smilergrogan wrote:Weyoun wrote:Another way to reason it out - sure, that perspiration quote was pretty famous, but I'm not sure how Ford would go about preserving a drop of sweat in a vial - even if it's really well sealed, it would likely evaporate within the vial at some point, since the air inside is probably around one atmosphere.
Equilibrium vapor pressure for water is about 3 kPa at room temp.; it would be lower for sweat due to Raoult's law (Vapor pressure depression of a solution). 1 drop of water is about 50 mg = about 0.003 moles. That's enough vapor to occupy about 2 liters volume at 3kPa pressure. So in an airtight vial of less than 2 L, some of the sweat wouldn't evaporate. In a typical size vial (10-20 mL), only about 1% or less of the sweat would evaporate. Much less than this if the vial were refrigerated, or if the sweat was collected on a humid day with partially or fully saturated air. And if it's not an airtight vial, then there's really not much point to try to collect either sweat or breath anyway.
Those are greats points. My thinking was that, even if sealed, a small drop of sweat would evaporate to saturate the air that was in the vial, so the water might still "disappear." I didn't realize how poorly room air can incorporate water vapor. But, then, when you think about it, at room temperature it should be that way, or else we might not have lakes, rivers, etc.
But I think your last point is the driving one - we can't really worry about the vapor pressure since it's not a closed container - and no real equilibrium can be established, at least not until the droplet has evaporated.
Back to the question, I think the original point still holds - a breath of air is ineffable - you can't see it, so who knows it is not there? You can CLAIM it has the breath. Whereas a droplet of sweat would really have to be seen - some trace salt in a vial won't please museum goers. "It used to be there, but it evaporated," doesn't impress.
