# What causes freezing point depression on a molecular level?

Jul 27, 2017

On the particle level...........?

#### Explanation:

The simplest answer is $\text{solute-solvent}$ interaction. Especially when an ionic salt dissolves in aqueous solution, the individual ions are solvated by water molecules......

$N a C l \left(s\right) \stackrel{\text{water}}{\rightarrow} N {a}^{+} \left(a q\right) + C {l}^{-} \left(a q\right)$

The individual sodium and chloride are present as their $\text{aquated ions}$, which is what we mean by $\text{(aq)}$, probably we deal with ${\left[N a {\left(O {H}_{2}\right)}_{6}\right]}^{+}$ as the dissolved species; and these solute/solvent interactions cause measurable changes to the bulk properties of the solution: i.e. $\text{boiling point elevation}$; $\text{freezing point depression}$; $\text{changes in osmotic pressure}$.

The observed change in these properties is proportional to the number of particles in solution, i.e. the concentration of the solute particles.

If we had equimolar sodium chloride and calcium chloride, how would these so-called colligative properties compare?