Steam distillation

[chiddler]

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By adding water or steam, the boiling points of the compounds are depressed, allowing them to evaporate at lower temperatures, preferably below the temperatures at which the deterioration of the material becomes appreciable.

Why does adding steam allow them to evaporate at lower temps?

 

[MedPR]

By adding water or steam, the boiling points of the compounds are depressed, allowing them to evaporate at lower temperatures, preferably below the temperatures at which the deterioration of the material becomes appreciable.

Why does adding steam allow them to evaporate at lower temps?

Steam distillation is used when the thing you are trying to purify has a really high boiling point. The boiling point of any mixture of liquids is going to be lower than the boiling point of the least volatile component of that mixture (which is what you are trying to purify in this case).

Steam is just heated water vapor. When you make the steam, you boil water in a separate beaker and allow it to pass into the mixture you are distilling. The steam is hotter than this mixture, so it condenses and releases heat. The released heat is enough to boil the mixture.

So if you hadn't mixed the unknown with water, its BP would be really high, and if it was heat sensitive (like organic compounds) you might heat it up too much and break it down into something different. Steam distillation is like an intermediate between normal distillation and vacuum distillation.

Also, I'm pretty sure steam is only used because its temperature is more easily controlled (since it's just heated, gaseous water) than the flame of a bunsen burner or a hot plate. The key is that mixing the target liquid with water lowers the BP of the target liquid.

 

[chiddler]

thanks i understand the general idea behind it. i just don't understand the how.

The boiling point of any mixture of liquids is going to be lower than the boiling point of the least volatile component of that mixture (which is what you are trying to purify in this case).

why is this?

 

[MedPR]

thanks i understand the general idea behind it. i just don't understand the how.



why is this?

Boiling point = temperature at which the vapor pressure equals or exceeds atmospheric pressure. Higher boiling point = lower vapor pressure, and lower boiling point = higher vapor pressure.

Water has a lower boiling point than the unknown, so it has a higher vapor pressure. By mixing it with the lower vapor pressure unknown, you increase the vapor pressure of the mixture, thereby lowering the boiling point.

 

[chiddler]

Boiling point = temperature at which the vapor pressure equals or exceeds atmospheric pressure. Higher boiling point = lower vapor pressure, and lower boiling point = higher vapor pressure.

Water has a lower boiling point than the unknown, so it has a higher vapor pressure. By mixing it with the lower vapor pressure unknown, you increase the vapor pressure of the mixture, thereby lowering the boiling point.

But mixing it with water will reduce the mole fraction of the compound which will proportionately reduce its vapor pressure by roult's law.

Should the desired compound be soluble with water?

 

[MedPR]

But mixing it with water will reduce the mole fraction of the compound which will proportionately reduce its vapor pressure by roult's law.

Should the desired compound be soluble with water?

No, should not be soluble with water. I've only read about steam distillation being done for high bp organic compounds. When you mix it with water, some of the molecules at the surface are water and some are the high bp organic, so the vapor pressure above the mixture is higher than the pure organic.

 

[chiddler]

No, should not be soluble with water. I've only read about steam distillation being done for high bp organic compounds. When you mix it with water, some of the molecules at the surface are water and some are the high bp organic, so the vapor pressure above the mixture is higher than the pure organic.

Roult's law describes this exactly. When you add liquid impurities to a liquid, then the surface will represent percent amount of each liquid. This is why vapor pressure decreases.

But here, this is not the case. Why does this model not reflect what is happening?

I'll agree that the vapor pressure is higher, but that is exactly what you would expect from those graphs like these

(except water is large vapor pressure, so one of those Pa or Pb will be much higher than the other)