Change of state

Every particle of matter is doing two things at once: moving (because it has kinetic energy) and being pulled toward its neighbours (because of interparticle attraction). Which of these two effects wins decides whether a substance is solid, liquid or gas, and changing which one wins is exactly what a change of state is.

Heat energy tips the balance. When a substance absorbs heat, its particles gain kinetic energy and move faster. Faster, more energetic particles collide harder and more often, and increasingly overpower the pull of interparticle attraction trying to hold them in place. The practical result is that interparticle space grows and particles gain freedom of movement. When a substance loses heat, the opposite happens: particles slow down, attraction regains the upper hand, and particles are pulled into a smaller space with less freedom to move.

The same rearrangement, run in five directions. Because there are three states and heat can flow in or out, there are five named changes you must be able to explain in these terms.

• Melting (solid to liquid): heat absorbed; particles vibrating about fixed positions gain enough energy to partly break free of them; interparticle space increases modestly; particles gain the ability to slide past one another.
• Freezing (liquid to solid): heat released; particles lose energy and slow down; attraction locks them into fixed positions arranged in a regular pattern; interparticle space decreases.
• Boiling/vaporisation (liquid to gas): heat absorbed; particles gain enough energy to escape attraction almost entirely; interparticle space increases enormously; particles move independently and fill any available volume.
• Condensation (gas to liquid): heat released; fast, widely-spaced particles slow down enough for attraction to pull them back together; interparticle space decreases sharply.
• Sublimation (solid to gas, or gas to solid, directly): heat absorbed or released in one step, with particles jumping directly between the tightly packed solid arrangement and the widely spaced gas arrangement, skipping the liquid arrangement altogether — camphor and solid carbon dioxide are the classic ICSE examples.

Note what does not change: the identity of the particles. A water particle in ice, liquid water and steam is chemically the same throughout — only spacing, freedom to move and the strength of attraction it experiences change, which is why every change of state is a physical change, not a chemical one.

There is no formula to derive here — ICSE Class 8 keeps change of state at a reasoning level, without latent heat values, specific heat capacity, or exact melting/boiling temperatures. But "no formula" does not mean "no logic," and exam answers are graded on exactly this chain of cause and effect.

Step 1. Heat is a form of energy. When a substance absorbs heat, that energy does not vanish — it is transferred to the particles of the substance, and particles store extra energy as increased kinetic energy, meaning faster, more vigorous motion (vibration in a solid, sliding in a liquid, free flight in a gas).

Step 2. Interparticle attraction is a force trying to hold particles at their equilibrium spacing. It does not switch off; it is always present, but its effectiveness depends on how much kinetic energy the particles have to fight against it. Slow particles stay well within the reach of attraction; fast particles increasingly break free of it.

Step 3. As heating continues, more particles reach the energy needed to overcome the attraction holding them in their current state, so change happens over a range rather than as a single-instant switch; once enough particles cross that threshold, the substance as a whole has changed state. The reverse logic explains cooling: removing kinetic energy lets attraction pull particles into closer, more ordered arrangements — gas condenses to liquid, liquid freezes to solid — though freezing does not stop motion altogether, since particles still vibrate about their fixed positions.

Step 4. Different substances need different amounts of heat to tip this balance because their interparticle attraction is stronger or weaker to begin with. Strong attraction means more energy is needed to overcome it, so that substance changes state under more extreme conditions than one with weak attraction — even though the underlying mechanism, motion versus attraction, is identical for all of them.

Question: A beaker of water is heated on a stove from room temperature until it boils away completely as steam. Explain, stage by stage, what is happening to the water particles, and then explain why a lid placed over a cold glass held above the steam becomes coated with water droplets.

Step 1. At room temperature, the water particles are in the liquid state — moderate interparticle attraction, moderate spacing, free to slide past one another but still held close together, which is why liquid water keeps a fixed volume but takes the shape of its container.

Step 2. As the stove supplies heat, the particles absorb this energy as kinetic energy and move faster, colliding more often and more forcefully; the water's temperature rises steadily, and increasing numbers of particles gain enough energy to overcome the interparticle attraction holding them in the liquid arrangement.

Step 3. At boiling point, this overcoming is essentially complete throughout the liquid: interparticle attraction is almost entirely defeated and interparticle space increases enormously. Particles escape as widely spaced, freely moving gas particles — steam. Vaporisation continues at a steady temperature until all the water is gone, since heat at this stage goes into breaking attraction rather than raising temperature further (a fact you will meet with numbers, as latent heat, only in higher classes).

Step 4. The cold lid held above the steam offers a surface far cooler than the vapour. Fast-moving steam particles strike it, lose kinetic energy and slow down sharply; slower particles can no longer resist interparticle attraction, which pulls them back into the closely spaced liquid arrangement. This reappearance of liquid water is condensation — the reverse of vaporisation, driven entirely by loss of heat.

The water on the lid is the same substance as the water that started in the beaker — no new substance was formed at any stage; only spacing, attraction and freedom of motion changed.

Competitive stretch

A related exam favourite: solid carbon dioxide ("dry ice") disappears directly into gas at room temperature, with no puddle ever forming. By the same reasoning, its particles absorb enough heat to jump straight from the tightly packed solid arrangement to the widely spaced gas arrangement — sublimation — because the liquid arrangement is never a stable resting point for this substance at normal room conditions. The particles are unchanged; only the arrangement skips a step.

• Thinking a change of state creates a new substance. Melting, freezing, boiling, condensation and sublimation are all physical changes — the same particles rearrange; no new chemical substance appears. Contrast this with a chemical change such as burning, which does form new substances.
• Believing particles stop moving completely on freezing. Particles in a solid still vibrate about fixed positions; only their freedom to travel from place to place is lost, not their motion altogether.
• Assuming every substance changes state at the same conditions. The amount of heat and the conditions needed differ from substance to substance because interparticle attraction strengths differ — do not treat one substance's behaviour as a universal rule.
• Confusing sublimation with melting followed by boiling. Sublimation is a single, direct jump between solid and gas with no intermediate liquid stage — describing it as "first it melts, then it boils" is incorrect and will lose marks.

Ask the tutor: Want to work through a few more everyday scenarios — like why a wet cloth dries faster on a hot windy day, or why boiling water stays at the same temperature while it boils — using this same kinetic theory reasoning?