Melting point/Boiling point confusion

[Ultimeaciax]

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For colligative properties, boiling point goes up when the solution has impurities. Thus, the melting/freezing point goes down. Does it mean that it's easier to melt or harder to melt? My guess would be easier to melt but harder to freeze?

For a solution containing exclusively pure stable molecules that experience high level of intermolecular interactions, why does it have a lower bp, higher mp than solution that has impurities or less stable molecules? Wouldn't stable = harder to boil = less likely to react at room temp?

This easier/harder to freeze/melt is hurting my head.

 

[kasho11]

Boiling point elevation, freezing point depression. For purposes of the MCAT it is safe to assume impurities will result in these two situations. It's gets more complicated in real life because the interactions between pure A and impurity B may be weaker or stronger than A on its own, so they could actually either raise or lower the MP or FP.

The best analogy I've found is to think of the impurities as anchors for melting point. The impurities interact with the pure compound and keep it anchored to that phase. Meaning it will be more difficult for the pure molecules to gain enough energy to escape from liquid into gas phase because they are being tethered by these additional interactions.

Likewise but slightly different for freezing/melting point depression, solids have a highly ordered structure to them and any impurities you introduce into a liquid solution that are trying to freeze will disrupt the formation of this ordered structure and anchor the molecules to the liquid phase. They will require additional energy to overcome this interference and form their desired ordered solid structure.


On a similar note I am still unsure of how vapor pressure depression works. So a liquid boils when its vapor pressure equals atmospheric pressure .... but how does a solution's vapor pressure change because of changes in applied temperature and pressure? For example let's say a solution has a vapor pressure of 500 torr, if you could lower the atmospheric pressure to 500 it would boil, but let's say it's at 760, how does increasing temperature affect the solution.. what is actually happening with the molecules as you heat them up in terms of vapor pressure.

 

[MedPR]

For colligative properties, boiling point goes up when the solution has impurities. Thus, the melting/freezing point goes down. Does it mean that it's easier to melt or harder to melt? My guess would be easier to melt but harder to freeze?

For a solution containing exclusively pure stable molecules that experience high level of intermolecular interactions, why does it have a lower bp, higher mp than solution that has impurities or less stable molecules? Wouldn't stable = harder to boil = less likely to react at room temp?

This easier/harder to freeze/melt is hurting my head.

BP elevation: It is harder to boil because the impurities strengthen the intermolecular bonds (solvent-solute > solvent-solvent). This is why you salt the water you boil your eggs in.. Not to flavor your eggs (shells are on, no salt gets onto the part you actually eat) but to raise the BP of the water.

FP depression: It is easier to MELT because the impurities disrupt lattice formation. It's the reason why you salt the roads before and during a big winter storm. Since freezing and melting are just the opposite process (heat of fusion = melting), the energy change is the same, but with opposite sign. So if you make something easier to melt, you also make it HARDER to freeze because now you have to lower the temperature more.

 

[SN2ed]

Thread closed. These types of questions belong in the MCAT Study Q&A forum.