What is a particle? (From Colloid Science)

　Ultrafine particles (a few nm) of oxides of metallic elements that are dispersed in water are colloidal particles.

　Soluble proteins of macromolecular organic matter, which appear to dissolve in water, are also colloidal particles.

The former are very much particles.

In the latter, the organic molecules are much smaller (a few nm in size) and are water-soluble and therefore compatible with water molecules. They do not grow in aggregation like hydrophobic particles in water. Nevertheless, from the perspective of colloid science, the latter are hydrophilic molecular colloidal particles.

In colloid science, ions and molecules (substances that pass through a semipermeable membrane) of approximately 1 nm or less are not particles※.

※  As explained below, "not particles" does not = "dissolved material"!

So what is "dissolved"?

Dissolved is a solid or gas dissolved in a solvent and present.

Dissolution is the process of putting a solid or gas into a solvent and keeping it in some form (ionic, molecular, or solid) homogeneous in the solvent for an extended period of time.

Colloids such as milk are also colloidal solutions because they remain homogeneous for long periods of time. Hydrophilic colloidal particles dissolve in water.

Thus, in physical chemistry and engineering, we do not dichotomize matter in water by particulate and dissolved.

This is where the concept of the distinction between particulate and dissolved matter differs in oceanography and other fields of the aquatic environment.