[ygao85]

Why is the freezing and boiling pt of a solution higher than those of a pure substance?

Thx for the help.

[Grumples]

Actually, the freezing point is lowered, and the boiling point is raised (usually).  They are, in fact, completely unrelated, however.

FP:
The freezing point of a substance is lowered when a solute is added because the solute disrupts the crystalisation process of the solvent (and vise-versa).  To effectively crystalize, the solvent particles must actually "push" the solute particles out of the way (if you ever drink frozen juice concentrate, look at the top of the can- there will be a syrupy, sugary ooze where all the sugar got pushed out of the ice into). Because it requires more energy to do this, you need to lower the temperature.

BP:
The definition of the term "boiling point" is when the atmospheric pressure = the vapor pressure of the substance.  VP in turn is connected to intermolecular forces: the stronger they are, the lower the VP.  When you add a solute (typically, although not always) you raise the amount of bonding forces in the solution; these additional forces make it harder for the solvent particles to escape (as gas), so less gas is released, and so the VP is less.  because the VP is lowered, it requires more energy (i.e. heat) to bring the VP up to 1 atmosphere. There is one caveat to this: if you add a higher VP liquid to a lower VP liquid (i.e. dissolve methanol to water) then the VP of the solution will increase, because two different substances are vaporizing (and the new substance, methanol, has a higher VP than water).  However, the VP of the water alone will, indeed, decrease.

[ksr985]

To find out exactly why the vapour pressure changes, when an impurity is added to a substance, i suggest you read up on  chemical potential, phase equilibria, and colligative properties.

[jdurg]

Another way to think about the BP elevation is that when you dissolve something in water, that substance dissolves because the water molecules are able to hydrate the substance that is dissolving.  That is, for every particle of solute that dissolves there are numerous water molecules around that solute which are attracted to that solute.  In order to get the water to boil, you have to not only break the intermolecular bonding beween the water molecules, but you also have to break the intermolecular bonding between the water molecules and the solute.  As a result, the boiling point increases.  This is easy to see if you dissolve a salt in some water and some sugar in another.  You'll notice that the salt water takes longer to boil because the intermolecular forces attracting the water molecules to the salt ions are much stronger than those attracting the water molecules to the sugar molecules.

[arnyk]

So generally, the stronger the intermolecular forces, the higher the BP.

[jdurg]

Yes!  It's pretty logical as well.  If something is strongly attracted to something else, it's not going to want to break apart.  Thinking in that manner really helps you understand the strength of certain intermolecular forces.  Gases like hydrogen and helium are held together by very weak Van der Waals forces.  As a result, their boiling points are very, very, very, very low.  Then we have things like phosphorus which have much stronger intermolecular forces so the atoms are held together much more tightly.  As a result, the boiling point is a bit higher.  Now take a look at ionic solids.  They are held together by ionic bonding, so their 'intermolecular' forces are even higher.  As a result, most ionic solids have a high melting/boiling point.  (Then again, I'm not even sure if an ionic substance can boil.  I'd have to look that up).  Now take a look at metals.  They are held together via metallic bonding, and as a result most of them have VERY high melting and VERY high boiling points.  This is all attributed to the strength of the intermolecular bonding.

[arnyk]

Also with hydrocarbons, alkanes will have a higher BP than alkenes (with the same # of carbons).  Since the single bonds in alkanes allow the chains to "tangle up" with each other, this creates even stronger intermolecular forces.  The double bonds in alkenes actually weakens the forces as the section of the chain without the double bond will move towards another molecule (let's say to its right side) and thus it is moving even farther away from the one on its left weakening the entire system overall.  Think about it in terms of saturated and unsaturated  fats, the unsaturated (akenes, alkynes) are liquid at room temperate because the intermolecular forces are not strong enough to form the solid.  Saturated fats (alkanes) on the other hand are solid at room temperature because of the strength of their IMF's.  The forces between the hydrogens and carbons I think are also Van der Waals.

[xiankai]

arent u reffering to the bonds in hydrocarbons and not those between their molecules? come to think of it, what are internal bonds called.

i was wondering, is the attraction between the molecules of ionic compounds electrostatic? because in liquid/aq. solution, they are dissociated into their respective ions, so that means their electrostatic IMF were overcome. the electrostatic bonds between the molecules of ionic compounds always seemed internal and not external to me :/

[arnyk]

In ionic compounds the electrostatic forces act both intermolecularly and intramolecularly.  Obviously the individual molecule is held together electrostaticly, but the compound itself (for example in a crystal lattice) is held together by the electrostatic differences between different molecules.  Like the Na+ from one molecule is attracted to the Cl- from another, which is what forms the lattice.

Although in solution they are dissociated, I believe that the ions still randomly form the compound for a "split second" before being broken up by the solvent.  The reason the BP is generally higher in solution is because, "In order to get the water to boil, you have to not only break the intermolecular bonding beween the water molecules, but you also have to break the intermolecular bonding between the water molecules and the solute" (jdurg 4).  Haha, so much for plagerism.