Thermal properties and temperature
Thermal expansion, thermometers, specific heat capacity and specific latent heat.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 2.2.1Describe thermal expansion of solids, liquids and gases and give everyday consequences.
- 2.2.2Recall and use E = mcΔθ for specific heat capacity.
- 2.2.3Describe melting, boiling and condensation in terms of latent heat.Supplement
8 minute read
Thermal expansion, heat capacity and latent heat
Thermal expansion
When a substance is heated, its particles gain kinetic energy and vibrate or move more, taking up slightly more space on average. This is thermal expansion. Gases expand the most for a given temperature rise, then liquids, then solids, because gas particles have the most freedom to spread out.
Everyday consequences: gaps are left in railway tracks and bridges to allow for expansion in hot weather; a bimetallic strip (two different metals joined together) bends when heated because the two metals expand by different amounts, which is used in thermostats.
Specific heat capacity
Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C. The equation is E = mcΔθ. A substance with a high specific heat capacity (like water, c = 4200 J/(kg °C)) needs a lot of energy to heat up, which is why water is slow to warm and slow to cool — useful for storage heaters and for regulating coastal climates.
Latent heat
When a substance melts or boils, energy is supplied but the temperature does not rise — instead, the energy is used to overcome the forces of attraction between particles as the state changes. This hidden energy is called latent heat.
The reverse is also true: freezing and condensing release energy to the surroundings, which is why condensing steam on skin causes a more severe burn than boiling water at the same temperature — the steam releases extra latent heat as it condenses.
Think of it like this
Latent heat is like the "toll" paid to change floor in a building with no lift — you supply energy just to break free of the current arrangement, and none of that energy shows up as extra speed (temperature) until the move is complete.
Worked examples
Method, step by step
Calculate the energy needed to raise the temperature of 0.50 kg of water from 20 °C to 100 °C. (c of water = 4200 J/(kg °C))
- 1Δθ = 100 − 20 = 80 °C
- 2E = mcΔθ
- 3E = 0.50 × 4200 × 80
E = 168 000 J (168 kJ)
Common misconceptions
- Believing temperature keeps rising while a substance melts or boils. It stays constant until the change of state is complete.
- Thinking a high specific heat capacity means a substance "holds onto" heat forever. It simply needs (and releases) a large amount of energy for each degree of temperature change.
- Assuming solids do not expand at all when heated. They do, just much less than liquids or gases for the same temperature rise.
In the exam
- A graph of temperature against time for a heated substance should show flat sections at the melting and boiling points — label these correctly if drawing or interpreting one.
- For E = mcΔθ questions, always identify Δθ carefully as the *change* in temperature, not a single reading.