Definitions

Melting point

The melting point of a substance is 'the temperature at which the two states, liquid and solid, co-exist in equilibrium'. Or to put it into plain terms, the temperature at which something melts.

The melting point is actually a very important physical property, as it may be used to ascertain the degree of purity of a substance. Pure substances have sharp, well defined melting points, whereas the addition of impurity both lowers and broadens the melting temperature.

Unike boiling points, the melting point remains virtually unaffected by the external air pressure.

Boiling point

The boiling point of a liquid is the temperature at which the vapour pressure of the liquid equals the atmospheric pressure. The atmospheric pressure is variable according to the elevation above sea level and the weather conditions. For this reason boiling points need to be measured at a specific atmospheric pressure. This is 1 atmosphere pressure, 1 x 10⁵ Nm^-2.

The boiling point is a measure of the strength of the interparticular forces within the body of a liquid. It is a more convenient comparison for many volatile substances, as their melting points are often too low.

---

^ top

Melting temperature

The change of state from solid to liquid takes place when the available energy is sufficient to literally shake the structure apart. In all solids the particles are held together by forces (as we should well know by now!). As energy is given to the particles, they vibrate more about their fixed positions in the solid, until eventually the vibrations are strong enough to tear them from these fixed positions. At this temperature the integrity of the solid structure breaks apart and the solid turns to liquid.

It is appropriate at this point to remind ourselves of the particular chacteristics of solids, liquids and gases in terms of motion and interparticular distance and bulk (real world scale) properties, in terms of shape and volume.

The melting point of a metal may be used to assess the strength of its interparticular forces. The melting point is a function of the ionic charge on the metal ions in the lattice and their size (radius).

• High charged ions = high melting points
• Small sized ions = high melting points

Example: Explain the difference in melting points between the values for sodium (98ºC) and magnesium (667ºC). Sodium from group I forms ions with a single plus charge. Each atom only provides 1 electron to the delocalised electron cloud, responsible for holding the ions together. Magnesium however forms ions with a 2+ charge, each atom providing two electrons to the charge cloud that holds the ions together. An additional factor increasing the lattice strength of magnesium, is the smaller size of its ion compared to that of sodium. The magnesium 2+ ion has a higher charge density and a greater attractive force for the delocalised electron charge cloud.

There is also a third, more unpredictable factor, which is the actual packing of the metal atoms. Different metals may use one of several different packing schemes, or even a mixture of more than one.

The strength of the metallic lattice depends on the charge on the ions within the lattice (as each metal atom provides the same number of delocalised electrons to the sea of electrons as the charge on the ion).

group 1		group 2
Li	180	Be	1287
Na	98	Mg	650
K	63	Ca	842

You can see that group 2 elements in general have much higher melting points associated with much stronger forces of attraction within the metal lattice. The anomalous value for magnesium in group 2 is due to the crystal structures of magnesium and calcium.

Metallic bonding and properties

---

^ top