[yulchick_90]

Hey 

Is the melting point of impure water the same/higher/lower than the melting point of pure water?

We did this lab, with snow..we added a scoop of salt at a time, mixed the snow with the salt and the tempreture droped with every scoop.. could someone explain why is that?

We did the same thing with antifreeze, the tempreture also droped..

Please explain why the drop in temp. occurs when salt is added? thanks a  lot

[Yggdrasil]

The answer has to do with entropy (the amount of disorder that a substance has).  The melting point of a substance is the temperature at which the solid and liquid form of a substance are at equilibrium.  From thermodynamics, we know that equilibrium corresponds to a state where ΔG = 0. 

Since ΔG = ΔH - TΔS, setting ΔG = 0 gives us the relation:

T_eq = ΔH/ΔS

So the melting point depends on the ΔH and ΔS of a process.

What happens when we add salt to pure water?  Adding salt to water increases its entropy, because it is now a mixture instead of a pure substance.  Since ice already has a lower entropy than water (because ice is ordered into a crystal lattice and water is not), increasing the entropy of water makes the difference between the entropy of ice and water greater (i.e. it increases ΔS).  According to the relation above, increasing ΔS should decrease T_eq.