Thermal Properties of Matter examines how substances store and exchange energy as heat. It connects particle-level behaviour to measurable quantities such as temperature change and change of state.
These notes cover temperature and internal energy, specific heat capacity, specific latent heat, and the differences between evaporation and boiling. The topic is a foundation for later topics on the gas laws and heat transfer.
What these notes cover
- •Temperature and Internal Energy
- •Celsius and Kelvin
- •Specific Heat Capacity
- •Heat Capacity
- •Change of State and Specific Latent Heat
- •Latent Heat of Vaporisation versus Fusion
- •Heating Curve of Ice
- •Evaporation versus Boiling
- •Factors Affecting the Rate of Evaporation
- •Common Mistakes
- •Summary
Key definitions
- Temperature
- A measure of the average kinetic energy of the particles in a body. It is measured in kelvin (K) or degrees Celsius (°C).
- Internal energy
- The sum of the internal kinetic energy (from the random motion of particles) and internal potential energy (from intermolecular forces) of all the particles in a body.
- Specific heat capacity (c)
- The thermal energy required to raise the temperature of 1 kg of a substance by 1 °C (or 1 K), without a change of state.
- Specific latent heat (l)
- The quantity l in Q = ml for a change of state at constant temperature: the specific latent heat of fusion (lf) for solid ↔ liquid, or of vaporisation (lv) for liquid ↔ gas. Unit: J kg⁻¹.
- Evaporation
- A process by which a liquid changes into a gas at any temperature, occurring only at the liquid surface, with no bubbles and no external heat source needed.
Key formulas
Celsius to kelvin
T = θ + 273
T is the temperature in kelvin; θ is the temperature in °C.
Thermal energy and specific heat capacity
Q = mcΔθ
Q is the thermal energy supplied or removed (J), m is the mass (kg), c is the specific heat capacity (J kg⁻¹ °C⁻¹), Δθ is the change in temperature (°C or K). Applies only when there is no change of state.
Heat capacity of an object
C = mc
C is the heat capacity of one specific object rather than per unit mass. SI unit: J °C⁻¹.
Thermal energy and heat capacity
Q = CΔθ
Used for one specific object with heat capacity C.
Thermal energy and specific latent heat
Q = ml
l is the specific latent heat of fusion (lf, solid ↔ liquid) or of vaporisation (lv, liquid ↔ gas), in J kg⁻¹. Applies when state changes at constant temperature.
Worked example
A 0.50 kg block of copper (c = 390 J kg⁻¹ °C⁻¹) is heated from 20 °C to 95 °C. Calculate the thermal energy absorbed.
- 1Find the change in temperature: Δθ = 95 − 20 = 75 °C.
- 2Substitute into Q = mcΔθ: Q = 0.50 × 390 × 75 = 1.46 × 10⁴ J.
Watch out: Q = mcΔθ applies only when there is no change of state. During melting or boiling Δθ = 0, so use Q = ml instead.
Common mistakes
Confusing heat capacity with specific heat capacity
Wrong: Treating heat capacity and specific heat capacity as the same quantity.
Right: Heat capacity (Q = CΔθ) is a property of one specific object; specific heat capacity (Q = mcΔθ) is a property per kg of the material, with C = mc.
Using Q = mcΔθ during a change of state
Wrong: Using Q = mcΔθ to find the thermal energy needed to melt or boil a substance.
Right: Temperature is constant during melting and boiling, so Δθ = 0 and Q = mcΔθ does not apply. Use Q = ml instead.
Saying evaporation removes heat
Wrong: Evaporation removes heat from the liquid.
Right: The more energetic particles escape, leaving behind particles with lower average kinetic energy, so the average temperature of the remaining liquid falls. No external heat is removed — the liquid cools itself.
Questions students ask
What is the difference between temperature and internal energy?
Temperature is a measure of the average kinetic energy of the particles in a body, while internal energy is the total kinetic and potential energy of all its particles. A body at a higher temperature does not necessarily have more internal energy, because internal energy also depends on the mass and state of the body.
Why does temperature stay constant during melting and boiling?
During a change of state, the thermal energy supplied is used entirely to overcome intermolecular forces of attraction, changing internal potential energy rather than the kinetic energy of the particles. This is why the heating curve of ice has flat plateaus at 0 °C and 100 °C.
Why is the specific latent heat of vaporisation greater than that of fusion?
Boiling requires completely separating particles into a gas, breaking all remaining intermolecular bonds and doing work against atmospheric pressure as the substance expands hugely in volume. Melting only needs to loosen the fixed arrangement of particles in a solid.
What is the difference between evaporation and boiling?
Evaporation occurs at any temperature, only at the liquid surface, with no bubbles, and is a slow process that needs no external heat source. Boiling occurs only at the boiling point, throughout the liquid, with visible bubbles of vapour, and is a rapid process that requires a continuous source of heat.
In the full PDF
- •Figure 1: heating curve of ice warmed at a constant rate from −10 °C until it becomes steam
- •Two further worked examples: explaining why a hot metal rod can have less internal energy than a barrel of water, and calculating the energy to melt 0.20 kg of ice
- •Full side-by-side comparison table of evaporation and boiling
- •All five factors that affect the rate of evaporation, each with its particle explanation
- •The remaining common mistake on temperature and internal energy
- •End-of-topic summary