1-Why is it that a solution of a material (say sugar) is water will raise the boiling point of the solution ?

2-Why does the the boiling point of the solution depends on the number of moles (of the material (eg sugar, salt ...)) ? (i see it must depend on the type of material, and the number of moles, but our textbook says that if you put 1 mole of ANY matterial (that does not get ionized and is in liquid or solid state) in 1 kg of water (as an example) then the boiling point of the solution will raise the same ammount no matter what is the matterial that you use !)

1)

As the substance itself will absorb energy, less energy will be avaliable to the water hence the boiling poinst of the solution will increase (i.e more energy needed to be provided for the same effect).

2)

Because, mol is the AMOUNT (whatever zillions of atoms) of the substance.

But i'm not sure about this, as i thought that different materials have different values for how much energy they would absorb for a given temperature change. Eg, every mole of copper will require less (or is it more?) energy for an increase of 1degree then say a mole of iron.

Then again, there may be something fiddly with energy equilibrium, heat capacities and molar ratios..

i'm sorry, but thats about the limit i can do without resorting to the textbooks (which i incidentally sold) and doing some test sums.. hmm... :/

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Originally posted by Jikx:
i'm not 100% sure about this.. but

1)

As the substance itself will absorb energy, less energy will be avaliable to the water hence the boiling poinst of the solution will increase (i.e more energy needed to be provided for the same effect).

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2)

Because, mol is the AMOUNT (whatever zillions of atoms) of the substance.

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just read thru my reply, realised it didn't make much sense and was probably wrong. Well, i am here to learn

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Originally posted by STAii:
Well the hydrogen bounds are re-made with the substance that you put in water instead of with water molecules themselves ...
And according to our textbook the BP depends only on the molality, and the substance that you are desolving IN.
So if you put 1 mole of sugar in water, or 1 mole of salt in water, you will get the same BP for both !
Any other explaination(s) ?

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Umm, each of the solute ions or molecules, as the case may be, is solvated by the water molecules, forming a cage like structure, called a clathrate, around the solute particle. There are hydrogen bonds formed between the solute and clathrae, between the clathrate and free water molecules, between the clathrate water molecules themselves, and between the free water molecules themselves. There's still plenty of hydrogen bonds between the free water molecules, otherwise you'd be talking about dissolving water in sugar instead of the other way around. In any case, dissolving materials in water changes the ionic strength of the solvent. And this depends on a property of the solute molecules known as "activity," and this would be drastically different between sugar and salt. What I'm saying is, I'm pretty sure your book is wrong. They don't spread sugar on snow to melt it.

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Originally posted by STAii:
When i said that the hydrogen bounds are re-made, i was talking only about the hydrogen bounds that have been broken.
I guess i understand your point, i will try to find some articles to proove that our book is wrong.
Thanks a lot.

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It's mostly wrong for compounds that associate or dissociate, that is, ionic compounds. For compounds that don't, like sugar, the effect on the ionic strength of the solution is small, and the boiling point elevation will depend on the molality.