Solids, liquids and gases

In a liquid the particles are still touching — only randomly arranged instead of following the regular pattern of a solid. Drawing them with visible gaps turns the diagram into a gas. A gas box should hold only a few particles, about seven at most, widely and randomly spaced; every particle must be drawn a consistent size, and a solid box need not be filled edge to edge.

While a pure substance melts or boils, its temperature stays constant even though heating continues; while it freezes, a cooling curve holds flat, not falling smoothly. The confusion is temperature (average kinetic energy of the particles) against thermal energy (what is transferred in or out). Energy keeps flowing throughout; the two are not the same quantity.

Boiling overcomes only the weaker forces between molecules; the bonds within each molecule are untouched. Boiling water gives steam, not hydrogen and oxygen — that would be a chemical change, and a change of state is a physical one. The substance leaves the beaker with exactly the formula it went in with, which is why only the state symbol changes in the equation.

An explain question on gas pressure needs every link: higher temperature, more kinetic energy, collisions with the walls that are both more frequent and more forceful, then a greater force per unit area. Stopping at "more energy" or "they move faster" leaves the two markable links unwritten, and those are exactly the two examiners report as missing.

Boiling happens throughout the liquid, only at the fixed boiling point, with bubbles inside it. Evaporating happens at the surface alone, at any temperature below the boiling point. A puddle drying on a warm day without bubbling is evaporating; calling it boiling that has not got hot enough scores nothing — the two differ in location as well as temperature.

Saying a liquid flows, or a gas fills its container, describes the bulk substance rather than its particles. When the question names separation, arrangement or motion, all three ideas must appear in particle language. Bulk properties are a consequence of the particle picture and earn credit as an extra sentence, never as a substitute for it.

Increasing the temperature increases a gas volume; increasing the pressure decreases it. Under time pressure the pair gets reversed. Anchor it physically: heating makes particles need more room, squeezing pushes them closer together. Check the stated constant too — the temperature effect assumes constant pressure, and vice versa.