Melting point & Boiling point

1. The LATTICE is the most important factor in determining a melting point.

How strong is a lattice? How easily can it be molten?
Melting means that the lattice is destroyed, and it costs a lot of energy to destroy a strong lattice.

The strength of an ionic lattice depends on the charges of the ions and of their mutual distances.
In the lattice of CaO (with charges 2+ and 2-) wil melt more difficultly than the lattice of NaCl (with charges 1+ and 1-). Apart from that, also the ions of CaO are smaller (so: closer toghether)
Small ions cause stronger lattices. They are closer toghether
Salts, in general, will have high melting points.

Also metalic lattices depend on charges and distances.
Between the metals exist rather some differences: in general a metalic lattice will be strong (high melting point), but ther are exeptions:
Mercury(l) is at normal temperatures a liquid. Lead, Tin, Lithium, Sodium, Potassium have no strong lattice. They melt easily.
Extremely strong are the lattices of: Chromium, Wolfframium and Vanadium. Also see table V)
2. VANDERWAALS-FORCES are the second factor in determining the melting points.

They exist in particular in molecular lattices, of which the strength depends on:
  1. the participation of Hydrogen bridges
  2. the participation of polar atoms (with δ- and δ+)
  3. the molecular masses. Big molecules cause higher melting points than smaller ones.
Substances with molecular lattices do not have high melting points.

So: melting points depend on the strength of the lattice:
  1. First check the presence of charge related forces; how big are the charges and the ionic rays?
  2. Check then the vanderwaals forces, they depend on the molecular masses.

Boiling points depend (in order of importance) on:
  1. Charge related forces between the mutual particles (are they ions or dipole molecules?)
  2. VanderWaals forces; so compare the M(olecular masses).
  3. The threedimensional shape of the particles:
    The more spherical a molecule is, the smaller is its surface, the smaller are the mutual attraction forces, and the lower the boiling point.
    Spherical particles escape more easily (this will boil more easily).

Gaseous substances at room temperature, in general, have no or little attraction forces between the particles. They are not polar and have no Hydrogen bridges.

Phase changes are not considered as chemical reactions. But mind: it is not always that easy to distinguish between chemical and physical processes.
There are substances that never reach a melting or boiling point. Long before the come to melting or boiling, the decompose (and that certainly is a chemical process).

Watch carefully the film on Youtube about heating of sugar,