From: ROY MAYEDA (roymayeda@yahoo.com)
Date: Sun Apr 04 2004 - 20:25:47 PDT
Message-ID: <20040405032547.72701.qmail@web20413.mail.yahoo.com> Date: Sun, 4 Apr 2004 20:25:47 -0700 (PDT) From: ROY MAYEDA <roymayeda@yahoo.com> Subject: Re: physcial change & why CO2 douses flames
Hi Treena.
I'll give it a go.
#1) Yes, melting and boiling points are physical
properties. Some materials do decompose on heating,
and may not boil.
As for the calcium chloride, it is not a freezing
point depression effect (which does not involve the
release of heat anyway). If I remember right, it is a
"heat of solution" issue. The dissolved calcium
chloride has a lower enthalpy (energy due to its
arrangement?)than the solid calcium chloride. The
energy must be given off, and is detected as heat.
(Other salts take in energy to dissolve, so the
surroundings get colder -- e.g. ammonium chloride and
water in instant cold packs.) The calcium chloride
should still be there when the water evaporates,
unless it was reacted with another material in the
experiment -- can't remember what else went into the
bag -- it's been about 10 years since I last did that
one.
#2) Well, carbon dioxide doesn't exactly inhibit
flames. It will displace air (which contains oxygen)
from around the fire, since its density is greater
than air. (If you haven't done this demo, try it:
React some baking soda and vinegar in a cup or beaker
to generate some CO2. Carefully pour the gas only
into another "empty" container. Then pour the
container of CO2 over a candle, extinguishing it.
Using a clear shallow vee-shaped trough and
extinguishing a series of candles down its length is
even more impressive. Even though students know that
the container was filled with some kinds of gases,
they will be amazed. Floating bubbles on a CO2 layer
is fun too, but less related to fire.) If CO2 is put
in the presence of a flaming material that is a
stronger reducing agent than carbon (has a greater
tendency to give off electrons than carbon), that
material will actually use the CO2 as an oxygen
source/electron sink. A great demo was done in a
session at the Exploratorium where powdered magnesium
was burned in a hollow between two sheets of dry ice.
The flame from the magnesium intensified briefly
before all of the magnesium was consumed. When
separated, we could see that the hollows had become
coated with carbon dust and magnesium oxide. This is
also why labs should have a "D" rated fire
extinguisher, which is to be used on metal fires.
Using a CO2 extinguisher would only intensify the
flames.
#3) As far as the Rutherford simulations, I'm not
familiar with that one (at least by that name), but I
believe Rutherford used some type of radioactive
material that exhibited alpha decay. When certain
nuclei break down, they give off alpha particles (2
protons, 2 neutrons). Not sure what he used, and my
references are still "in a box somewhere". I've been
away from my chem room so long, I've started to forget
some of this stuff.
Hope this helps.
Roy Mayeda
"Hey, the roadside ditches not only route drainage,
they keep cars from flying off the highway into the fields."
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