Phase Transitions

Home Phases of Water Melting Evaporation Boiling Humidity

Matter exists in different states.  It can, for example, be in a solid state, a liquid state, or a gaseous state.  These states are called phases.  The atoms and molecules making up the matter move differently in different phases.

  • In solid the atoms vibrate about an equilibrium positions but are unable to flow or diffuse.  The intermolecular forces are very strong, and impulsive forces from collisions with thermal-energy atoms are on the average, much weaker.

  • In liquids the atoms or molecules can flow past each other, but they maintain close contact.  Some ordering persists over the range of a few molecular diameters.  The intermolecular forces are not negligible compared to the impulsive forces from collisions with thermal-energy atoms and molecules.

  • In gases the atoms or molecules move freely.  Their average spacing is much larger than the atomic or molecular diameter and the intermolecular force become negligible compared to the impulsive forces from collisions with thermal-energy atoms and molecules.  The behavior of most gases is well described by the ideal gas law, PV = NkT.

Solid Liquid Gas
 

In general, heat flows from an object of higher temperature to an object of lower temperature.  Temperature is the quantity that indicates whether or not heat will flow and in what direction it will flow.  When heat flows from a hotter to a cooler object, the temperature of the hotter object decreases, while the temperature of the cooler object rises.  The average kinetic energy of the molecules in the hotter object decreases, while the average kinetic energy of the molecules in the cooler object increases.  But when an object changes phase, its temperature does not change, even though heat is added or removed.  The melting of ice and the boiling of water are familiar examples.  During a change of phase the temperature does not change, but the internal energy does.  The internal energy is the sum of the kinetic energy of the molecules and the chemical potential energy of the molecules.  During a change of phase, the average kinetic energy of the molecules stays the same, but the average potential energy changes.

  • The processes represented by the red arrows require energy input.

 

  • The processes represented by the black arrows release energy.