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ChemWiki: The Dynamic Chemistry E-textbook > Development Details > Approaches > Demos > Hot and Cold Paks

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Hot and Cold Paks


Exothermic reactions produce heat and endothermic reactions absorb heat from its surroundings.


  • 1. Plastic baggie
  • Metal twist tie
  • 25 g iron metal mesh 100
  • 1 g salt
  • 1 tablespoon small vermiculite
  • 5 mL water
  • Ziploc plastic baggie
  • 20 g sodium thiosulfate (fills one 50 mL beaker)
  • 125 mL warm tap water
  • 250 mL beaker
  • Regular warm ice pack


Place iron powder in baggie, add salt and mix contents by shaking (hold baggie closed). Add the vermiculite and mix again. If possible, do these steps before demonstration. Add the water to the baggie and seal with twist tie; squeeze and shake baggie. After a minute or so, a noticeable amount of heat will be produced. Have a volunteer verify.

Pick a new volunteer for cold pak. Place warm water in ziploc baggie and have volunteer verify it is warm. Put the sodium thiosulfate in baggie and zip closed. Shake bag. Have volunteer verify it is now cold. Walk around so others can feel both baggies.


I have two problems right now. My hands are very cold from being outside, like when you're at a football game. I need something to warm my hands. But I hurt my knee yesterday getting out of my car and I have to keep a cold pack on it. So I also need a cold pack. Let's see if I can make both.


An exothermic reaction gives off heat, like our hot pak. The iron powder and the oxygen in the bag react to form iron oxide. This is called oxidation. The salt sped up this reaction and is therefore a catalyst. The vermiculite made sure the heat stayed in the baggie. The iron oxide that is formed is a compound.

Fe + O2 ---> Fe2O3 + heat

As for the cold pak, that is an endothermic reaction. In an endothermic reaction, a substance takes heat from its surroundings. The sodium thiosulfate needs energy to dissolve, so it takes heat from its surroundings, in this case the warm water. The heat is absorbed into the sodium thiosulfate and the water is now cold.


  • Dump water down sink then discard baggie. Discard warm pak in garbage.


  • Unknown


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Last modified
10:32, 2 Oct 2013



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