You're likely thinking of the classic "volcano" science experiment where you mix sodium thiosulfate crystals with water. However, the water doesn't actually get cold; it gets warm.

Here's why:

* Dissolving is an exothermic reaction: When sodium thiosulfate crystals dissolve in water, they release heat into the surroundings. This is because the energy needed to break the bonds in the crystals is less than the energy released when new bonds form between the ions and water molecules.

* The "volcano" effect: The reaction is often accompanied by a visible reaction where the water seems to boil or bubble. This is due to the rapid formation of sulfur dioxide gas, which is also a result of the dissolution process.

So, instead of getting colder, the water actually gets warmer. This is a great example of how chemical reactions can release energy in the form of heat.