Reaction

A reaction is an observable transformation in which materials change. At the macroscopic level we recognize a reaction by what can be seen, heard, measured, or felt: a color change, a gas given off, heat released or absorbed, mass that has shifted from one place to another. If none of those things is present, we have no grounds for claiming a reaction has taken place.

The temptation is to talk about reactions the way the books talk about them, as something happening in some invisible realm we cannot see. For our purposes that is the wrong level. A reaction here is not a story told about the unseen; it is an event you can point to. Something was one way. Then it was another way. The "how" of that transformation, explained in terms we cannot observe, is the concern of a different kind of chemistry. The "what," grounded in what the senses and the instruments report, is the concern of this one.

A reaction can be treated as a system: reactants as inputs, products as outputs, observable changes as the process, and surroundings as feedback. The iron on the railing is an input, the rust is an output, the change is a slow darkening of the surface over months, and the weather is the feedback. The candle is an input, the wax-on-the-table and soot-in-the-air and warmth-in-the-room are the outputs, the burning is the process, and the draft of air is the feedback. Describe a reaction this way, and it stops being a mystery and starts being a thing we can measure.

Every reaction is one face of the universal process. The engine underneath is a difference in chemical potential; the reaction proceeds because the difference is there to close, and stops when it has closed. Iron and the air around it sit at different potentials, and the iron slowly becomes rust. Wood and air sit at different potentials, and the wood burns. The chemistry looks various from the outside, but the shape is one shape, seen many times.