March 29, 2024, 11:16:32 AM
Forum Rules: Read This Before Posting


Topic: Fledgling Industrial Hygienist here with a Volatility Question  (Read 1146 times)

0 Members and 1 Guest are viewing this topic.

Offline g36jordan

  • Very New Member
  • *
  • Posts: 2
  • Mole Snacks: +0/-0
My apologies, I am not sure if this is the right sub for this question but I figured it would be applicable.

I am looking for an explanation of vapor pressure as it relates to how to make a determination on how "volatile" a chemical is. I always understood the higher the vapor pressure, the more a chemical will naturally volatilize even at STP. How then, if the pressure exerted by air is 760 mmHg at STP, can something like Methylene Chloride (considered to be a pretty volatile chemical) volatilize into the air with a VP of about 353 mmHg? Wouldn't its vapor pressure need to increase to beyond 760 mmHg before the molecules jump into the air? How does this work? Something like formaldehyde, with a VP > 1 atm, is something I can understand, but in what way are chemicals, at STP, with vapor pressures less than 760 mmHg, still volatile?

I realize I must sound like an idiot, but I figured I, as a non-chemist, would ask the chemists.

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27635
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Fledgling Industrial Hygienist here with a Volatility Question
« Reply #1 on: May 06, 2020, 06:50:13 PM »
Whatever evaporates pushes out the air. Total pressure stays the same, just the (local) composition changes.

Unless you work with a closed container, then the pressure will build up.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline g36jordan

  • Very New Member
  • *
  • Posts: 2
  • Mole Snacks: +0/-0
Re: Fledgling Industrial Hygienist here with a Volatility Question
« Reply #2 on: May 07, 2020, 11:27:04 AM »
I am still confused, if evaporation can occur with a vapor pressure less than 760 mmHg then why does water need to boil (aka get up to a vapor pressure of 760 mmHg) to evaporate? Or can it evaporate at less VP?

Offline Enthalpy

  • Chemist
  • Sr. Member
  • *
  • Posts: 4041
  • Mole Snacks: +304/-59
Re: Fledgling Industrial Hygienist here with a Volatility Question
« Reply #3 on: May 07, 2020, 12:52:35 PM »
Evaporation occurs before boiling. This happens after a rainfall for instance.

If the vapour pressure exceeds the pressure of the surrounding gas (for instance the atmosphere), then the vapour can form bubbles in the liquid. The liquid boils.

Sponsored Links