Re: Pinhole Digest #918 - 05/13/02

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From: George Gigiolio (ggigiolio@urbanschool.org)
Date: Mon May 13 2002 - 12:34:56 PDT 

Message-id: <fc.000f7611002a704a3b9aca00345017c2.2a714d@urbanschool.org>
Date: Mon, 13 May 2002 12:34:56 -0700
Subject: Re: Pinhole Digest #918 - 05/13/02
From: "George Gigiolio" <ggigiolio@urbanschool.org>

pinhole@exploratorium.edu writes:
>Pinhole Digest #918 - Monday, May 13, 2002
>
> a question on heat
> by "Steve Miller" <nanodog2@hotmail.com>
>
>
>----------------------------------------------------------------------
>
>Subject: a question on heat
>From: "Steve Miller" <nanodog2@hotmail.com>
>Date: Sun, 12 May 2002 19:33:25 -0700
>
>
>Hi everyone -
>
>Here's one of those questions that is so basic, yet I, at least, can't
>answer it.
>
>For water to turn to steam it must absorb 540 calories/gram. This is heat of
>vaporization. But what's going on with the water molecules. Why does it need
>to absorb this heat to vaporize? Are the water molecules adhering - and the
>energy breaks these bonds? What's really going on?
>
>(Ditto question for the phase transition from ice to water)
>
>Thanks for the help,
>
>Steve Miller
>
>
>
>
>
The heat of vaporization states that this is the energy that it takes to change 1 gram of water, at 100 degrees Celsius, from liquid to vapor. at this point there is no net temperature change because all of the energy is being used to break the intermolecular forces between water molecules. In this case they are hydrogen bonds. So what does this mean? ...Water's oxygen atom has a large attraction for hydrogen's electrons (electronegativity) therefore making water more negative. this overall large
electronegativity of the oxygen atom makes the water molecule polar. (slightly negative). This negative charge on water's oxygen atom attracts hydrogen atoms from other water molecules therefor creating an intermolecular force called hydrogen bonding. This is the bond that needs to be broken between water molecules. So the temperature does not rise at this point, all the energy is used to break these bonds. In summary water has a slight charge to it. (negative) this polarity is what attracts
water molecules to itself, heat is used to break these bonds.

A very similar event occurs when ice melts, but here the energy is used not to completely separate the water molecules but, to separate them from there compact state. Here (like above) heat is used to increase the kinetic energy between the molecules and and rearrange the water molecules from a compact state to a let's say a less compact state. The hydrogen bonds are further apart but, not totally broken.

Hope this helps
George Gigiolio
Urban

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