Revision summary

Connect the property to the particle model

Solid

Particles are close and vibrate near fixed positions. Forces keep them near those positions, giving a fixed shape and volume under ordinary conditions. Close spacing makes compression difficult.

Liquid

Particles are close and irregularly arranged; attractions keep them close while they move past one another. The liquid flows and changes shape but keeps nearly the same volume and is difficult to compress.

Gas

Widely separated particles move randomly through the available volume. Attractions are negligible for most of the time between collisions. Large spaces make compression easy compared with liquids and solids.

Particles may be atoms or molecules. A crystalline model's regular arrangement is not a claim about every solid. Gas expansion changes particle spacing, not the size of each molecule.

Temperature and measurement

Temperature symbols include θ (theta) and T, commonly used for Celsius and kelvin temperatures respectively. State the unit, °C or K; a symbol or number alone does not give a complete temperature reading.

A temperature rise means increased average random kinetic energy of all the particles. Individual speeds remain varied; solid particles vibrate. Ratios of Celsius readings are not particle-energy ratios.

Choose a thermometer with a suitable range and resolution. Immerse its sensing region correctly, keep it clear of the vessel, allow for response time and read an analogue scale at eye level. Repetition does not correct poor sensor placement or response lag.

Combined: internal energy

Internal energy contains the total kinetic energy of random particle motion and the total potential energy between particles. Temperature relates to an average, not that total. More of the same material can have more total random kinetic energy at the same temperature. Motion of the whole body is a separate kinetic store.

Combined: four changes of state

• Melting: solid to liquid, energy enters.
• Solidification: liquid to solid, energy leaves.
• Boiling: liquid to gas, energy enters.
• Condensation: gas to liquid, energy leaves.

For a pure substance undergoing the stated transition at fixed pressure, temperature stays constant while the proportions in each state change. The potential-energy contribution changes while average random kinetic energy remains unchanged. Particles keep moving, and energy transfer continues during the plateau.