OCR Specification focus:
‘Internal energy increases as temperature rises; during phase change temperature remains constant.’
Understanding how temperature affects internal energy and the processes of phase change is essential for explaining the physical behaviour of matter. This topic links microscopic molecular motion to macroscopic temperature changes and state transitions.
Temperature and Internal Energy
Temperature is a measure of the average kinetic energy of the particles in a substance. It reflects how fast the particles are moving or vibrating.
Internal energy is the total energy stored within a system due to both the random motion (kinetic energy) and the intermolecular forces (potential energy) between particles.
Internal Energy: The sum of the random kinetic energy and potential energy of the particles within a system.
As a substance is heated and its temperature rises, the average kinetic energy of its particles increases, and therefore its internal energy increases. However, not all energy supplied necessarily increases temperature — especially during a phase change.
Heating and Temperature Change
When energy is transferred to a substance without changing its state, it causes an increase in temperature. The amount of temperature change depends on the specific heat capacity of the substance.
EQUATION
—-----------------------------------------------------------------
Specific Heat Capacity (c): Q=mcΔθQ = mc\Delta\thetaQ=mcΔθ
QQQ = energy supplied (J)
mmm = mass of the substance (kg)
ccc = specific heat capacity (J kg⁻¹ K⁻¹)
Δθ\Delta\thetaΔθ = change in temperature (K or °C)
—-----------------------------------------------------------------
This equation describes how much energy is needed to raise the temperature of a given mass by one degree. A high specific heat capacity means more energy is required for the same temperature rise, indicating strong particle interactions.
For example, water’s high specific heat capacity allows it to absorb large amounts of heat with minimal temperature change, which has important environmental and biological implications.
Phase Changes and Constant Temperature
When a substance undergoes a change of state — for instance, melting, boiling, or condensing — its temperature remains constant even though energy continues to be transferred. This energy does not increase the average kinetic energy of the particles but instead changes their potential energy by overcoming or forming intermolecular forces.
Phase Change: A physical transformation between solid, liquid, and gaseous states that involves energy transfer without a change in temperature.
During melting or boiling, the energy supplied increases the potential energy of the particles as bonds are broken, allowing greater freedom of movement.
Schematic panels show energy input for melting and vaporisation and energy release for freezing and condensation. The diagrams emphasise that phase change occurs at constant temperature; the supplied or removed energy alters intermolecular bonding rather than particle speed. Minor extras (e.g. sublimation) appear but reinforce the same latent-heat principle. Source.
During freezing or condensation, energy is released as particles form stronger bonds and potential energy decreases.
Latent Heat
The energy involved in changing the state of a substance at constant temperature is called latent heat. The amount of energy required depends on the mass of the substance and the type of phase change.
EQUATION
—-----------------------------------------------------------------
Specific Latent Heat (L): Q=mLQ = mLQ=mL
QQQ = energy supplied or removed (J)
mmm = mass of the substance (kg)
LLL = specific latent heat (J kg⁻¹)
—-----------------------------------------------------------------
Two types of latent heat are commonly studied:
Specific latent heat of fusion (Lₓ): energy required to change 1 kg of a substance between solid and liquid phases.
Specific latent heat of vaporisation (Lᵥ): energy required to change 1 kg of a substance between liquid and gas phases.
Even though the temperature remains constant, the internal energy of the system still changes because potential energy changes while kinetic energy remains constant.
Microscopic Interpretation of Phase Changes
At the microscopic level:
In solids, particles vibrate around fixed positions with strong bonds and little spacing.
In liquids, particles are close together but can move past each other, giving fluidity.
In gases, particles move freely and rapidly, with negligible intermolecular forces.
As heat is applied:
Temperature rises in the solid as kinetic energy increases.
When melting begins, added energy breaks intermolecular bonds — temperature remains constant.
After all solid has melted, further heating increases kinetic energy again, raising temperature.
At the boiling point, energy is used to separate particles into the gas phase — temperature remains constant.
The constancy of temperature during phase change demonstrates that energy is being used to alter molecular arrangement, not molecular speed.
Cooling and Phase Change
When a substance cools, the process reverses:
As energy is removed, particle motion slows, reducing kinetic energy and temperature.
