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.

Pure: observation and inference

Illumination and a microscope reveal smoke specks moving irregularly. The specks are suspended particles, not individual air molecules. Changing unequal molecular collisions explain their movement and provide evidence for random molecular motion. A common directional drift alone may be fluid flow.

Pure: wall collisions and pressure

Gas particles collide with walls and exert forces. Their average force per unit wall area is pressure, acting on every wall. At fixed volume and particle number, higher temperature gives greater typical speeds, more frequent wall collisions and greater force contributions, so pressure rises.

At unchanged temperature, compressing a fixed quantity increases the wall-collision rate per unit area and raises pressure without requiring faster or smaller molecules. State the conditions before making a prediction.