Question

I'm 16. I study chemistry in college along with biology, physics, and maths, and I have a general interest in all of them.

I always found this interesting: sometimes when I left the bathtub, even when I forgot to use soap, the bath would still have a somewhat noticeable, murky tint in the water, indicating that something had dissolved in it, even though I didn't take the time to rub it off manually. When we get in the bath, how does the dirtiness (small amounts of mud, bacteria, pen markings etc.) dissolve in the bath alone without any of our skin dissolving as well so that we can leave clean? Maybe dead skin might dissolve, that would make sense, is there an adaptive mechanism on the skin involved in this sort of process of hygiene? The same thing would happen if I put any other object in the water with something on its surface and the object would come out clean when the thing on the surface dissolved.

How does the water break the attraction? Some sort of cohesive force, between the atoms/molecules of the thing (dirtiness) on the surface of the object and the atoms/molecules of the object (our bodies) and separate them to make it dissolve? And how can it differentiate between the cohesion of the skin and the dirtiness (what determines solubility, if both polar and non polar molecules on our skin will dissolve in the water)?

Answer 1

The apparent cleaning effect of water alone is due to its ability to rinse away loosely attached dirt without dissolving the skin, whereas soap's amphiphilic structure makes it more effective at removing oils and grime. Dead skin cells may also be rinsed away, adding to the murky water, but the skin remains intact as it's more resilient.

Step-by-step explanation:

When you take a bath, the water alone can sometimes remove certain types of grime or surface dirt from your skin or other objects. This can happen without the use of soap because some of this dirt is not strongly attached to the skin or object surfaces and can be rinsed away with water. Water's properties, such as its ability to wet surfaces and its action as a solvent, can help in the cleaning process. However, water alone is not very effective at removing oily substances, which is why soap is commonly used.

Soap has a special structure that makes it an excellent cleaning agent. The molecules of soap are amphiphilic, meaning they have both a hydrophilic (water-loving) and a hydrophobic (water-fearing) part. The hydrophobic part is the long hydrocarbon chain that can interact with nonpolar substances like oils, while the hydrophilic part is polar and interacts with water. When soap is added to water, the hydrophobic tails of the soap molecules interact with hydrophobic particles of dirt or grease, forming micelles that encapsulate the dirt and allow it to be washed away as the micelles disperse in the water.

This mechanism of cleaning with soap and water is different from the natural process of skin shedding. Your skin continually sheds dead skin cells, which can also be rinsed away during a bath, contributing to the murky water you mentioned. However, your skin does not dissolve in the water because the bonds holding your skin cells together are stronger, and the skin is designed to be resilient against the aqueous environment.