Comments on: Let Me Tell You About My Sacks http://nat.org/blog/2005/09/let-me-tell-you-about-my-sacks/ Mon, 07 May 2012 20:05:57 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: James Scott-Brown http://nat.org/blog/2005/09/let-me-tell-you-about-my-sacks/comment-page-1/#comment-6284 James Scott-Brown Thu, 31 Dec 2009 17:18:46 +0000 http://www-new.nat.org/blog/?p=768#comment-6284 These products are often marketed as hand-warmers. They are essentially just a supersatured solution of sodium acetate in water. When you click the disc, the sodium acetate crystallises; this is an exothermic process, so gives out heat. When you heat the pad in boiling water, the crystals dissolve again, and the pad returns to its previous liquid state. When it has cooled to room temperature, more sodium acetate is dissolved than is soluble at room temperature (the solution is supersatured) - but it cannot recrystallise until triggered by clicking the disc. These products are often marketed as hand-warmers. They are essentially just a supersatured solution of sodium acetate in water. When you click the disc, the sodium acetate crystallises; this is an exothermic process, so gives out heat.

When you heat the pad in boiling water, the crystals dissolve again, and the pad returns to its previous liquid state. When it has cooled to room temperature, more sodium acetate is dissolved than is soluble at room temperature (the solution is supersatured) – but it cannot recrystallise until triggered by clicking the disc.

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