Follow the soda immediately with ½ cup of white vinegar. You will hear it—a hiss, a crackle, like a soda can opened in a library. Quickly, while the reaction is active, cover the drain with a drain plug or a wet rag. You want that expanding foam to go down , pushing against the clog, not up into your sink.

Unclog a Drain Naturally: The Baking Soda & Vinegar Method