A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Does soap need time to work?

February 24, 1995

Dear Cecil:

When I was a little girl my Daddy used to say, Edna, girl, when you're in the shower, leave the soap on you for a little while. Soap needs time to work, so you leave it on you so's you don't stink like your mama. I follow his advice to this day but sometimes, like in the women's showers after aerobics, all the women look at me funny, standing there motionless, all soapy, whistling. Cecil, there's no instructions on soap, and I don't know who else I can ask. Does soap need time to work?

Cecil replies:

Your Daddy sounds like quite a character, Edna. I bet he's the life of the trailer park. I had your question posted to the net and the consensus is that, with the possible exception of antibacterial soaps, the time you spend soaping and scrubbing is more than sufficient for soap to do its thing without your having to wait additional time. Although I did get this caveat from Tom M. of the University of Oregon: "If you're using detergents to permeabilize a sample for immunohistochemistry or in-situ hybridization, time is very important, as the rate of diffusion into and out of the sample is an important consideration. Others should be consulted about detergent uses in protein purification/analysis or other extraction protocols." Maybe that was what Daddy had in mind.

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