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Effect of a soluble volatile impurity on the boiling point?

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a_zed24

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Hello everyone, sorry if this is not the right place to ask this question.
Just wanted to know how does a soluble, volatile impurity affect the boiling point of a liquid? I know it depends on the difference between the boiling points of the liquid and the impurity, but i'm afraid i still didn't get the point.
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Doogie.Howser

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What determines the boiling point of a liquid?
If a soluble impurity is dissolved in said liquid, how does this change the factor that determines boiling point?
If something is volatile, what does this mean, and therefore, do you think it has an affect on what determines a boiling point?

These are questions you should ask yourself if you don't understand something. Break down what the question is asking. Google away.
 
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Doogie.Howser

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I only say it this way because the MCAT tests critical thinking skills. It tests your ability to take what you know and apply it to some scenario you haven't studied before.

To answer your question, boiling is achieved when atmospheric pressure equals vapor pressure. The soluble compound in solution interrupts the primary compound's ability to escape as vapor, therefore lowering it's ability to escape, decreasing its vapor pressure, and increasing it's boiling point. Maybe you've heard of dropping some salt in a pot of boiling water and it takes longer to boil. Review colligative properties.
 
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a_zed24

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What determines the boiling point of a liquid?
If a soluble impurity is dissolved in said liquid, how does this change the factor that determines boiling point?
If something is volatile, what does this mean, and therefore, do you think it has an affect on what determines a boiling point?

These are questions you should ask yourself if you don't understand something. Break down what the question is asking. Google away.

Yeah, I've appreciated your first answer, because while reading those questions I confirmed my small hypothesis:
1)The attraction forces between the molecules of the liquid determine the boiling point
2)Maybe the soluble, volatile impurity will make weaker intermolecular bonds with the original liquid. These weak bonds will then be easier to be broken while heating up the liquid, and the BP will decrease.
And of course, if the impurity will estabilish stronger bonds with the liquid's molecules, then the final BP will be more than the original one. So,
3) adding a volatile impurity to another liquid will affect the BP according to the nature of the impurity.


However, I've always thought that adding salt, which is soluble and non-volatile, would increase the BP! Because the salt's ion on the surface will block the H2O molecules from escaping, thus lowering the vapor pressure...correct me if I'm wrong.
 
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AdaptPrep

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However, I've always thought that adding salt, which is soluble and non-volatile, would increase the BP! Because the salt's ion on the surface will block the H2O molecules from escaping, thus lowering the vapor pressure...correct me if I'm wrong.
You are correct. The bp would increase with the addition of an impurity (freezing point depression and boiling point elevation). Adding salt would lower the vapor pressure. Therefore, you need a higher temperature in order to increase the vapor pressure to the level of the atmospheric pressure to make it boil. The formula is ΔTb=kb⋅m⋅i (where Tb is change in boiling temp, kb is the boiling point elevation constant for the solvent, m is the molality of the solution, and i is the number of particles formed when the solute dissolves). The freezing point depression formula is similar: ΔTf=kf⋅m⋅i. For water, kb is 0.515 and kf is 1.86.

Adding non-volatile impurities make it harder to freeze and to boil.

Of course, your original question was dealing with adding volatile impurities. This is where Raoult's law comes in...
 
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