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The physics and the chemistry of the heat pad

The physics and the chemistry of the heat pad Flexing a metallic disk triggers the crystallization of the supercooled sodium acetate solution contained in commercial heat pads. Many mechanisms have been proposed to explain the apparent nucleation of crystalline material. In this paper a simple experiment is described that demonstrates that nucleation is triggered by preserving seed crystals clamped between opposing metal surfaces. An explanation for the retention of the crystalline particles is the elevated melting point caused by very high local pressures. A series of thermophysical properties of the sodium acetate solution is also measured, including the available enthalpy upon crystallization of the supercooled substance, and liquid and solid phase specific heat capacities. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png American Journal of Physics American Association of Physics Teachers

The physics and the chemistry of the heat pad

American Journal of Physics , Volume 76 (6) – Jun 1, 2008

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References (18)

Publisher
American Association of Physics Teachers
Copyright
Copyright © 2008 American Association of Physics Teachers
ISSN
0002-9505
eISSN
1943-2909
DOI
10.1119/1.2830533
Publisher site
See Article on Publisher Site

Abstract

Flexing a metallic disk triggers the crystallization of the supercooled sodium acetate solution contained in commercial heat pads. Many mechanisms have been proposed to explain the apparent nucleation of crystalline material. In this paper a simple experiment is described that demonstrates that nucleation is triggered by preserving seed crystals clamped between opposing metal surfaces. An explanation for the retention of the crystalline particles is the elevated melting point caused by very high local pressures. A series of thermophysical properties of the sodium acetate solution is also measured, including the available enthalpy upon crystallization of the supercooled substance, and liquid and solid phase specific heat capacities.

Journal

American Journal of PhysicsAmerican Association of Physics Teachers

Published: Jun 1, 2008

Keywords: crystallisation; enthalpy; melting point; nucleation; sodium compounds; specific heat

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