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Hvad er der i håndvarmere?
Fra : SatinaB
Vist : 1418 gange
300 point
Dato : 03-04-08 17:53

Jeg har søgt og ledt overalt, men har ikke fundet et svar på, hvad det er der er i håndvarmere. De der små nogen hvor man knækker en metaldims, og det flydende noget inde i krystaliseres og bliver varmt.
Er der nogen her der kan svare på det?

 
 
Accepteret svar
Fra : transor

Modtaget 310 point
Dato : 03-04-08 18:14

Håndvarmere virker ved at smeltevarmen gives tilbage når væsken størkner.

Der er en del væsker som kan underafkøles. Bl. a. glaubersalt, så vidt jeg husker.
Det vil sige at den bliver ved med at være væske, selv om den køles ned under stoffets smeltepunkt.
Der skal være en urenhed , en krystal eller en forstyrrelse til før størkningen begynder og varmen frigives. Klikket med dimsen er en af måderne.

Underafkøling kendes også med rent vand. Blandt andet som regn, der fryser når det når jorden. Isslag.


Kommentar
Fra : vagnr


Dato : 03-04-08 18:21

Hvis du er god til enegelsk er der et svar her."
Kids, there is a cool (okay, not really) product available on the market for a reusable heat pack/handwarmer that is loaded with chemistry-in-action ability. It's called a "Zap Pac Heat Pack" (contact info below). While a monetary investment is required, it dramatically and safely showcases the phenomena of both supercooling and an exothermic reaction.
These products consist of a concentrated aqueous salt solution together with a flexible metallic activator strip (usually stainless steel) in a sealed, flexible container. Sodium acetate (natriumacetat på dansk) and calcium nitrate are examples of suitable salts (Zap Pacs use the former). These salts are much more soluble in hot water than in cold water. The flexible metal strip is bent back and forth a few times, whereupon a white cloud of crystals begins to precipitate. Within seconds, the entire pack is filled up with solid crystalline needles of sodium acetate without any solution left, and the temperature raises to 130°F for about 30 minutes. Because heat is released upon this precipitation, it is called an exothermic reaction (the opposite is called an endothermic reaction).

Supercooled liquids can be cooled below their normal freezing point without turning solid. Then, at the flick of button, the supercooled liquid is triggered to solidify (crystallize) and at the same time release large amounts of heat. Salt solutions that have been processed in such a way that their temperature can be lowered well below their solidification (or melting) temperature and still remain in liquid are defined as supercooled or metastable liquids. The triggering device initiates the rapid solidification of the solution. In the case of salt solutions that release or absorb large amounts of energy during phase changes (common table salt sodium chloride does not do this), the solidification process is a rapid crystallization that releases large amount of heat at the salt solution's normal melting temperature.

The activator is a thin metal piece with ridges and a specially roughened surface. The flexing causes metal-to-metal contact that releases one or more very tiny particles of metal from the roughened surface. This acts as a nesting site for one crystal deposited from the solution and (voila!) all of the crystals fall out instantly.

These heat packs are reusable because, by re-heating the pack in boiling water for a few minutes, the salt re-dissolves and the pack again contains a clear solution. (Be sure to have an adult partner help you with this part). Best of all, the activator strip can be reused dozens of times!"

http://membership.acs.org/C/Chicago/ChmShort/CS02.html.



Godkendelse af svar
Fra : SatinaB


Dato : 03-04-08 19:04

Tak for svaret transor.
Det var rart at få det at vide. Ikke at jeg stadig forstår det, men nu er jeg heller ikke den store kemiker :)
Men svaret er nok til at jeg ikke behøver spekulere over det mere :)

Kommentar
Fra : SatinaB


Dato : 03-04-08 19:05

Også en tak til Vagnr for svaret :)

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