The opposite of alkalize, a term that refers to making substances more basic or alkaline, is acidify. Acidification is the process of increasing the acidity of a solution, making it more acidic or sour. Antonyms for alkalize could also include other terms like neutralize or balance, which refer to adjusting the pH level of solutions to make them less acidic or less basic. It's important to note that while alkalizing and acidifying agents can be used for various purposes ranging from cleaning agents to health remedies, it's essential to exercise caution and follow instructions carefully to prevent harm to oneself or the environment.