Solution , Solubility and Colligative Property?
Let's say we have some ice.



Beautiful structure. Ice stays solid at some temperatures because of its neat, tight lattice. Ice stacks this over and over until there's no more ice left to stack.

If I were to put a lot of ions between this stack, the stack might bend or even break; it isn't as reinforcing as it once was. The stack isn't as durable anymore, so we would expect it to melt a lot easier - there's too much structural stress for it to be a solid.

The freezing point of solutions depends, at least in this case, on how much dissociation occurs. If we can push more ions into solution, it's going to have a harder time making a sturdy lattice - a harder time freezing.

$\ce{C6H6O6 -&gt; C6H6O6}$ It doesn't dissociate at all.

I'm not sure how much urea dissociates, but I assume it would be like glucose.

$\ce{NaCl -&gt; Na+ + Cl-}$ It dissociates into 2 ions.

$\ce{K2SO4 -&gt; K+ + K+ + SO4^{2-}}$ It dissociates into 3 ions.

$\ce{K2SO4}$ dissociates the most, so it's freezing point is lowest. Hope this helps!