States of Matter

Matter exists in three states: solid, liquid, and gas. The state of a substance depends on the temperature and pressure, and can be explained by the arrangement, motion, and energy of its particles.

Particle model

Property	Solid	Liquid	Gas
Arrangement	Regular pattern, closely packed	Irregular, closely packed	Random, widely spaced
Motion	Vibrate about fixed positions	Move around each other	Move rapidly in all directions
Forces between particles	Strong	Weaker than solid	Very weak (negligible)
Spacing	Very close	Close (slightly more than solid)	Far apart (about 10x the particle diameter)
Density	High	High (slightly less than solid)	Very low
Shape	Fixed	Takes shape of container	Fills container
Volume	Fixed	Fixed	Expands to fill container
Compressibility	Almost incompressible	Almost incompressible	Easily compressed

Changes of state

• Solid to liquid: melting (energy absorbed, temperature constant at melting point)
• Liquid to solid: freezing (energy released, temperature constant at freezing point)
• Liquid to gas: boiling (energy absorbed, temperature constant at boiling point) or evaporation (occurs at any temperature, only from the surface)
• Gas to liquid: condensation (energy released)
• Solid to gas: sublimation (energy absorbed, skips the liquid state)

Energy and changes of state

During a change of state, the temperature stays constant even though energy is being supplied (or removed). The energy goes into changing the potential energy of the particles (breaking or forming bonds), not their kinetic energy. This is why latent heat exists.

Brownian motion

Brownian motion (the random, jerky movement of smoke particles or pollen grains in a fluid) provides evidence for the particle model. The visible particles are being struck by invisible, fast-moving molecules in the fluid.