Why is it that when I wash my hands (or other, naughtier body parts) in cold water they seem to rinse cleaner than when I use hot water? It doesn’t matter whether I use bar or liquid soap, hand soap or dish detergent, or what brand of soap it is (I do use vegetable-based glycerin soap for the most part so I don’t know if animal vs vegetable based products is a factor). It also doesn’t matter where I wash my hands. If the water comes from the hot tap, I feel a filmy residue. Under the cold there is none. If I wash with the hot then turn the cold on either with or without turning the hot off) then the film rinses off. The only difference I can identify is the temperature of the water (and possibly the pipe through which it travels althouth the recurrence of the phenomenon at multiple locations leads me to discount that variable as significant). So what’s up with my soap?
What kind of soap are you using?
Water softeners are often hooked up to treat only the hot water. The sodium ions found in soft water don’t form complexes with soap and skin as readily as do the Calcium, Magnesium, Iron ions in hard water. Your skin feels slicker without a coating of metal bearing goo, but the slickness is easily confused with leftover soap.
Semi-Relevant Links
Can soap ever get dirty?
What’s more important in cleaning, soap or hot water?
The mailbag answer (the second link) does say:
but never goes into detail why. My guess is that the solubility of soap in water (see Cecil’s column–the top link) is directly related to the temperature.
I’ve got to admit I’ve never experienced the residue left by hot water as described by in the OP, so I’ve nothining to add there.
Regarding the question of hot vs. cold posed by crc the answer is simple. Pretty much everything, particularly fats, are more soluble in warm water. By using warm water the gunk floats off the surface being cleaned and is then flushed away or wiped off more effectively. This displacement from the surface also helps the surfactant in soap more effectively dissolve fats. It’s the same reason warm water is used to wash dishes.