Sec 3 Physics Thermal Properties of Matter notes

The internal energy of a body is the grand total of its particles’ energies: the kinetic energy of their random motion (bigger when hotter) plus the potential energy stored between them as they attract and repel (bigger when they are pulled further apart). Heating raises internal energy — sometimes in the kinetic account (temperature rises), sometimes in the potential account (the substance changes state), often in both.

Keeping the two accounts separate explains everything in this chapter. A thermometer reads only the kinetic account. Energy that flows in while the temperature stays put (melting, boiling) must be going into the potential account — stretching the particles apart against their attractions. Cooling is the reverse bookkeeping.

The heat capacity of a body is the energy needed to raise the whole body by 1 °C. The specific heat capacity (c) normalises it: the energy needed to raise 1 kg of the substance by 1 °C, in J/(kg·°C). Rearranged into the working formula: E = mcΔθ — energy equals mass times specific heat capacity times the temperature change.

Water’s specific heat capacity, 4200 J/(kg·°C), is enormous for an everyday liquid: a kettle must supply 4200 J just to warm one kilogram by one degree. That is a feature, not a bug — it is why water cools engines, carries heat around central-heating systems, and keeps Singapore’s climate mild near the sea: a huge energy change buys only a small temperature change.

A pure substance melts and boils at fixed temperatures (its melting and boiling points). While ice melts at 0 °C, a thermometer in the ice-water mixture holds steady even though energy flows in: the energy is potential-account spending, pulling particles apart. The same hold applies at the boiling point: bubbling water stays at 100 °C until the last of it has boiled away.

Boiling and evaporation both turn liquid into vapour, but they are different processes. Boiling occurs throughout the liquid (bubbles form at the bottom), only at the boiling point, and needs an external energy supply. Evaporation happens only at the surface, at any temperature below the boiling point, and — crucially — takes its energy from the liquid itself, which is why evaporation cools what remains: sweating is the body’s own air-conditioning.

Latent heat is the energy absorbed or released during a change of state with no temperature change — "latent" because it hides from the thermometer. The specific latent heat (l) normalises it to 1 kg: E = ml. Two flavours matter: the specific latent heat of fusion (melting/solidifying; for water ≈ 3.34 × 10⁵ J/kg) and of vaporisation (boiling/condensing; for water ≈ 2.26 × 10⁶ J/kg). Remember: values like these are given in the question — the exam carries no data booklet, but it always supplies what it wants you to use.

In particle terms: melting must pull particles partly free of the lattice (potential energy up, kinetic account unchanged), and boiling must separate them almost completely — which is why vaporisation’s latent heat is several times fusion’s. Freezing and condensing run the film backwards and release exactly the same energies: steam scalds far worse than boiling water because each condensing kilogram hands over 2.26 × 10⁶ J on contact.

A cooling curve plots temperature against time as a hot substance cools. The signature shape: a steep fall while the liquid cools; a flat plateau at the freezing point while the liquid solidifies (the released latent heat exactly offsets the energy lost to the room); then a further fall as the solid cools. The plateau is the thermometer’s blind spot for latent heat — the freezing point can be read off it to the degree.

Heating runs the same graph backwards: a rise, a flat melting plateau (solid + liquid coexisting), another rise through the liquid stage, then a hold at the boiling point. Reading questions hand you such a curve and ask: What is the melting point? Where is the substance partly solid and partly liquid? Why is one section steeper? — the answers live in the slopes and plateaux.