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Have you ever seen how a salt heater works? You just press it - and the cap­sule with flu­id heats up and turns sol­id! Strange­ly enough, this heater works by us­ing ice - only hot ice.

Safe­ty pre­cau­tions
Be care­ful when work­ing with heat­ing de­vices.
Ob­serve safe­ty rules when work­ing with these sub­stances: wear pro­tec­tive gloves, glass­es and mask.
Warn­ing! Only un­der adults su­per­vi­sion.

Reagents and equip­ment:
* bak­ing soda (sodi­um bi­car­bon­ate, 77 g);
* 9% so­lu­tion of acetic acid (662 ml);
* wa­ter (51 ml);
* saucepan;
* hot plate;
* glass con­tain­er;
* beaker;
* spat­u­la.

Step-by-step in­struc­tions
Sprin­kle bak­ing soda into the saucepan and pour vine­gar over it. Stir thor­ough­ly. Put the re­sult­ing so­lu­tion on the hot plate and evap­o­rate un­til it turns dry, and try to break up the lumps of the re­sult­ing pow­der. Then put the pow­der in the glass con­tain­er, add wa­ter and heat in a wa­ter bath un­til it dis­solves com­plete­ly. Move the re­sult­ing so­lu­tion to the beaker and al­low to cool. Touch with your fin­ger. Ob­serve the for­ma­tion of crys­tals and the re­lease of heat.

Pro­cess­es de­scrip­tion
In the re­ac­tion of acetic acid and sodi­um bi­car­bon­ate, sodi­um ac­etate forms. It has a very in­ter­est­ing prop­er­ty–it dis­solves in its own crys­tal­lized wa­ter when heat­ed. A sat­u­rat­ed so­lu­tion forms, which when it cools is very un­sta­ble and crys­tal­lizes from any im­pact, for ex­am­ple the touch of a fin­ger.
How­ev­er, in our ex­per­i­ment we dried the sodi­um ac­etate out com­plete­ly, so it was eas­i­er to cal­cu­late the amount of wa­ter need­ed. This is so crys­tal hy­drate forms–a sol­id salt with wa­ter mol­e­cules in a crys­tal lat­tice. For ev­ery 100 g of sodi­um ac­etate, around 66 g of wa­ter is re­quired.
NaH­CO₃ + CH₃­COOH = CH₃­COONa + CO₂ + H₂O
CH₃­COONa•3H₂O(sol­id) = CH₃­COONa(so­lu­tion)

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