# Understanding of Kinetic theory of matter

1. Mar 18, 2007

### greenfloss

Is this correct? -

The density of substances decreases for the transition between solidà liquid, and liquid--> Gas (exception to this rule is for water, where the soli--> liquid transition causes the substance to contract instead of expand, hence increasing density)

[at higher pressures, the change is smaller for the general rule, but for water’s special case, lower pressures will make the change smaller]

2. Mar 18, 2007

### HallsofIvy

Staff Emeritus
Roughly speaking, yes, that is correct. The "temperature" of a material depends upon it internal molecular motion. The higher the temperature, the greater the molecular motion. In a solid, that motion is restricted by bonds between the molecules. Melting occurs when the molecular motion becomes great enough to break the molecular bonds. "Boiling" occurs when the molecular motion becomes great enough that molecules leave the liquid. Water, and a few other materials, form large crystals when the solidify so that the solid is less dense than the liquid form.

Kinetic theory also, by the way, explains "evaporation"- a liquid disappearing when well below "Boiling temperature". The temperature is based on the average molecular motion. There are always a few "outliers" that will have enough speed to escape the liquid. That, of course, reduces the average speed and so the temperature but then the environment raises it back to the "ambient temperature" so a few more molecules leave, ---.

3. Mar 18, 2007

Thank you :)