A colligative property is governed by a law that does not depend on the nature of the solute, but only the nature of the solvent and the number of (moles) of particles (ions or molecules) of solute.
→ Cryoscopic temperature decrease → Ebullioscopic temperature increase → Osmotic pressure
An ionic substance releases by "molecule" 2,3 or more ions. A fraction of the ions are not dissociated in the solution such that the effective number of particles is still less than 2,3 ..: If $n$ is the number of ions that can be produced and $\alpha$ is the degree of dissociation (The ratio of the number of "molecules" dissociated and the number of "molecules" present before dissociation) then: For one "molecule" initially present, there remain $1-\alpha$ "molecule" undissociated and there will be $n\alpha $ ion produced, so we will have in all: $i$ $=$ $1-\alpha+n\alpha$ $=$ $1+\alpha(n-1)$ particles!
For one "molecule" initially present, there will be at all after dissociation: $i = 1 + \alpha (n-1) $ (Van't Hoff´s factor) particles (ions and undissociated molecules) $ \alpha $ = the degree of dissociation (= $\frac{dissociated \;number} {total\;number}) $ $ n $ = the number of ions
The solution $\mu = 0.050$ iron(III) chloride $FeCl_3$ has $ i = 3.4 $ This means that - Per mole of $FeCl_3 $ introduced, there are $ 3.4 moles of particles (ions $Fe^{3 +}$, $Cl^-$ and undissociated "molecules" $FeCl_3)$ in this solution. - The degree of dissociation is: $\alpha = \frac{3.4 - 1} {3} = 0.80$, thus - $ 80 \% $ of "molecules" are dissociated.