The big idea: Everything is made of tiny particles (atoms or molecules) that are always moving and pulling on each other.
Internal energy is the total energy stored in those particles. It is made of two parts:
- their random kinetic energy (KE) — the energy of the particles jiggling and moving about (this sets the temperature); - their intermolecular potential energy (PE) — the energy stored in the forces between the particles (this depends on how far apart they are).
Internal energy — the definition: Internal energy = total random KE of the particles + total intermolecular PE of the particles.
Random KE = energy of the particles' motion. Intermolecular PE = energy stored in the bonds/forces holding particles together or apart.
For a real gas both parts count. (An ideal gas is a model with no forces between particles, so its internal energy is the random KE only.)
Spot it: Temperature tells you about the average random KE of the particles — not the PE.
So heating something up always raises its KE part; the PE part changes mainly when the state changes (solid ↔ liquid ↔ gas).
The particle model pictures matter as particles with random KE (motion) and intermolecular PE (forces between them). It explains the three states of matter and why solids are usually denser than liquids.
| State | Particle spacing | Particle motion | Shape & volume |
|---|---|---|---|
| Solid | Very close, in a fixed pattern | Vibrate about fixed positions | Fixed shape, fixed volume |
| Liquid | Close, but free to move past each other | Move/slide around | No fixed shape, fixed volume |
| Gas | Far apart | Move fast in all directions | No fixed shape, fills the container |
Why solids are denser than liquids: Density = mass packed into a volume. In a solid the particles are held in a tight, regular pattern — packed closer together than in the same liquid.
More particles in the same space → more mass per volume → higher density. So melting a solid usually makes it less dense (the particles spread out a little).
Density links a substance's mass to its volume. It is a simple quotient (one quantity divided by another), and it is given in the data booklet:
- density (kg m⁻³)
- mass (kg)
- volume (m³)
[Diagram: phys-formula-triangle] - Available in full study mode
Worked example — comparing solid and liquid density
200 g of solid wax fills 2.0 × 10⁻⁴ m³. When melted, the same 200 g fills 2.2 × 10⁻⁴ m³. Find the density in each state and say which is denser.
Solution
- Start with the given formula (mass in kg: 200 g = 0.20 kg):
- Solid wax — put in the numbers:
- Liquid wax — same mass, larger volume:
Final answer
The solid (1.0 × 10³ kg m⁻³) is denser than the liquid (9.1 × 10² kg m⁻³) — the particles are packed closer in the solid.
Memorize terms 3x faster
Smart flashcards show you cards right before you forget them. Perfect for definitions and key concepts.
How this is tested: This micro is tested as short explain / identify questions.
- Paper 1A: a quick MCQ — e.g. identify what makes up the internal energy of a real gas (random KE + intermolecular PE). - Paper 2: 'explain why most substances are denser as solids than as liquids', and reading a density–temperature graph for water to identify the temperature of maximum density.
Classic trap: saying internal energy is only the kinetic (temperature) part, and forgetting the intermolecular PE.
Water is the odd one out: Most liquids get steadily denser as they cool. Water does not.
Water is densest at about 4 °C. Cool it further and it gets less dense, and ice is less dense still — so ice floats.
IB-style question — temperature of maximum density
The graph shows how the density of a sample of water changes as it is warmed from 0 °C. From the graph, identify the temperature at which the water has its maximum density, and state what this tells you about ice compared with liquid water.
Solution
- Read the temperature at the highest point of the curve (the densest water):
- Below 4 °C the density falls again, and solid ice is less dense still — so ice is less dense than liquid water and floats on it.
Final answer
Maximum density is at about 4 °C. Because water is less dense below 4 °C (and ice less dense still), ice floats on water.