vapor pressure boiling point visualization

[dudewheresmymd]

Hey everyone,
I sometimes mix up vapor pressure/boiling point questions.

1) Do they mean atmospheric pressure in the question below? If they were referring to vapor pressure, when vapor pressure decreases shouldn't boiling point increase? can someone clarify what's going on in distillation?

2) when we decrease the vapor pressure of a solution vs. when we go up in the mountains or down in a valley, how do we figure out the new boiling point of the solution, (ie. is it a movement along the curve of the graph, or do we "shift the entire curve up or down", or do we draw a new line from the horizontal?) I'm assuming for vapor pressure it's a shift, and for the mountain/valley questions, you drop a horizontal line at the new pressure? Can someone clarify this for me?

---

Members don't see this ad.

 

[DrknoSDN]

1) Do they mean atmospheric pressure in the question below?

Yes, vacuum distillation will lower the pressure inside the apparatus below atmospheric.

If they were referring to vapor pressure, when vapor pressure decreases shouldn't boiling point increase?

With lower pressure things are more easily converted to gas during distillation so BP decreases.

can someone clarify what's going on in distillation?

Your separating two liquids that have different boiling points. You boil the mixture very slowly and the substance that boils more easily turns into a gas and then condenses in a condensing tube and ends up in a different collection flask because it boils first.

2) when we decrease the vapor pressure of a solution vs. when we go up in the mountains or down in a valley, how do we figure out the new boiling point of the solution, (ie. is it a movement along the curve of the graph, or do we "shift the entire curve up or down", or do we draw a new line from the horizontal?) I'm assuming for vapor pressure it's a shift, and for the mountain/valley questions, you drop a horizontal line at the new pressure? Can someone clarify this for me?

The horizontal line in your picture at 101,300 Pascals is atmospheric and it shows the boiling temperature for the liquids. If you lower the pressure (lower the line) the boiling points follows the path of each chemical.
[edit]If you Decrease the vapor pressure 'artifically' by adding a solute or something, then you shift the curve right thus increasing the boiling point at all pressures. An example is a BP increase by adding salt to water for cooking.

 

[dudewheresmymd]

If you increase the vapor pressure 'artifically' by adding a solute or something, then you shift the curve right thus increasing the boiling point at all pressures. An example is a BP increase by adding salt to water for cooking.

Awesome thanks. And that's a typo right? We always lower vapor pressure when adding a solute? thereby shifting the curve to right/ "Down"?