During freezing or condensation, energy is released as bonds form, again at constant temperature.
Internal energy decreases because both kinetic and potential energies are lower after solidification or liquefaction.
These processes illustrate the conservation of energy: the energy lost by a cooling substance is gained by the surroundings.
Graphical Representation: Heating Curve
A heating curve illustrates how temperature changes with energy input:
A heating curve for water showing temperature versus energy added. Sloped sections indicate rising temperature within a single phase; flat sections at 0 °C and 100 °C show phase changes where temperature remains constant while latent heat changes the particles’ potential energy. The uncluttered labels make the latent heat plateaus easy to identify at OCR level. Source.
Sloped sections indicate temperature rise where kinetic energy increases.
Flat sections represent phase changes where temperature remains constant and potential energy changes.
Key points on a heating curve:
Each plateau corresponds to a latent heat process (fusion or vaporisation).
The length of each plateau reflects the amount of energy required for that phase change.
Temperature of water versus heat added, highlighting latent heat plateaus at the melting and boiling points. The dashed line illustrates an example energy path (extra detail beyond the OCR syllabus), but the core message remains the constant temperature during phase change. The axes and labels are clear and uncluttered for study use. Source.
This visual relationship reinforces the specification principle: “Internal energy increases as temperature rises; during phase change temperature remains constant.”
FAQ
The specific latent heat of vaporisation is usually much higher than that of fusion because far more energy is needed to completely separate particles in the liquid phase into a gas.
In fusion (melting), particles only need to overcome some intermolecular forces to move freely but still remain close together.
In vaporisation, particles must break almost all attractive forces to move independently, requiring a greater increase in potential energy per kilogram of substance.
Yes, changes in pressure shift the temperature at which a substance changes state.
Increasing pressure raises the melting point for most substances and lowers the boiling point for gases.
For example, water boils below 100 °C at high altitudes where atmospheric pressure is lower.
These effects occur because pressure alters the energy balance needed for particles to separate or come together during a phase change.
During sublimation, a solid transforms directly into a gas without passing through the liquid phase.
Both kinetic and potential energy of particles increase: kinetic energy rises as temperature and particle motion increase, while potential energy rises as intermolecular bonds are broken entirely.
Although the temperature may remain constant during the phase transition itself, the total internal energy change is significant because the molecular separation becomes extreme.
At the boiling point, the energy supplied is used exclusively to overcome the attractive forces between liquid molecules.
This energy increases the potential energy of the molecules.
The kinetic energy (and therefore temperature) remains unchanged until all the liquid becomes gas.
Any further energy input simply accelerates the phase change rather than increasing temperature.
Only after the entire substance has vaporised does additional heating raise the temperature of the gas.
Energy is released to the surroundings when a gas condenses or a liquid freezes.
The particles lose potential energy as intermolecular forces strengthen and bonds form.
The released energy equals the energy that was required during vaporisation or melting of the same mass.
Thus, energy conservation is maintained: the system’s loss of internal energy becomes a gain in the surroundings’ thermal energy.
Practice Questions
Question 1 (2 marks)
Explain why the temperature of a substance remains constant while it is melting, even though energy is still being supplied.
Mark scheme:
1 mark: States that the energy supplied is used to break or overcome intermolecular bonds/forces between particles.
1 mark: States that this energy increases the potential energy of the particles rather than their kinetic energy, so temperature does not rise.
Question 2 (5 marks)
A student heats a 0.50 kg block of ice at 0 °C until it completely melts and then warms the resulting water to 20 °C.
(a) Name the two processes that occur during this experiment. (2 marks)
(b) Describe and explain how the internal energy of the substance changes during each process. (3 marks)
Mark scheme:
(a)
1 mark: Melting (fusion).
1 mark: Heating (temperature rise) of liquid water.
(b)
1 mark: During melting, temperature remains constant while potential energy of particles increases as bonds are broken.
1 mark: During heating of liquid water, kinetic energy of particles increases as temperature rises.
1 mark: Overall internal energy increases in both processes — due to increases in potential energy (during melting) and kinetic energy (during heating).
