The depression in freezing point of water observed for the same amount of acetic acid, trichloroacetic acid and trifluoroacetic acid increases in the order given above. Explain briefly.

 Depression in freezing point is a colligative property that depends on the number of solute particles in a solution. Since the order of depression is 

Acetic acid < Trichloroacetic acid < Trifluoroacetic acid

It means the number of ions produced in the solution increases in the same order. 

This increase in ionization relates directly to the acidic strength of each acid: 

Trifluoroacetic acid (CF3COOH) has the strongest electron-withdrawing inductive effect due to the highly electronegative fluorine atoms, making it the strongest acid. It ionizes to the largest extent in water, producing the highest number of ions. 
Trichloroacetic acid (CCl3COOH) is weaker than the trifluoro derivative because chlorine is less electronegative than fluorine, resulting in a moderate degree of ionization. 
Acetic acid (CH3COOH) is the weakest acid because the methyl group (CH3) is electron-donating effect, which hinders ionization. It produces the fewest ions.

Because a greater degree of dissociation results in more solute particles, the depression in freezing point is maximum for trifluoroacetic acid and minimum for acetic acid.