Thermal Properties of Matter: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 10 · Read time: ~9 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
Internal energy
Internal energy is the energy stored in a system. It is the total kinetic energy of the particles' random motion plus the total potential energy between the particles.
Heating a substance raises its internal energy. If the temperature rises, the extra energy goes mainly into kinetic energy. During a change of state, it goes into potential energy instead.
Heat capacity and specific heat capacity
Heat capacity (C): the energy needed to raise the temperature of a whole object by 1°C. Unit: J/°C.
Specific heat capacity (c): the energy needed to raise the temperature of 1 kg of a substance by 1°C. Unit: J/(kg °C). Q = mcΔθ.
Water has a high specific heat capacity, so it heats up and cools down slowly. This is why coastal areas have milder temperatures than inland areas.
Trap: Heat capacity is for a whole object, specific heat capacity is per kilogram. A large pot and a small pot made of the same metal have the same specific heat capacity but different heat capacities.
Melting, boiling, evaporation and latent heat
During melting and boiling, energy is absorbed with no change in temperature. During solidification and condensation, energy is released, again at constant temperature.
Latent heat: the energy absorbed or released during a change of state, without a change in temperature. Specific latent heat (l): the energy for 1 kg of the substance. Q = ml. Use the latent heat of fusion for melting and the latent heat of vaporisation for boiling.
Particle explanation: the energy supplied is used to overcome the attractive forces between particles, increasing their potential energy, not their kinetic energy. That is why the temperature stays constant.
Boiling vs evaporation: boiling happens at a fixed temperature (the boiling point), throughout the liquid, with bubbles, and needs a continuous energy supply. Evaporation happens at any temperature, only at the surface, and causes cooling because the most energetic particles escape.
Trap: The most common mark lost: saying the temperature rises during melting or boiling. Energy is still being supplied, but it goes into breaking particles apart, so the temperature holds steady until the change of state is complete.
Reading a cooling curve
A cooling curve plots temperature against time as a substance cools. Sloping sections show the temperature falling within one state. A flat section shows a change of state (for example liquid to solid) at constant temperature, while latent heat is released.
Trick: Before calculating, label each section of the graph. Sloped section: use Q = mcΔθ. Flat section: use Q = ml. Most errors come from applying the wrong formula to the wrong part of the graph.
Worked Example
A heater supplies 50,400 J of energy to a 0.60 kg block of ice at 0°C. The specific latent heat of fusion of ice is 336,000 J/kg, and the specific heat capacity of water is 4,200 J/(kg °C).
(a) Calculate the energy needed to melt all of the ice. [2]
Q = ml
= 0.60 × 336,000
= 201,600 J
(b) Determine the final state of the ice-water mixture after the 50,400 J has been supplied. [3]
50,400 J is less than the 201,600 J needed to melt all the ice, so only part of it melts and none of the energy raises the temperature.
Mass melted = Q ÷ l = 50,400 ÷ 336,000 = 0.15 kg.
The mixture ends as 0.15 kg of water and 0.45 kg of ice, still at 0°C.
(c) Calculate the further energy needed, after all the ice has melted, to warm the 0.60 kg of water from 0°C to 20°C. [2]
Q = mcΔθ
= 0.60 × 4,200 × 20
= 50,400 J
Why this question is a good test of the topic: it forces students to check whether there is enough energy to finish a change of state before any temperature rise can happen, then switch formulas once the substance is in a single state.
Frequently Asked Questions
What is internal energy?
The total kinetic energy of the random motion of the particles plus the total potential energy between them.
What is the difference between heat capacity and specific heat capacity?
Heat capacity is the energy to raise a whole object's temperature by 1°C. Specific heat capacity is the energy to raise 1 kg of a substance by 1°C.
Why does temperature stay constant during melting?
The energy supplied is used to overcome the forces between particles, increasing their potential energy rather than their kinetic energy, so the temperature does not rise until all of it has melted.
What is the difference between boiling and evaporation?
Boiling happens at the boiling point, throughout the liquid, with bubbles. Evaporation happens at any temperature, only at the surface, and cools the remaining liquid.
What does the flat part of a cooling curve show?
A change of state at constant temperature, while latent heat is being released.
Still mixing up specific heat capacity and latent heat?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author.