What is specific heat capacity, and what happens to energy during a change of state?
Specific heat capacity and the equation for energy, internal energy, and the energy needed for a change of state.
A focused answer to the WJEC GCSE Science Double Award Unit 6 topic on heating, covering specific heat capacity and its equation, internal energy, and the energy needed during a change of state when the temperature stays constant.
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What this dot point is asking
WJEC Double Award Unit 6 wants you to describe specific heat capacity and use its equation, explain internal energy, and explain the energy involved in a change of state.
Specific heat capacity
A substance with a high specific heat capacity needs more energy to warm up (and stores more energy), which is why water is used in heating systems and to cool engines.
The energy equation
To use it, substitute the values and multiply. The same equation finds the energy released when a substance cools down.
Internal energy
When you heat a substance without changing its state, the energy increases the particles' kinetic energy, so they move faster and the temperature rises.
Energy during a change of state
This is why, on a heating graph, the temperature rises, then flattens during melting, rises again, then flattens during boiling.
Reading a heating graph
A heating (or cooling) graph plots temperature against time as a substance is heated steadily. The sloping parts show the temperature rising as the particles gain kinetic energy, and the flat parts show the changes of state, where the energy is breaking forces between particles instead of raising the temperature. The first flat part is melting (at the melting point) and the second is boiling (at the boiling point). Being able to identify the states and the changes of state from the shape of a heating graph, and to read the melting and boiling points, is a common data-handling skill.
Why water is good for storing heat
Water's high specific heat capacity (4200 J/kg degrees C) means it can absorb or release a lot of energy for a small change in temperature. This makes water useful for storing and carrying heat - for example in central-heating systems and car cooling systems - and it is why the sea warms up and cools down slowly compared with the land. The high value is why heating a large tank of water takes a lot of energy and time. Linking a high specific heat capacity to the ability to store a lot of energy is a common application of the idea.
Try this
Q1. What is the specific heat capacity of a substance? [1 mark]
- Cue. The energy needed to raise the temperature of 1 kg by 1 degree C.
Q2. What happens to the temperature while a substance is boiling? [1 mark]
- Cue. It stays constant.
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 style4 marksCalculate the energy needed to heat 2 kg of water by 30 degrees C. The specific heat capacity of water is 4200 J/kg degrees C.Show worked answer →
A Unit 6 calculation worth 4 marks. Use (energy = mass x specific heat capacity x temperature change) (1 mark). Substitute , , : (1 mark for substitution) (2 marks for the answer). Markers reward the equation, substitution and the answer with units. A common error is to forget to multiply by the mass.
WJEC style3 marksExplain why the temperature stays constant while a substance is melting, even though it is still being heated.Show worked answer →
A Unit 6 explain question. Reward: during melting the energy supplied is used to break the forces between the particles (overcoming the bonds holding the solid together) (1); it is not used to make the particles move faster (1); so the temperature stays constant until all the solid has melted (1). Markers credit the energy breaking the forces/changing the arrangement rather than raising the temperature. A common error is to say the heating has stopped.
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