According to the Encyclopedia Britannica, a chemical reaction is “a process in which one or more substances, the reactants, are converted to one or more different substances, the products.” Indeed, you’d be hard-pressed to find any scientist who disagrees with this definition. Chemical reactions describe the change of some substance to another substance, that’s just a fact of life.
However, there are two parameters to the “turn something into something else” generalization. First and foremost, when I say “something”, I’m not talking about turning a chair into, say, a dolphin. That’s not chemistry, that’s alchemy. Or magic. It depends on who you ask. Chemical reactions describe the changes between chemical compounds, which are made up of elements from the periodic table of the elements:
If you take any basic-level chemistry class, you’re familiar with this boxy layout. Anything on here is fair game to smash together for chemical reactions, though most of the fun, reactive chemicals are made up of one element from the left half of the table and one element from the right half. These ones are called ionic compounds, and they tend to react violently or with some sort of visual change that make them exciting to watch. The more boring chemicals are combinations of the elements on the right half of the table, called covalent. These ones tend to be more stable and don’t react as violently, save for a few rare cases.
The second parameter to chemical reactions is similar to the first one: you can’t make substances out of elements that were not present in the first part of the equation. Old-time alchemy, whose main goal was to turn lead into gold, was doomed to fail because you can’t turn one metal into another. Lead is a pure substance, and unless alchemists added chemicals to it that contained gold, they would never get pure gold out of a lump of lead. When working with chemical reactions, what you start with is rearranged into something different-looking, but still made up of the same elements.
However, you’ll see that in action on the next post. This one is long enough to describe a sentence-long definition of chemical reactions. I hope the two parameters outlined above make the definition a little clearer.