Melting Point and Impurities

[shefv]

Why does the melting point always decrease due to impurities?

Can we not expect the impurity to pull the melting point towards its own mp? I mean that if an impurity is a lot larger and makes stronger intermolecular and intramolecular bonds than the substances itself, wouldn't the melting point of the impure compound be larger than the mp of the pure substance? I am confused about why does it always decrease?

I understand that the impurity disrupts the lattice structure of the pure compound but aren't there new bonds forming which are always stronger than the previously existing bonds? This makes me think that the melting point would then deviate towards the melting point of the impurity.

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[NextStepTutor_1]

The melting point decreases due to impurities because the bonds that the solvent makes with the added solute are stronger than the bonds between molecules within the solution. This is why the boiling point increases as well. A good way to remember this is that solutes/impurities increase the tendency for a liquid to remain a liquid.
Let's put a face on this with an example.
Salt dissolved into water (and this is key, if the solute doesn't dissolve then it won't have any affect), the boiling point increases. Likewise, this is the reason salt is put on ice during the winters where roads get icy. The ice decreases the melting point of the water it dissolves into and thus prevents the road from becoming as icy as it would without the salt.

We typically think of the boiling point elevation or melting point depression in terms of the solvent, or substance that's already a liquid, and thus when stronger bonds are made with the dissolved solute, we can say the bp increases and mp decreases of the solvent.

 

[Labrat07]

@NextStepTutor_1
Your explanation made a lot of sense to me but your example of Ice with melting is a bit strange. It explain the theory but it doesn't flow. In term of Ice and Salt, I'd think Freezing point, salt and Ice. Adding salt (impurities in there) will lower the freezing point (opposite of boiling point). It'll be harder to freeze = harder to be icy. Not trying to be a smart ass. Just discussing so I can understand deeper.

 

[Cawolf]

@Labrat07

Freezing point and melting point are the same thing.

 

[NextStepTutor_1]

^ Bingo. This is sometimes not directly stated, but mp and fp are the exact same number and refer to the exact same phenomenon (just differing in the direction of said phenomenon, i.e. liquid to solid, or solid to liquid).

 

[Labrat07]

I got it now. For odd reasons, I alway thought freezing is liquid to ice, melting is solid to liquid and limit to that. Make sense in real life logically but not quite true.

Thank you, both.

 

[Cawolf]

Your definitions are correct @Labrat07. It's just that the temperature at which they are occur is the same, they are simply phase changes in opposite directions. Colligative properties alter these points, not specific phase changes.