Represent the differences between solid, liquid, and gas phases using a particulate-level model.
The three phases differ in how much freedom the particles have, and the CED describes each explicitly:
Solids can be crystalline — particles in a regular 3-D structure — or amorphous, with no orderly arrangement (glass). In both cases individual particle motion is limited to vibration, and there is no overall translation relative to one another. Structure is influenced by interparticle interactions and by how well the particles can pack.
Liquids: particles are in close contact but continually move and collide. Their arrangement and motion depend on the nature and strength of the forces between them, including polarity, hydrogen bonding, and temperature.
A frequently tested consequence: the solid and liquid phases of a substance have similar molar volumes, because in both the particles are in constant close contact. The gas phase is hundreds of times larger in volume for the same amount of substance.
Gases: particles are in constant motion; their collision frequency and average spacing depend on temperature, pressure, and volume. Because of the constant motion and the minimal effect of interparticle forces, a gas has neither a definite volume nor a definite shape.
Drawing conventions that earn credit on particulate questions: solids in a regular touching array; liquids touching but disordered; gases sparse, randomly placed, with the same number of particles as before unless the amount actually changed.
A sealed rigid container holds 1.0 mol of water. Compare the volume the water occupies as ice at −10 °C, as liquid at 25 °C, and as steam at 150 °C, and explain the pattern at the particulate level.
Ice ≈ 19.7 mL; liquid ≈ 18.0 mL; steam ≈ 34 700 mL at 1 atm.
Ice and liquid water have similar molar volumes because in both phases the molecules remain in constant close contact — hydrogen bonding holds them at essentially fixed separations. Ice is in fact slightly larger because its hydrogen-bonded lattice is open, which is why ice floats; this is an unusual case, as most substances contract on freezing.
Steam occupies roughly 1900 times the volume because the molecules have enough kinetic energy to overcome the intermolecular attractions entirely. They are now separated by distances far larger than the molecules themselves, with mostly empty space in between, and the forces between them have almost no effect.