Modern residential plumbing often utilizes various types of plastic, which are highly sensitive to thermal stress. PVC (Polyvinyl Chloride):

). Exposing them to boiling temperatures causes several structural issues:

The safety of this practice depends entirely on what your pipes are made of:

Every sink has a P-trap—the curved pipe under the basin that holds water to block sewer gas. P-traps are often made of thin metal (like chrome-plated brass) or plastic. Because they hold standing water, pouring boiling water into the sink means that water will sit in the P-trap, soaking the same small section of pipe with intense heat. Thin metal P-traps can warp, and plastic ones can soften and droop, leading to leaks.