Why You Should Never Mix Different Drain Cleaners

This article was originally posted on RealClearScience.

Your humble correspondent had quite the predicament over the weekend. I had burned one of those fake logs in the fireplace, and after the fire went out I closed the flue and went to bed. Only the fire didn’t go out. I awoke the next morning to a condo full of smoke and my wife in a semi-panic.

Airing out the house proved problematic. A storm was moving through Seattle (great timing!). Thus, opening the front and back doors created a miniature tempest inside our home. Worse, wind kept coming down into the fireplace, blowing ash all over the house. And we couldn’t close the flue because the fake log was still smoldering.

So, we did the only thing we could think of: We threw the burning log in the bath tub and doused it with water. That worked, but it created an entirely new problem. As the fake log disintegrated, it clogged the tub. Unfortunately, the sodium hydroxide-based drain cleaner we used to unclog it wasn’t up to the job.

Because I hate shopping, I asked my wife to please go to the store to get some more drain cleaner. But, since I minored in chemistry, I chose to give her a brief lecture first. (I’m sure she appreciated it.) I reminded her not to buy any drain cleaner that contained something other than sodium hydroxide. Why?

Drain cleaners aren’t created equal. Some are bases (e.g., sodium hydroxide), others are bleach (i.e., sodium hypochlorite), and others are acids (e.g., sulfuric acid or hydrochloric acid). Not all of these chemicals play nicely together. For instance, here is the reaction of sulfuric acid with sodium hydroxide:

Sulfuric acid + sodium hydroxide (base) –> sodium sulfate (salt) + water
[H₂SO₄ + 2 NaOH –> Na₂SO₄ + 2 H₂O]

The products are salt and water. That sounds harmless enough, but acid-base reactions are alsoexothermic. That means they give off heat. If you mix together an acid drain cleaner with a base drain cleaner, there is a good chance that scalding hot water will come shooting out of the drain. And since you probably didn’t mix the acid and base in stoichiometric proportions, the hot water will probably be tainted with leftover acid or base. Unpleasant.But there are worse reactions out there.

Many people use Liquid Plumr, a drain cleaner that contains both sodium hydroxide and sodium hypochlorite (bleach). If you mix bleach with a drain cleaner containing hydrochloric acid, you are in for a real treat:

Hydrochloric acid + bleach –> water + table salt + chlorine gas
[2 HCl + NaClO –> H₂O + NaCl + Cl₂]

Chlorine gas at a low concentration is an irritant. At a high concentration, it can kill you. It was actually used as a chemical weapon during World War I. Here’s a video of the reaction between hydrochloric acid and bleach. The yellowish gas you see is chlorine. (Do I really need to tell you not to try this at home?)

If you have ever wondered why drain cleaner labels warn against mixing different chemicals together, now you know why!

(Image: Skatebiker via Wikimedia Commons)