Let’s say I buy an X amount of paint, which has a total mass of 5kg (or 5lb). I spread the whole content on my wall and 24hrs later the stuff has “dried” and my wall has taken on a new shade.
So chemically what has actually happened to that liquid that weighed 5kg/lb? Did it change phase to a solid, adhered to my wall, and then added 5kg/lb to the weight of the wall?
In: Chemistry
The simple version is that paint is actually a polymer, so it’s all polymerizing when it dries.
The details depend on what kind of paint, but in general, much of the wet paint is a liquid carrier (usually water and/or an organic solvent) that evaporates off.
So you are adding weight to the wall, but once dry, not as much as you applied.
Depending on what kind of paint, different things happen.
Talking about walls, this is most likely a water-based paint. When these dry, it is mostly just evaporation that occurs, which then allows the polymer in the paint to harden. For latex paint specifically, the hardening (curing) process actually takes days to weeks beyond when it has dried. It will continue getting harder, and thus more durable.
Oil-based paint is a little different as it isn’t water evaporating, but is still a similar concept. There are some compounds in there that will evaporate, but the chemical change happens as a result of exposure to oxygen. This causes the compounds to change and harden. Depending on what kind of painting you are doing with oil-based (for example practical painting vs fine art painting) it can take much longer for these to dry compared to water-based
Paint typically has a level of solvent in it that evaporates away. So your wall doesn’t gain 5 kg of mass it gains 4 and the other kg goes out into the world as a vapor. The solvent dissolves the solid pigments so it’s liquid, but once the solvent is gone the solid pigments stay.
If you ever opened an old can of paint and see the top layer of the can is really watery and the bottom is thick and gooey. That’s why you have to shake and stir it.
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