Water evaporating from my hand in the cold.

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    Cold hand Water
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Discussion Overview

The discussion centers around the phenomenon of water evaporating from a hand in cold conditions, exploring the underlying physical principles of evaporation and boiling points. Participants examine the factors influencing evaporation, such as temperature, humidity, and molecular motion.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant describes observing visible evaporation of water from their hand while playing in the snow, prompting the question of why this occurs.
  • Another participant explains that evaporation happens due to the random motion of water molecules, with some achieving the energy needed to transition to vapor.
  • A different viewpoint suggests that the hand acts as a heat source, heating the water while the cold air and low humidity facilitate diffusion from a higher concentration (the hand) to a lower concentration (the air).
  • A question is raised about why the boiling point of water is presented as a fixed value rather than a formula that accounts for varying conditions.
  • One participant provides a technical definition of boiling point in terms of vapor pressure and atmospheric pressure, noting that evaporation can occur below boiling point under certain conditions.
  • A reference to a phase diagram is made, indicating that while there is no formula for boiling point, such diagrams can provide useful information.
  • Another participant suggests that research on boiling point variations with elevation and barometric pressure can yield additional insights.

Areas of Agreement / Disagreement

Participants express differing views on the nature of boiling points and evaporation, with some agreeing on the principles of molecular motion and heat transfer, while others question the representation of boiling points as fixed values. The discussion remains unresolved regarding the specifics of boiling point definitions and their applications.

Contextual Notes

Participants reference concepts such as vapor pressure and phase diagrams without fully resolving the implications of these terms or their interdependencies. The discussion includes assumptions about temperature and humidity that are not explicitly defined.

wasteofo2
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Recently I was playing in the snow without gloves. I made a snowball and the residual water on my hand began evaporating and was visible, much the way your breath is in the cold.

Why did the water do this?
 
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Water evaporated there for the same reason it evaporates anywhere - due to the random motion of the water molecules, some reach the energy needed to boil.
 
There are a couple of things going on. First your hand is a source of heat, so you were heating the water, then I am betting that it was pretty cold out and the humidity was pretty low. So in essence the water diffused from the high concentration in your hand to the lower concentration in the air.
 
So then why is the boiling point of water always given as an absolute number instead of there being some sort of formula to decide when water boils/evaporates?
 
b.p. --- vapor pressure equals one atmosphere. Vapor pressure increases with temperature; lower than b.p., evaporation occurs without boiling so long as the partial pressure of water in a one atmosphere total gas pressure atmosphere above liquid water is less than the vapor pressure; higher than b.p. requires that pressure be maintained at least as great as the vapor pressure for the liquid phase to exist.
 
Originally posted by wasteofo2
So then why is the boiling point of water always given as an absolute number instead of there being some sort of formula to decide when water boils/evaporates?
No formula, but there is a chart: PHASE DIAGRAM
 
Do some googling and you will find a lot of information on the variation of boiling point with elevation and barametric pressure.
 

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