Home

Home
Common Compounds
Exam Guide
FAQ
Features
Glossary
Construction Kits
Companion Notes
Just Ask Antoine!
Simulations
Slide Index
Toolbox
Tutorial Index

FAQ
Introduction
Measurement
Matter
Atoms & ions
Compounds
Chemical change
The mole
Gases
Energy & change
The quantum theory
Electrons in atoms
The periodic table
Chemical bonds
Solids
Liquids
Solutions
Acids & bases
Redox reactions
Reaction rates
Organic chemistry
Everyday chemistry
Inorganic chemistry
Environmental chemistry
Laboratory
History of chemistry
Miscellaneous


Home :FAQ :Chemistry of everyday lifePrint | Comment
Previous Question Next Question

How do dry chemical heatpacks work?


All liquids release heat when they freeze. For example, the water in an ice cube tray releases heat into the freezer compartment as the ice cubes solidify. The molecules relax into a nice, stable, hydrogen-bonded network once they lose that excess thermal energy (which makes them rather jittery!).

It isn't as easy to make an ice cube as you might expect. If the water is very pure and air-free, you must cool it several degrees below its freezing point to get it to freeze. The ice crystals need some sort of nucleus to form around- a scratch, an impurity, a dust speck- or you get supercooled water. When supercooled liquids freeze, their temperature rises to the melting point as they freeze.

That's the basic idea behind most of the chemical handwarmers. A flexible plastic pillow pad is filled with a liquid (sodium acetate trihydrate) that is supercooled below its freezing point of 54°C.

When you flex a tiny metal disk embedded in the pad, rough surfaces on the disk are exposed that allow crystals to grow. The sodium acetate instantly begins to crystallize as the pad warms to a toasty 54°C.

Author: Fred Senese senese@antoine.frostburg.edu



General Chemistry Online! How do dry chemical heatpacks work?

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/consumer/faq/how-do-handwarmers-work.shtml