OCR GCSE Physics A (J249) — Matter
Every time you boil a kettle, the water stops getting hotter once it hits , even though the kettle is still transferring electrical energy to it. This happens because the energy is no longer being used to raise the temperature, but to change the state of the substance.
To understand this, we must look at internal energy, which is the total energy stored by the particles within a system. It is made up of two parts: the kinetic energy of the particles moving around, and the potential energy stored in the intermolecular bonds between them.
When a substance is heated and its temperature rises, the thermal energy increases the kinetic energy store of the particles. However, during a change of state at the melting or boiling point, the temperature remains perfectly constant. The supplied energy is entirely used to break or weaken the intermolecular forces, increasing the potential energy store while the kinetic energy remains unchanged.
The reverse is true when a substance cools down to freeze or condense. This is an exothermic process where particles move closer together and form bonds. As these bonds form, potential energy decreases and thermal energy is released to the surroundings, keeping the temperature constant until the transition is complete. During all these physical changes, the number of particles does not change, meaning mass is always conserved.
If you leave a glass of water and a block of iron in the sun, the iron heats up much faster, yet melting that iron requires immense energy. Understanding thermal physics requires you to know exactly when energy causes a substance to heat up and when it causes a state change. We use two different measurements for these processes: specific heat capacity and specific latent heat.
The word "latent" means hidden, referring to the fact that thermal energy is being transferred without any visible change on a thermometer. The table below compares the two concepts:
| Feature | Specific Heat Capacity | Specific Latent Heat |
|---|---|---|
| Definition | Energy required to raise the temperature of of a substance by . | Energy required to change the state of of a substance with no change in temperature. |
| Energy transfer | Increases or decreases the kinetic energy of the particles. | Increases or decreases the potential energy of the particles. |
| Temperature | Continually changes (rises or falls). | Remains completely constant. |
| State | Remains the same. | Changes (e.g., solid to liquid). |
When calculating the energy transfer for a temperature change, we use the specific heat capacity formula:
When calculating the energy transfer for a state change, we use the specific latent heat formula:
Melting an ice cube takes a fraction of the energy required to boil away the same mass of liquid water. Because different state changes require different amounts of energy to overcome intermolecular forces, specific latent heat is split into two distinct types.
The specific latent heat of fusion is the thermal energy required to change of a substance from a solid to a liquid at its melting point, with no change in temperature. Here, energy is simply used to weaken the solid bonds so particles can flow past each other.
The specific latent heat of vaporisation is the thermal energy required to change of a substance from a liquid to a gas at its boiling point, with no change in temperature. This value is always significantly higher than the latent heat of fusion for the same substance because all intermolecular bonds must be completely broken to separate the particles into a gas.
Calculate the thermal energy required to completely boil of water at . (Specific latent heat of vaporisation of water = )
Step 1: State the correct formula for a state change.
Step 2: Substitute the known values into the equation.
Step 3: Calculate the final answer and include the correct units.
Why does a graph of temperature against time for a melting and boiling substance look like a staircase rather than a straight line? These graphs, known as heating curves or cooling curves, provide a visual representation of how internal energy changes over time.
When plotting temperature against time (or energy supplied), the sloping sections show a single state being heated. The gradient of this slope depends on the specific heat capacity of the substance — a shallower slope means it takes more energy to raise the temperature.
The horizontal plateau sections show where the state changes are occurring. The first plateau at a lower temperature represents melting, while the second plateau at a higher temperature represents boiling. The length of the plateau corresponds directly to the specific latent heat; a longer horizontal line means more energy is required to complete the transition.
Students often state that temperature increases during a change of state because energy is constantly being supplied; remember that the temperature remains completely flat (constant) until the entire substance has changed state.
In 6-mark questions asking you to explain a heating curve, examiners will award marks for explicitly stating that thermal energy increases the potential energy store (by breaking/weakening bonds) during the plateaus, not the kinetic energy store.
When defining specific latent heat, you will lose marks if you forget to include the phrases '1 kg' (or unit mass) and 'without a change in temperature'.
Always check the units of mass in specific latent heat calculations; if you are given the mass in grams, you must divide by 1000 to convert it into kilograms before multiplying by L.
If asked why the specific latent heat of vaporisation is higher than fusion, explicitly state that vaporisation requires breaking ALL intermolecular bonds, whereas fusion only requires weakening them.
Internal energy
The total energy stored by the particles in a system, calculated as the sum of their randomly distributed kinetic and potential energies.
Kinetic energy
The energy particles possess due to their motion or vibration, which increases as temperature rises.
Potential energy
The energy particles possess due to their position and the intermolecular bonds between them, which changes during a change of state.
Exothermic
A process that releases thermal energy to the surroundings, such as freezing or condensing.
Specific heat capacity
The amount of energy required to raise the temperature of 1 kg of a substance by 1°C.
Specific latent heat
The amount of energy required to change the state of 1 kg of a substance with no change in temperature.
Specific latent heat of fusion
The thermal energy required to change 1 kg of a substance from a solid to a liquid at its melting point with no change in temperature.
Specific latent heat of vaporisation
The thermal energy required to change 1 kg of a substance from a liquid to a gas at its boiling point with no change in temperature.
Heating curves
Graphs showing how the temperature of a substance changes over time as thermal energy is steadily supplied.
Cooling curves
Graphs showing how the temperature of a substance changes over time as it releases thermal energy to its surroundings.
Put your knowledge into practice — try past paper questions for Physics A
Internal energy
The total energy stored by the particles in a system, calculated as the sum of their randomly distributed kinetic and potential energies.
Kinetic energy
The energy particles possess due to their motion or vibration, which increases as temperature rises.
Potential energy
The energy particles possess due to their position and the intermolecular bonds between them, which changes during a change of state.
Exothermic
A process that releases thermal energy to the surroundings, such as freezing or condensing.
Specific heat capacity
The amount of energy required to raise the temperature of 1 kg of a substance by 1°C.
Specific latent heat
The amount of energy required to change the state of 1 kg of a substance with no change in temperature.
Specific latent heat of fusion
The thermal energy required to change 1 kg of a substance from a solid to a liquid at its melting point with no change in temperature.
Specific latent heat of vaporisation
The thermal energy required to change 1 kg of a substance from a liquid to a gas at its boiling point with no change in temperature.
Heating curves
Graphs showing how the temperature of a substance changes over time as thermal energy is steadily supplied.
Cooling curves
Graphs showing how the temperature of a substance changes over time as it releases thermal energy to its surroundings.
