vapor pressure

[osimsDDS]

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Is this correct for vapor pressure:

If you add solute that dissociates in solution the boiling point will increase and therefore the vapor pressure will DECREASE...???

is this correct...thanks

And also when does the vapor pressure increase then?

 

[Chga]

This is correct

 

[osimsDDS]

now what about an alloy solution of 2 metals, why is the melting point lower, wouldnt you expect it to be higher just like a solution...

I read this in kaplan and now iam TOTALLY confused...someone please explain thanks

 

[dleodyddlek]

My way of seeing this is

if you dissolve salt into water, boiling point goes up [due to lower vapor pressure, or tendency of water escaping] but freezing point goes down [salt all over the street when it snow 🙂 ]

samething for alloy. freezing/melting point goes down when more substrate is mixed. don't get confused about boiling and freezing point.

 

[Jake8]

It looks like you are equating melting point and boiling point. The two are not the same an impure solution should have a lower melting point and a higher boiling point.

edit: dleodyddlek beat me to it with a good analogy too

 

[Mamona]

now what about an alloy solution of 2 metals, why is the melting point lower, wouldnt you expect it to be higher just like a solution...

I read this in kaplan and now iam TOTALLY confused...someone please explain thanks

I am confuse as well. Does it have something to do with the phases, like in the first one vapor pressure is from gas-liquid equilibrium and the alloy solution is probably in the liquid-solid equilibrium????

Hopefully someone can help us....

 

[dleodyddlek]

I am confuse as well. Does it have something to do with the phases, like in the first one vapor pressure is from gas-liquid equilibrium and the alloy solution is probably in the liquid-solid equilibrium????

Hopefully someone can help us....

gas-liquid equilibrium is boiling/condensing point
liquid-solid equilibrium is melting/freezing point

be careful. two different story so dont get mixed up :]

how about memorizing this way [i don't know if it'd be misleading or something if so, let me know]

adding more substrate like dissolving salt in water or mixing metals to make alloy or whatever. it will expend its tendency of staying as liquid.


ex) from pure water

_________F_____________B__________
.....Solid...........Liquid................Gas.....

by adding salt or something

____F_________________________B____
Solid................Liquid.....................Gas..


arg.. it's not that clear but hope you get what i'm trying to say :]

 

[jdent]

Well for an alloy the crystal lattice is not as organized as for a pure metal. A pure metal with a nice lattice will take more energy to dissociate, while a alloy isn't arranged so well and will take less energy to dissociate.

 

[rebma]

I am confuse as well. Does it have something to do with the phases, like in the first one vapor pressure is from gas-liquid equilibrium and the alloy solution is probably in the liquid-solid equilibrium????

Hopefully someone can help us....

i was confused about this a while ago too, but i researched it and i'll try to explain what i understand.

water boils when its vapor pressure equals atmospheric pressure. so if you add a non-volatile solute to a volatile solute, less volatile molecules on the surface of the liquid are coming in and out of the liquid phase. therefore, the solution's vapor pressure decreases. if the vapor pressure decreases it will take a lot more heat to bring it's v.pressure to boiling- so boiling point increases.


as for freezing point depression- when u add a solute to a solvent or mix two different compounds, it would take a longer time for the solution to freeze because of the interactions between the compounds or because their sizes are different making it harder for the solution to become compact. --- not too sure about this one