Phase Changes

A change of phase - or change of state of matter - is not really a chemical change, not a chemical reaction (although the difference is not always that clear).
When internally something happens with the particles of a substance (they really change), than you have got a chemicalreaction. But when those particles only change in position, vibration, energy etc, and inside they do not really change, than we prefer to talk about a physical change. Physical changes can be changes of phase.
Thus, particles in a lattice (a solid) can change structure in different ways:
  1. When melting (when heating a dry substance).
  2. When dissolving (always in a solvent like water)
When a substance with an ionic lattice melts, ((s) → (l)) or dissolves ((s) → (aq)), than the consequence is that the ions start to move freely.
Well known phase changes are:

(s) (l) melting, solidification, freezing
(l) (g) boiling or condensation
(l) (g) or (s) (g) evaporation
(g) (s) sublimation

It can be fun to look at some Youtube films about this topic. For example: the following

Phase changes are not always real chemical reactions; they belong to the physical phenomena. Mind that something like boiling is really something different from decomposition!!! People sometimes confuse these concepts.
There are also substances that, at heating, do nog melt of boil, but decompose, like sugar.

A couple of examples of phase changes:
  1. melting of ice: H2O(s) H2O(l)
  2. evaporation of gasoline: C8H18(l) C8H18(g)
  3. melting of Lead: Pb(s) Pb(l)