What cause changing of melting point more?

Question

Can someone plz tell me that is adding covalent bond compound or ionic bond compound will decrease the melting point more??

I want to do a lab that is about ice melting, and i'm adding salt, sugar, and ingredients on the ice to see which will cause the ice to melt faster. What my teacher said is that the answer is in the structure of the ionic and covalent compound, but i don't know why, would some one explain it to me?? thanks

Answer 1

That depends among others on solubility of the compound in water, but generally a compound which dissociates produces more moles of dissolved molecules or ions than a compound which stays inert.

So, if the compound dissociates into ions, it would be an ionic bond compound. However, adding an acid molecule which has only covalent bonds that dissociates upon dissolution, or an ionic compound which doesn't dissociate (or dissolve, e.g. chalk soap) shows that only the nature of the dissolved phase matters, not the nature of the compound before adding it.

Answer 2

Further info. Sugar dissolves in water giving only sugar molecules. Salt (NaCl) dissolves giving Na+ and Cl- ions = two ions per molecule. CaCl2 dissolves giving Ca++ plus two Cl- ions = three ions per molecule. Although other factors are involved, the more "bits" the solute molecule becomes, the greater the depression of freezing (= melting) point.