why salt elevate boiling point

[boomboom2dolla]

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Can anybody explain how adding salt will increase the bp?
Kaplan says when vapor pressure of a solution is less than vapor pressure of pure solvent a higher temperatur is needed for vapor pressure to equal atmorspheric pressure.


So a salt/water solution has less vapor pressure than water by itself. So it needs a higher temperature to boil. why?

 

[doc3232]

stronger interactions in the solution. Less favorable to lose them if the water is dried out and only salt is left.
Entropy is lost also.

 

[PooyaH]

when salt is added to water, an endothermic reaction occurs, where Na+ and Cl- ions interact with water molecules. As doc3232 mentioned since there are stronger interactions, the boiling point increases as the vapor pressure decreases. Also know that the freezing point of water would decrease as well. Since those ions interfere with the intramolecular forces between the water molecules the freezing point is decreased. When vapor pressure decreases, so does the freezing point!

 

[nze82]

The way I would answer this question is as follows...
When salt dissolves in water, or any other solution for that mater, the vapor pressure of that solution drops compared pure solution.
Also, by definition, we know that solution boils when:
Vapor Pressure = Atmospheric Pressure


If dissolving salt lowers the vapor pressure, and we want to bring up the vapor pressure up, so that it equals the atmospheric pressure for the solution to boil, then we must provide more heat (NOTE: vapor pressure only depents on temprature). This results in an elavated boiling point.

 

[faerielynx]

stronger interactions in the solution. Less favorable to lose them if the water is dried out and only salt is left.
Entropy is lost also.

hmmm... i thought if you mix two solutions together it would increase entropy... but then that would cause weak interactions... can you explain the entropy part... 🙁 i get everything else.

 

[doc3232]

hmmm... i thought if you mix two solutions together it would increase entropy... but then that would cause weak interactions... can you explain the entropy part... 🙁 i get everything else.

I am saying entropy will be lost when the salt particles are reunited when you boil off the water of a pan of salt water for example.
But this has a small effect.

Just know that there are favorable interactions with the salt that basically cement the water particles together so they cannot escape the solution.

 

[Cofo]

A salt has an intermolecular force that is IONIC...which is the strongest intermolecular force, thus it will require more heat to break those bonds, and enter into the liquid-gaseous equilibrium (boiling point).

 

[nze82]

A salt has an intermolecular force that is IONIC...which is the strongest intermolecular force, thus it will require more heat to break those bonds, and enter into the liquid-gaseous equilibrium (boiling point).

Not quite sure if this is an accurate statement. Although it's true that intermolecular interactions between salt components are ionic, but when it comes to boiling you're breaking the bonds between the solvent molecules (not salt molecules) so that they can escape the liquid phase and join the gas phase. So, I don't see how the fact that the intermolecular force between salt components is ionic plays a role in elevating the boiling point.

 

[dl9006]

the fact that salt has ionic bonding seems irrelevant imo

salts dissolve in water and result in two aqueous species.

heating the water is not what breaks the bond in the salt.


i would have thought it's just the fact that you're adding more atoms/molecules into the same amount of solvent which allows the solvent to absorb more heat before boiling since the salt species also absorb energy. (ie increase the heat capacity of solvent)

 

[Cofo]

Not quite sure if this is an accurate statement. Although it's true that intermolecular interactions between salt components are ionic, but when it comes to boiling you're breaking the bonds between the solvent molecules (not salt molecules) so that they can escape the liquid phase and join the gas phase. So, I don't see how the fact that the intermolecular force between salt components is ionic plays a role in elevating the boiling point.

The speaker wasn't completely specific, but the solution is AQUEOUS. So, salt does play a pretty big role. 😎

 

[Cofo]

to dl...
correction: one aqueous species. But now SOME Na+ will be attracted to the O- ion, ....and SOME Cl- ion will join with the H+ ion. The aqueous solution now has covalent bonding, and ionic bonding because not all NaCl molecules will be dispersed throuhgout solution. Thus more heat (energy) will be required to get the AQUEOUS solution to its boiling point because it's not a pure solvent anymore....it has salt in it now.

 

[dl9006]

to dl...
correction: one aqueous species. But now SOME Na+ will be attracted to the O- ion, ....and SOME Cl- ion will join with the H+ ion. The aqueous solution now has covalent bonding, and ionic bonding because not all NaCl molecules will be dispersed throuhgout solution. Thus more heat (energy) will be required to get the AQUEOUS solution to its boiling point because it's not a pure solvent anymore....it has salt in it now.

perhaps i'm missing something here but i think you may be trying to say the same thing that i was trying to say. lol


i think when you dissolve anything(ie say NaCl in this case) into a solvent(ie water), you're basically putting more species that can absorb energy into the same amount of space.

more species that can absorb energy= more energy required to heat to it's boiling point

vs pure solvent= less species that can absorb the energy thus quicker boiling


thus by adding salts into a solvent, you're increasing it's heat capacity since it's better able to absorb the heat energy and more resistant to temperature changes.