What Happens in a Reaction

Five parts on chemical change itself, starting with the test that decides whether one has happened at all — and it is not how dramatic the change looks. A kettle boiling is the noisiest thing in the room and nothing about the water has altered; a nail rusting in a shed is silent and has genuinely become something else. The question is only whether a substance is present at the end that was not there at the start. The second part takes the accounting that follows, through two observations that appear to contradict each other: a candle shrinks as it burns and a rusting nail gets heavier, and neither has created or destroyed anything — seal the container and the balance does not move, because a reaction rearranges atoms and never touches a nucleus. The middle part reads a chemical equation properly, since the parts that look decorative are the ones carrying the information: the arrow means becomes and never equals, the state symbols say where everything is, and the two numbers that look alike mean entirely different things. The fourth part turns that into the rule for balancing, and insists on the step that is nearly always skipped — a coefficient states how much of a substance there is, a subscript states which substance it is, and that asymmetry is the whole reason only one of them may be changed. It is also, despite the shared word, nothing whatever to do with balancing in algebra. The last part asks what a reaction will feel like: every one of them breaks bonds, which costs energy, and makes bonds, which releases it, so whether a flask warms or cools is decided by which total is larger.