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Q & A: boiling and evaporation

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Most recent answer: 10/22/2007
Q:
okay, so I was in chemistry and this tottaly random question popped into my head that had nothing to do with the lesson - Does the rate at which something evaporates have anything to dowith the boiling point? for example, if water evaporates faster than oil ((oil you’d put into a car engine)) then does water have a lower tempurature boiling point??? Just a thought I haad hoped to get cleared up thanks!
- Katrine (age 17)
NWHS, Canada
A:
Yes, you're right. A fluid boils when the rate of evaporation into a vapor bubble (at some room pressure) exceeds the rate at which molecules rejoin the fluid from the bubble. Obviously, other thing being equal, faster evaporation makes it easier to boil, so that boilng can start at a lower temperature.

One way to picture this is with a crude mechanical model. If molecules stick together well, it's hard for them to evaporate, and they have to be heated more before they boil.

However, although the boiling point is very well defined (for a given pressure), the rate of evaporation in general will depend on how much wind there is on the surface, how well the fluid is stirred, etc.

Mike W.

(published on 10/22/2007)

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