Question

Which has the lowest freezing point?

Answer 1

Pure water has the lowest freezing point among the others.

Answer 2

Answer: 0.5 ionic $AlCl_3$

Explanation: Depression in freezing point takes place when a non volatile solute is added to the solvent.

Pure water freezes at 0 degree C. If a non volatile solute is added to water then water freezes below 0 degree C.

The equation used to calculate depression in freezing point is:

$\Delta T_b=imk_f$

where, $\Delta T_b$ is the depression in freezing point,

i is the Van't hoff factor,

m is molality and $k_f$ is the freezing point depression constant.

We have aqueous solutions for all the compounds so $k_f$ value will be same. Molality is also same for all of them.

So, the only deciding factor is the Van't hoff factor.

sodium chloride gives one sodium and one chloride ion and so the value of i = 2

Calcium chloride gives one calcium and two chloride ions and so the value of i = 3

Aluminium chloride breaks to give one aluminium ion and three chloride ions and so the value of i = 4

Last one is a molecular compound that does not give ions and so the value of i = 1

The highest value of i is for aluminium chloride and so it has lowest freezing point.