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Why is baking soda slippery?
July 5, 2012 11:53 AM   Subscribe

Solutions of baking soda (sodium bicarbonate) and potassium metabisulfite in water are both slippery to the touch. Why is that?
posted by Prince_of_Cups to Science & Nature (3 answers total) 2 users marked this as a favorite
 
Alkaline solutions are slippery or soapy to the touch, due to the saponification of the fatty substances on the surface of the skin.
posted by fake at 11:58 AM on July 5, 2012 [3 favorites]


Fake has it, but just to confirm: those are both alkaline solutions, and they'll turn the oil on your skin into soap which makes them feel slippery.

The industrial version of this is classically done with lye (sodium hydroxide) and rendered animal fat or plant oils to make washing soap but there's a whole suite of related reactions and the ones that happen on your hands with baking soda and potassium metabisulfite are among them. You will also experience this if you get bleach on your hands, because bleach is a solution of one or another chlorine-based alkali, typically sodium hypochlorite.

Note that stronger alkalines (bases) will give you chemical burns and will saponify your flesh as well as just the oil on the surface of your skin. This is why you want to wear gloves when working with bleach or dran-o or other things like that.
posted by Scientist at 12:05 PM on July 5, 2012 [3 favorites]


To be a little more specific about why that skin-oil soap is slippery:

Soap molecules are long hydrocarbon chains (which are hydrophobic, which pretty much means "won't mix with water") with a hydrophyllic (water-loving) bit on one end. When you mix them with water, these molecules self-organize into big spherical bunches with the hydrophobic tails on the inside, away from the water, and the hydrophillic heads on the surface of the sphere, facing the watery environment. These structures are called micelles. In that illustration, I think the micelle is organized around a little bit of greasy dirt, which illustrates how soap gets you clean.

But, back to the slipperiness. These micelles act like tiny ball bearings in the water, reducing the friction between your thumb and forefinger. It's probably more complicated than this--I imagine that the micelles disperse and reform--but the critical point is that they are filling microscopic surface features and preventing the intimate contact which allows them to "stick"/experience friction.
posted by pullayup at 3:00 PM on July 5, 2012 [6 favorites]


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