Three states: arrangement, separation, motion
Describe each state by three properties. Solid: regular lattice, very close/touching, vibrate about fixed positions — fixed shape and volume. Liquid: irregular, touching (as close as a solid), flow past one another — fixed volume, no fixed shape. Gas: random, widely spaced/far apart, rapid random motion changing speed and direction on each collision — no fixed shape or volume.
Internal energy vs temperature
Internal energy = total KE + total PE of all particles. Heating usually raises the average KE → temperature rises; temperature measures the average KE only, not the total. During a change of state the temperature holds constant because the energy raises the particles' PE (weakening/breaking forces), not their KE. Kelvin ∝ average KE, 0 K = absolute zero; convert with .
Brownian motion is evidence for particles
Brownian motion (Robert Brown, 1827): visible particles (pollen, smoke) suspended in a fluid move in a random zig-zag path. Explanation: tiny, fast-moving, invisible fluid molecules bombard the visible particle unequally from all sides, so the net force keeps changing direction. It is evidence that fluids are made of small, fast particles. State that the molecules' speed and direction change on each collision — that is the source of the randomness.
Drawn from real examiner reports.
Gas: state motion AND separation
A 2-mark "describe the particles in a gas" needs BOTH: (1) they move rapidly in random directions, AND (2) they are widely spaced / far apart. "Random motion" alone scores 1; "far apart" alone scores 1. Give both the separation and the motion to earn both marks.
June 2024 Paper 2P Q8(c)(i) — majority of candidates gained the random-motion mark; fewer gained the second mark for particle separation
Boiling vs evaporating: bulk vs surface
The mark scheme distinguishes these by WHERE they happen, not speed. Boiling occurs throughout the bulk of the liquid (bubbles form below the surface) and only at the boiling point. Evaporating occurs at the surface only and at any temperature. "Boiling is faster" or "boiling needs more energy" earn nothing — state the location.
June 2024 Paper 2P Q8(c)(ii) — the locational distinction (bulk vs surface) was the key mark that many candidates missed
Temperature is not internal energy
Temperature measures the average KE per particle; internal energy is the total KE + PE of ALL particles. A large iceberg at 0 °C holds more internal energy than a hot cup at 20 °C — far more particles — yet is colder. More particles at lower KE can still mean greater internal energy.
Energy is still absorbed while melting
During melting or boiling the temperature is constant, so students wrongly assume no energy is transferred. Energy is still absorbed — it raises the particles' potential energy, weakening or breaking the forces between them, not their KE. That is why a heating curve shows a flat plateau.
Say "kinetic energy", not just "energy"
When temperature rises, write that the kinetic energy of the particles increases — not merely "energy increases". Dropping the word "kinetic" loses the specificity mark, because temperature is defined as the average KE of the particles, not their PE or total energy.
Recurring across 4PH1 sittings — June 2024 Paper 2P Q8(b)(i) noted a minority omitted "kinetic" and wrote only "energy increases", losing the specificity mark
Liquid particles: not fixed, not far apart
Two liquid slips: calling the particles "fixed" or "vibrating in fixed positions" (that is a solid — liquid particles flow past one another), and calling them "far apart" (they are close together and touching, like a solid). Liquid particles are irregular, touching and free to move.
Give all three particle features
For any "describe the particles in a [state]" question, write one line each on arrangement, separation and motion. This guarantees the 2–3 marks and stops you omitting an element the mark scheme needs.
Annotate the heating-curve plateau
On a temperature–time graph, label both axes (quantity + unit), name each plateau (melting/boiling) at the correct temperature, and annotate it "energy raises PE, not KE, so temperature is constant". On the slopes write "KE increases → temperature rises".
Brownian motion: molecules do the hitting
Explain the visible particle's jerky path by the invisible molecules: they are small and fast, collide with it unequally from all sides, so the net force keeps changing direction. Do not say only "the particle moves randomly" — name the unequal bombardment by molecules.
Convert °C to K with +273
When a temperature is needed in kelvin, use (e.g. 27 °C = 300 K); apply +273 to each value. Absolute zero is 0 K, the point of minimum particle KE, and the Kelvin scale is proportional to average KE.
Define internal energy.
