Bicarbonate of soda and vinegar down a drain

Does it work?

Unresolved

The chemical half of the claim is refuted by settled chemistry. The mechanical half is untested, and calling it refuted would assert a finding nobody has made.

This is a verdict on whether the technique is effective. Whether it is safe on a particular surface is a separate question, answered below.

Where not to use it

The proposed mechanism

The explanation below is what is offered for this technique. No evidence establishes that this is what actually happens, so treat it as a plausible story rather than as a finding.

The claimed mechanism has two parts and they need separating. The chemical part is that the reaction attacks the blockage; that is established to be wrong, because the reaction is between the two ingredients and consumes both, leaving sodium acetate solution. The physical part is that the evolved carbon dioxide agitates or pressurises the obstruction loose. That part is untested rather than disproved: it is not chemically absurd, but no source has tested it, and a domestic drain is open to atmosphere at both ends, so there is no obvious reason the gas should preferentially act on the blockage rather than escape past it. A plausible-sounding physical story is not a recommendation.

Background

This hack circulates relentlessly, and the counter-claim — that you should never do it — recurs almost as often as the hack itself. Both deserve an answer, and the honest one is narrower than either. What happens chemically is settled: acetic acid and sodium hydrogen carbonate neutralise each other, giving sodium acetate, water and carbon dioxide. The foam is the gas. When it stops, the acid that could have dissolved scale has been consumed by the base, and the alkali that could have helped with fat has been consumed by the acid. What is left in the pipe is a dilute, near-neutral salt solution. Whether the gas does any mechanical work is a separate question, and no source tests it. An open drain is not a sealed vessel, so the gas has somewhere to go. What IS established is the cause of the problem being treated. Slow kitchen drainage is usually fats, oils and grease solidifying on the pipe wall, and an extension service states that hot water and soap do not eliminate them. A near-neutral salt solution has less claim on fat than soap does.

What it does not do

• The two ingredients neutralise each other, so the mixture is a weaker cleaner than either used alone
• Nothing in the mixture addresses fats, oils and grease, which is what usually slows a kitchen drain
• Whether the gas does any useful mechanical work in a pipe is untested by any source
• It does nothing about a physical obstruction such as hair bound up with soap residue
• It leaves standing liquid in a drain, which matters if any other product is added afterwards

What to use instead

• Dry soil removal — Taking the loose material away before any liquid touches it. Skipped almost universally, and it is the step that decides whether the rest works.
• Household drain — The one thing in a house you clean without being able to see, reach or test it — which is why almost every drain hack is really a guess.