I am planning on making some homemade soap soon and have read online that breathing in the fume generated from the reaction between lye and water is not recommended because it is "hazardous."

I am a little bit confused because I thought the reaction between lye and water is simply a highly exothermic dissolution reaction and therefore the "fumes" would consist of only steam?

                           NaOH + H2O -> OH- + Na+ + H2O + HEAT

If this fume is corrosive, does that mean there is some gaseous hydroxide as well generated from the heat? Why else would this fume be dangerous?


2 Answers 2


The fumes will contain trace amounts of NaOH. The fumes are microscopic water droplets, and these droplets will still be caustic, as they are essentially a solution of sodium hydroxide.

There is no real chemical reaction going on here, but rather a dissociation of the sodium and hydroxide ions, which is an exothermic process. Solid sodium hydroxide is 'hazardous', the resulting solution is 'hazardous' and the fumes above the dissolving lye are 'hazardous'.


Add the water to the lye - the slow reaction with slow diffusion means there are no fumes. Misinformation about this (reference Soap Queen's alarmist and inaccurate instructions about mixing water and lye) has been pervasive on soap making websites

  • $\begingroup$ Do you have any reference for this? $\endgroup$ Jun 16, 2021 at 20:06

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