What you'll learn
Heating a substance stores energy in it, and different materials need different amounts of energy to heat up — that is what specific heat capacity measures. For AQA GCSE Physics you need to understand internal energy, how heating changes it, how to calculate the energy needed to change a substance's temperature, and the related idea of specific latent heat during changes of state. This guide covers internal energy, specific heat capacity and its equation, the required-practical link, and specific latent heat. By the end you should be able to define internal energy, use the specific heat capacity equation, and explain why temperature stays constant during a change of state.
Key terms and definitions
Internal energy — The total kinetic and potential energy of all the particles in a substance.
Temperature — A measure of the average kinetic energy of the particles.
Specific heat capacity — The energy needed to raise the temperature of 1 kg of a substance by 1 °C.
Specific latent heat — The energy needed to change the state of 1 kg of a substance with no change in temperature.
Change of state — A change between solid, liquid and gas.
Joule (J) — The unit of energy.
Kinetic energy — The energy a particle has because of its movement.
Latent heat of fusion / vaporisation — The energy to melt / boil 1 kg of a substance.
Core concepts
Internal energy
The particles in a substance are constantly moving and interacting. The internal energy of a substance is the total of the kinetic energy (from the particles' movement) and the potential energy (from their positions and the forces between them) of all the particles. Heating a substance transfers energy to its particles and so increases its internal energy. This increased internal energy either raises the temperature or changes the state of the substance.
Heating and temperature
When you heat a substance and its temperature rises, the kinetic energy of the particles increases — they move faster. Temperature is a measure of the average kinetic energy of the particles, so a higher temperature means faster-moving particles. How much the temperature rises for a given amount of energy depends on the substance and its mass.
Specific heat capacity
The specific heat capacity of a substance is the energy needed to raise the temperature of 1 kg of it by 1 °C. Substances with a high specific heat capacity, such as water, need a lot of energy to heat up (and release a lot as they cool), while substances with a low specific heat capacity heat up quickly. The energy needed is calculated using:
change in thermal energy = mass × specific heat capacity × temperature change
E = m c ΔT
where E is energy in joules (J), m is mass in kilograms (kg), c is the specific heat capacity in J/kg°C, and ΔT is the temperature change in °C.
Required practical link
You can measure the specific heat capacity of a material, such as a metal block, by heating it with an electric heater. You measure the mass of the block, use a heater to supply a known amount of energy (from the power and time, energy = power × time), and record the temperature rise with a thermometer. Rearranging E = m c ΔT gives the specific heat capacity: c = E ÷ (m ΔT). To improve accuracy, the block is often insulated to reduce heat loss to the surroundings, which would otherwise make the measured value too high.
Changes of state and specific latent heat
When a substance changes state — melting, boiling, freezing or condensing — energy is transferred but the temperature stays constant. This is because the energy is used to change the potential energy of the particles (breaking or forming the bonds between them), not their kinetic energy, so the temperature does not change while the state is changing.
The energy needed to change the state of 1 kg of a substance without a change in temperature is the specific latent heat:
energy for a change of state = mass × specific latent heat (E = m L)
There are two types: the specific latent heat of fusion (melting or freezing) and the specific latent heat of vaporisation (boiling or condensing).
Why temperature is constant during a change of state
During melting or boiling, all the energy supplied goes into overcoming the forces between the particles rather than making them move faster. Because temperature depends on the average kinetic energy, and the kinetic energy is not increasing, the temperature stays constant until the change of state is complete. This produces the flat sections seen on a temperature–time heating graph.
Why water has a high specific heat capacity and why it matters
Water has an unusually high specific heat capacity of about 4200 J/kg°C, which means it takes a lot of energy to heat it up and it releases a lot of energy as it cools. This has important practical effects. It is why water is used in central heating systems and car cooling systems — it can carry a large amount of thermal energy. It is also why the sea heats up and cools down more slowly than the land, which moderates the climate of coastal areas. When you see a substance with a high specific heat capacity, expect it to resist temperature change, which is exactly what makes water so useful for storing and transferring energy.
Heating and cooling graphs
A useful way to picture this topic is a temperature–time graph as a substance is heated steadily. While the substance is in a single state (solid, liquid or gas), the temperature rises steadily as its particles gain kinetic energy — the steepness depends on the specific heat capacity, with a higher capacity giving a gentler rise. At a change of state (melting or boiling), the line goes flat, because the energy is being used to change the state at constant temperature. When the change is complete, the temperature rises again. Reading these graphs — identifying the sloping sections as heating within a state and the flat sections as changes of state — is a common exam skill that ties internal energy, specific heat capacity and latent heat together.
Worked examples
Example 1: Using the specific heat capacity equation
How much energy is needed to raise the temperature of 2 kg of water by 30 °C? The specific heat capacity of water is 4200 J/kg°C. E = m c ΔT = 2 × 4200 × 30 = 252,000 J.
Example 2: Finding the temperature change
500 J of energy is supplied to 0.1 kg of a metal with a specific heat capacity of 500 J/kg°C. Find the temperature rise. Rearranging E = m c ΔT: ΔT = E ÷ (m c) = 500 ÷ (0.1 × 500) = 500 ÷ 50 = 10 °C.
Example 3: Latent heat
How much energy is needed to melt 0.5 kg of ice? The specific latent heat of fusion of ice is 334,000 J/kg. E = m L = 0.5 × 334,000 = 167,000 J.
Example 4: Explaining a flat line on a heating graph
A heating graph shows temperature rising, then staying flat, then rising again. Explain the flat section. The flat section is where the substance is changing state (for example melting). The energy supplied is used to overcome the forces between particles rather than increasing their kinetic energy, so the temperature stays constant until the change of state is complete.
Common mistakes and how to avoid them
A common error is confusing specific heat capacity with specific latent heat. Specific heat capacity is for a temperature change (E = m c ΔT); specific latent heat is for a change of state at constant temperature (E = m L). Choose the right equation.
Students often use grams instead of kilograms. Mass must be in kilograms in these equations, so convert grams to kilograms (divide by 1000) first.
Another mistake is thinking the temperature rises during melting or boiling. During a change of state the temperature stays constant, because the energy changes the particles' potential energy, not their kinetic energy.
When measuring specific heat capacity in the practical, remember that heat loss to the surroundings makes the measured value too high, so insulation improves accuracy.
Finally, be careful with the temperature change ΔT — it is the difference between the final and starting temperatures, not the final temperature itself.
Exam technique for "Internal energy and specific heat capacity"
For calculations, write down the equation (E = m c ΔT or E = m L), convert the mass to kilograms, substitute, and give the answer in joules. Rearranging to find c or ΔT is common, so practise all versions.
For explanation questions, define internal energy as the total kinetic and potential energy of the particles, and explain that heating increases it. Be ready to explain why temperature stays constant during a change of state, using the idea of energy going into potential energy rather than kinetic energy.
For the required practical, describe measuring the mass, supplying a known energy with a heater, measuring the temperature rise, and using insulation to reduce heat loss. Use precise terms — internal energy, specific heat capacity, specific latent heat — throughout.
Quick revision summary
- Internal energy is the total kinetic and potential energy of the particles; heating increases it.
- Temperature measures the average kinetic energy of the particles.
- Specific heat capacity: energy to raise 1 kg by 1 °C. E = m c ΔT (mass in kg).
- Specific latent heat: energy to change the state of 1 kg at constant temperature. E = m L (fusion for melting, vaporisation for boiling).
- During a change of state the temperature stays constant, because energy goes into potential energy (breaking bonds), not kinetic energy.
- In the practical, insulate the block to reduce heat loss, which would otherwise make c too high.