From: pauld@exploratorium.edu
Date: Mon May 13 2002 - 12:37:38 PDT
Message-Id: <200205131937.g4DJbaH28475@isaac.exploratorium.edu> From: pauld@exploratorium.edu Subject: Re: pinhole re: a question on heat Date: Mon, 13 May 2002 12:37:36 US/Pacific
Those 5'th graders did a pretty good job of getting to the basic differences
between evaporation and boiling.
Evaporation happens at the surface. and boiling happens within the liquid.
Put some water into an evacuated chamber, ignore what happens at first.
When you come back you find liquid water in part of the chamber, and water vapor
in the other part. The water vapor has a pressure, the "vapor pressure."
If you increase the temperature, the vapor pressure increases exponentially.
At room temperature the vapor pressure of water might be 3% of an atmosphere of
pressure.
Thus water at room temperature evaporates until it comprises 3% of the
atmosphere.
At 100 °C the vapor pressure is 1 atmosphere.
This is a large enough vapor pressure that bubbles of gas can form within the
liquid. The liquid boils. Also the amount of water vapor leaving the surface
greatly increases.
The liquid is a liquid because the water molecules are bound together.
To break these bonds requires energy whether the bonds are broken during boiling
or evaporation.
Paul D
> I know that there will be far more complete and
> thorough answers to your questions from the other
> pinholers but wanted to mention how this question came
> up with some fifth graders. We were talking about
> evaporation and heat in salt ponds. I was trying to
> get them to the understanding that evaporation changes
> concentration of a confined amount of sea water and
> they immediately started tripping on how the water
> evaporates. They dedided on their own that the water
> turns to "steam" because it got hot from the sun. When
> I asked them if there was any difference between
> evaporating water and boiling water they thought about
> it and said "only how much water is steaming" and
> "boiling comes from the bottom of the liquid" (they
> felt that the bubbles that came from the bottom of the
> pot made the "steam"). Listening to the three fifth
> graders they began to convince me that heating the
> surface water molecules was essentially the same as
> boiling them in a given volume, but it worried me I
> was missing something profound and important by
> agreeing with them. I always remember how important
> "boiling points" were and I realized that I couldn't
> give a clear technical definition of what "steam" and
> "boil" (in "Focus on Physical Science" middle school
> text it is defined as a "rapid change from a liquid to
> gaseous state") mean. Terms such as "boiling", "steam"
> and "evaporation" are used many ways, I'm not at all
> sure I am using them correctly, but I was so impressed
> with the fifth graders digging in on this question.
> That's why I look forward to the technical responses
> to your excellent question!
>
> With firm handshake,
> Raleigh
>
> __________________________________________________
> Do You Yahoo!?
> LAUNCH - Your Yahoo! Music Experience
> http://launch.yahoo.com
>
> ---------------------------------------------------------------------------
>
> To unsubscribe from pinhole, send an email to requests@exploratorium.edu
> with the words 'unsubscribe pinhole' (without the quotes) in the SUBJECT
> of the email.
>
> To subscribe to the digest and only get 1 combined message a day, send an
> email to requests@exploratorium.edu with the words 'subscribe digest
> pinhole' (without the quotes) in the SUBJECT of the email.
>
> Check out what your colleagues have written on Pinhole in the Pinhole
> archives at: http://saturn.exploratorium.edu/ti/alumni/pinhole.html
> ---------------------------------------------------------------------------
>
>
>
--------------------------------------------------
This message was sent using Exploratorium web mail
http://www.exploratorium.edu/
This archive was generated by hypermail 2.1.3 : Mon Aug 05 2002 - 09:21:41 PDT