Effects of impurities in solution on Boiling point

[Chocolatebear89]

I understand that if you put solutes into a solution it raises the boiling point solution. Does that mean any impurity in a solution will cause the boiling point to rise?

For instance, if I have a solution of dichloromethane, which is polar. If I add butane, will that increase the boiling point even though butane is not polar, and possibly have less interaction?

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[Rabolisk]

Not all impurities in a solution will cause the boiling point to rise. For example, a mixture of ethanol and water will have less boiling point than pure water, because alcohol itself boils at a lower temperature than water. Raoult's Law describes this phenomenon, although it only describes "ideal" mixtures. Ethanol and water deviate from Raoult's Law, resulting in a minimum boiling azeotrope somewhere at somewhere around 96% alcohol. This solution boils at lower temperature than both pure ethanol and pure water.

 

[sangria1986]

Not all impurities in a solution will cause the boiling point to rise. For example, a mixture of ethanol and water will have less boiling point than pure water, because alcohol itself boils at a lower temperature than water. Raoult's Law describes this phenomenon, although it only describes "ideal" mixtures. Ethanol and water deviate from Raoult's Law, resulting in a minimum boiling azeotrope somewhere at somewhere around 96% alcohol. This solution boils at lower temperature than both pure ethanol and pure water.

Also the law only works for nonvolatile solutes. Since ethanol can boil away it is volatile. Also ethanol can have h-bonding. This means ethanol can dissolve
in water. Thus the rule won't apply. Generally anything that can dissolve or boil away in a solution cannot follow raolts law

 

[Rabolisk]

There are examples of volatile solute and solvent solutions that closely follow Raoult's Law, like benzene and toluene, and they dissolve in each other. An ideal solution is a solution in which the interactions between the two liquids are equal to the interactions within the two liquids. Benzene and toluene are so similar that they are considered ideal. All solutions assume dissolution, whether they follow Raoult's Law or not. It is true though that given non-volatile solutes, you can generally assume that Raoult's Law will hold, at least for dilute solutions.

 

[sangria1986]

There are examples of volatile solute and solvent solutions that closely follow Raoult's Law, like benzene and toluene, and they dissolve in each other. An ideal solution is a solution in which the interactions between the two liquids are equal to the interactions within the two liquids. Benzene and toluene are so similar that they are considered ideal. All solutions assume dissolution, whether they follow Raoult's Law or not. It is true though that given non-volatile solutes, you can generally assume that Raoult's Law will hold, at least for dilute solutions.

After rereading the section I saw that indeed volatile solute and solutions can follow ideal law for Raolt...however the equation is modified if I am correct and takes into consideration the individual partial pressures of the solute and solvent...

In anycase, I dont think its neede for the MCAT. However, to modify what I said above, if a solute and solution are exothermic in solution formation (Hf<0) then the vapor pressure decreases since the solute is more similar to the solvent.

However, in the example you provided i think the delta G for that formation is 0 so indeed it follows Raolts law.