Melting: solid to liquid. Freezing: liquid to solid. Vaporization: liquid to gas on boiling. Evaporation: liquid to vapour at any temperature, from the surface. Condensation: gas to liquid. Sublimation: solid directly to gas. Deposition: gas directly to solid.
Thermal expansion: increase in size of matter on heating. Linear expansion: increase in length. Volume expansion: increase in volume.
Coefficient of linear expansion (α): fractional change in length per kelvin. Coefficient of volume expansion (β): fractional change in volume per kelvin. For solids β = 3α.
Real expansion of a liquid: expansion of the liquid itself. Apparent expansion: expansion seen when the container's expansion is ignored. Real = apparent + expansion of container.
Latent heat: heat absorbed or released during a change of state without any change in temperature. Latent heat of fusion: heat to change unit mass solid to liquid at melting point (water: 3.3 × 10⁵ J kg⁻¹, 336 J g⁻¹). Latent heat of vaporization: heat to change unit mass liquid to gas at boiling point (water: 2.26 × 10⁶ J kg⁻¹).
Superconductor: material with zero resistance below its critical temperature (near absolute zero).
Bimetallic strip: two different metals joined; bends on heating because one expands more.
Formulas
ΔL = α L₀ ΔT. ΔV = β V₀ ΔT. β = 3α.
Q = m Lf (melting), Q = m Lv (boiling). Boiling point of water 100 °C (212 °F, 373 K), melting point 0 °C.
Must-know points
Kinetic theory of expansion: on heating, particles vibrate with bigger amplitude and push each other apart, so the solid expands.
Why temperature stays constant during change of state: the supplied energy is used to break or weaken bonds between particles (potential energy), not to increase kinetic energy.
Three differences between boiling and evaporation: (1) boiling occurs at one fixed temperature, evaporation at any temperature; (2) boiling occurs throughout the liquid with bubbles, evaporation only at the surface; (3) boiling is fast and needs a heat source, evaporation is slow and cools the liquid.
Factors increasing evaporation: higher temperature, larger surface area, lower humidity, air movement over the surface.
Evaporation causes cooling: the fastest molecules escape, so the average kinetic energy (temperature) of the rest falls. Used in refrigerators (refrigerant cycling between liquid and gas, non-CFC).
Uses and effects of expansion: gaps in railway tracks and expansion joints in bridges and pavements (avoid bending and cracks); sagging cables; bimetallic strips in thermostats, fire alarms; fitting metal tyres on wheels.
Gas pressure: particles hit the container walls, force per unit area. In a tyre on a hot drive, particles move faster so pressure rises.
Superconductivity: no energy loss; limits: needs extremely low temperatures; uses: powerful magnets, MRI, maglev trains, lossless cables.
Past-paper questions with model answers
State three differences between boiling and evaporation (asked in every set): use the three above.
State three factors that affect the rate of evaporation: temperature, surface area, humidity, wind speed (any three).
What is linear expansion of solids? Why are gaps left between railway tracks (5 marks): define, give ΔL = αL₀ΔT, explain that tracks expand in summer and would bend without gaps.
Thermal expansion of liquids, real and apparent (5 marks): define both, explain with a flask filled with liquid (level first drops as flask expands, then rises), real = apparent + container.
Steel track 10 m, 15 °C to 30 °C, α = 1.2 × 10⁻⁵: ΔL = 1.2 × 10⁻⁵ × 10 × 15 = 1.8 × 10⁻³ m = 1.8 mm.
Heat to melt 15 kg ice: Q = 15 × 3.3 × 10⁵ = 4.95 × 10⁶ J.