Acids, Bases and Salts

Five parts on a family of substances that behave far more predictably than their reputation suggests. It opens by refusing to define an acid by taste or by danger, and works instead from the puzzle that lemon juice, vinegar, fizzy drinks and battery acid all do the same things despite having nothing else in common — with the decisive clue being that none of them does any of it while dry. Water is not a bystander: dissolving releases hydrogen ions, and those ions are the whole family resemblance. The second part puts a number on how many, and spends its time on the feature of the pH scale that is easiest to miss — each whole step is a factor of ten, so the difference between ordinary rain and damaging acid rain is one number on a chart. The third sorts the three things an acid reacts with and finds a salt under all of them, so the information is in the second product: hydrogen from a metal, water alone from a base, and the fizz of carbon dioxide from a carbonate. The fourth asks why getting a clean sample of that salt takes several stages when the reaction itself takes seconds, and the answer is that each stage solves a problem the one before it created — which is why the order cannot be rearranged. The last part separates two words that are routinely used as synonyms and are not: strong is about what fraction of an acid releases its hydrogen ions and cannot be changed, while concentrated is about how much is dissolved and changes the moment water is added.