To acidify is to become an acid, or to make something become an acid — more loosely, to make it more acidic than it was. A chemist acidifies a liquid by adding acid. A cook may acidify a sauce with lemon or vinegar, though in the kitchen people often just say add lemon. Scientists also say the ocean is acidifying: seawater is slowly becoming more acidic as it takes in carbon dioxide from the air. The change can be small; acidify does not always mean “turn into a strong, dangerous acid,” only “make more acid” or “make more acidic.” It is a verb of change, used in labs, in cooking, in talk about soil, and in talk about the climate.