system
Given the following post, respond as if you're commenting on Reddit:
human
Can someone explain me the chemistry behind stains and why some stains cannot be removed?
gpt
**TL;DR - Some stains are held in place by weak chemical bonds but others are held by strong covalent chemical bonds that won't be broken by traditional solvents or surfactants (soaps).**

In principle a stain is a bond between the surface of the original material and the molecules of the stain agent. That bond strength can vary greatly depending on the original surface and the stain agent. Some surfaces have very little microscopic surface area (e.g. a laminate countertop) whereas others like a cotton thread (hundreds of individual fibers) have a lot of area. Some materials are also porous (e.g. wood) and that adds extra surface area.

The stain agent reacts with the available surface area in some way. An example of a weak interaction might be a solvent that dries up and leaves a film of stain on surface, like most colored liquids dried on metal. If you add solvent the stain will move from the surface back into the liquid. If the surface is porous or has a lot of area, then it might take a lot of time for the stain to all dissolve, because the solvent has to get into the pore and the stain has to then get out of the pore.

Some stain materials will form stronger bonds with the original surface and won't just dissolve into their original solvent. In those cases people add a surfactant (e.g. soap) or cosolvent to help increase the solubility of the stain in the cleaning solution vs the adsorption of the stain to the surface.

Sometimes the reaction is so strong that most normal solvents won't break the bond between the stain and the surface. This is the case with most dyes which are engineered to covalently bond to fabrics and not dissolve into a solvent poured onto the fabric. If the bond is strong enough it may not be possible to break the surface-dye bond without also damaging the material that makes the surface. An example of this would be bleach which is good at destroying dye molecules, but also eats away at the underlying material - although at a much slower rate.