substance by 1 °C".
Wrong or incomplete unit for c
State the unit of in full: J/kg °C. Dropping "/kg" (writing J/°C) gives heat capacity; dropping "/°C" (writing J/kg) is dimensionally wrong; writing just "J" or "joules" scores zero. The energy answer , by contrast, is in J — do not tag it with J/kg °C.
Substituting Q where c belongs
Do not substitute the energy where belongs — they are different quantities. In June 2024 Paper 2P Q8(b)(iii) some candidates put in for , and others rearranged wrongly. Write the formula, then the correct rearrangement (), before substituting numbers.
June 2024 Paper 2P Q8(b)(iii): some candidates confused the energy transfer with the specific heat capacity (substituted Q for c); others failed to rearrange correctly.
Leaving mass in grams, not kg
Convert mass to kilograms before substituting — SHC uses J/kg °C. Leaving mass in grams (e.g. 500 instead of 0.50) makes 1000 times too small. Sense-check the answer against typical values: water ≈ 4200, metals ≈ 400–900 J/kg °C.
(general exam technique)
Multi-step SHC routine
SHC questions combine and . Routine: (1) convert units (time → s, mass → kg); (2) write both formulae; (3) find ; (4) substitute into ; (5) rearrange for the unknown. Numbered steps earn ECF marks.
Sanity-check the size of c
Sanity-check your : water ≈ 4200 J/kg °C, metals ≈ 400–900. If you get 60 000 or 0.75, you probably left time in minutes or mass in grams. A value orders of magnitude off flags a missed unit conversion.
Explain a low result with heat loss
In "why is the measured too high/low?" questions, use heat loss: some heater energy warms the container and surroundings, so the water gets less than . Using then makes come out too low. Improve with insulation and a lid.
Give both elements when defining c
When defining , always give both elements and the unit: "the energy to raise the temperature of 1 kg by 1 °C, unit J/kg °C". Missing "1 kg" turns it into heat capacity. Practise writing the full sentence so you do not drop half of it under pressure.
| Quantity | Symbol | Formula | Unit |
|---|---|---|---|
| Energy transferred | joule, J | ||
| Specific heat capacity | J/kg °C | ||
| Energy from heater | joule, J | ||
| Power | watt, W |
Where = mass (kg), = specific heat capacity (J/kg °C), = temperature change (°C or K), = power (W), = time (s).
Mark-scheme definition: Specific heat capacity is the energy required to raise the temperature of 1 kg of a substance by 1 °C. Both the per-kilogram and per-degree conditions must be stated for full marks.
Key values to remember:
Define specific heat capacity.
A block of aluminium has a mass of 2.0 kg. Its specific heat capacity is 900 J/kg °C.
Calculate the energy needed to raise the temperature of the block by 15 °C.
Give the unit of your answer.
