Calculate the energy to raise 0.20\,kg of water by 30\,^C (c = 4200\,J/kg\,^C). [2 marks]

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What are specific heat capacity and specific latent heat, and how are they calculated?

Specific heat capacity and its equation, specific latent heat of fusion and vaporisation, and heating and cooling curves.

A focused answer to WJEC GCSE Physics topic 1.8 on kinetic theory, covering specific heat capacity and its equation, specific latent heat of fusion and vaporisation, why temperature stays constant during a change of state, and heating curves.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-14

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this topic is asking
Specific heat capacity
Specific latent heat
Why temperature is constant during a change of state
Try this

What this topic is asking

WJEC wants you to define and use specific heat capacity, define specific latent heat, and explain heating and cooling during changes of state. This is part of topic 1.8 Kinetic theory in Unit 1 of WJEC GCSE Physics (3420).

Specific heat capacity

Measuring the specific heat capacity of a material, by heating it with an electric heater and recording the energy supplied and the temperature rise, is a specified practical. In the experiment an insulated block of the material is heated by an electric immersion heater of known power, the energy supplied is found from $E = Pt$ (power times time), the temperature rise is read from a thermometer, and the mass is measured on a balance. Rearranging $E = cm\Delta\theta$ then gives $c = \dfrac{E}{m\Delta\theta}$ . The main source of error is heat lost to the surroundings, which makes the measured specific heat capacity a little too high, so the block is lagged with insulation to reduce this loss. Water's high specific heat capacity is the reason it is used in central heating systems and to cool engines: it can carry a lot of energy without its own temperature rising too far.

Specific latent heat

Why temperature is constant during a change of state

Energy to heat and then melt

Calculate the energy needed to heat $0.50\,\text{kg}$ of ice at $0\,^\circ\text{C}$ until it has all just melted to water at $0\,^\circ\text{C}$ . The specific latent heat of fusion of ice is $3.3 \times 10^5\,\text{J/kg}$ .

step 1: Choose the right equation

There is no temperature change, only a change of state, so use $E = mL$ (not $E = cm\Delta\theta$ ).

step 2: Write down the values

Mass $m = 0.50\,\text{kg}$ and latent heat of fusion $L = 3.3 \times 10^5\,\text{J/kg}$ .

step 3: Substitute and calculate

$E = mL = 0.50 \times 3.3 \times 10^5 = 1.65 \times 10^5\,\text{J}$ .

step 4: State the answer

About $1.65 \times 10^5\,\text{J}$ (165 kJ) is needed to melt the ice, all at $0\,^\circ\text{C}$ . Use $E = mL$ for state changes and $E = cm\Delta\theta$ for temperature changes.

Try this

Q1. Calculate the energy to raise $0.20\,\text{kg}$ of water by $30\,^\circ\text{C}$ ( $c = 4200\,\text{J/kg}\,^\circ\text{C}$ ). [2 marks]

Cue. $E = cm\Delta\theta = 4200 \times 0.20 \times 30 = 25\,200\,\text{J}$ .

Q2. State what specific latent heat of vaporisation means. [1 mark]

Cue. The energy needed to boil $1\,\text{kg}$ of a substance without a temperature change.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20184 marksCalculate the energy needed to raise the temperature of

2.0\,\text{kg}

of water from

20\,^\circ\text{C}

80\,^\circ\text{C}

. The specific heat capacity of water is

4200\,\text{J/kg}\,^\circ\text{C}

Show worked answer →

A topic 1.8 Calculate question. Use $E = cm\Delta\theta$ where $c = 4200\,\text{J/kg}\,^\circ\text{C}$ , $m = 2.0\,\text{kg}$ and the temperature change $\Delta\theta = 80 - 20 = 60\,^\circ\text{C}$ (1 mark for the temperature change). Substitute: $E = 4200 \times 2.0 \times 60$ (1 mark) $= 504\,000\,\text{J}$ (2 marks for the calculation and the unit joules), which is about $5.0 \times 10^5\,\text{J}$ . Markers reward the correct temperature change, the substitution and the answer with units. A common error is to use the final temperature instead of the change.

WJEC 20223 marksExplain why the temperature of a melting solid stays constant even though it is still being heated.

Show worked answer →

A topic 1.8 Explain question. During melting the energy being supplied is used to break the bonds between particles and rearrange them from solid to liquid (1 mark), rather than to make them move faster (1 mark). Because the average kinetic energy of the particles does not increase, the temperature stays constant until all the solid has melted (1 mark). Markers reward the energy breaking bonds, the kinetic energy not rising, and the constant temperature. A common error is to say no energy is being absorbed during melting.

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Sources & how we know this

WJEC GCSE Physics specification (3420) from 2016 — WJEC (2016)