When you mix chemicals together and they react, it is known as a spontaneous reaction. Most spontaneous reactions give off heat, called exothermic reactions, but some reactions absorb heat, called endothermic reactions. When you mix baking soda and vinegar together, the atoms in the baking soda and vinegar need to rearrange to form the final products: carbon dioxide, water, and dissolved sodium acetate. In the case of this reaction, the rearranging takes energy, which is absorbed from the environment. As energy is absorbed, the solution becomes cold.