The question of whether water is wet has long been a popular, paradoxical debate. Physical chemistry offers a definitive answer, clarifying the true nature of this common liquid. The resolution rests on a precise scientific definition of “wet” and an understanding of water’s molecular behavior. By examining the forces that govern liquid-solid interactions, we can determine why water is the agent of wetness but not the recipient of the description itself.
Understanding the Condition of Being Wet
The condition described as “wetness” is not an intrinsic property of a liquid, but rather a description of an interaction. Scientifically, something is designated as wet when a liquid adheres to the surface of a solid material and remains there. Therefore, wetness requires two components: the liquid doing the wetting and an external solid surface being wetted.
This concept relies on adhesion, the attractive force between molecules of two different substances, such as water and glass. When water is poured onto a surface, the state of being wet describes the external object and its retention of the liquid. A singular substance, like a body of water, cannot be described as wet because it lacks the necessary external surface to which it must adhere.
The degree to which a surface becomes wet depends on a balance of molecular forces. If the attractive forces between the liquid and the solid surface are strong, the liquid will spread out and coat the surface. Conversely, if these forces are weak, the liquid will minimize contact and form spherical droplets. Wetness is a relational description of a successful liquid-solid bond.
The Internal Forces of Water
To understand water’s role in this interaction, we must look at its molecular structure. A water molecule (H2O) consists of one oxygen atom bonded to two hydrogen atoms, forming a bent shape. Due to the oxygen atom’s higher electronegativity, it attracts the shared electrons more strongly than the hydrogen atoms.
This uneven sharing of electrons creates a polar molecule, where the oxygen end acquires a slight negative charge and the hydrogen ends acquire slight positive charges. These opposing charges allow water molecules to form weak attractions with neighboring water molecules, known as hydrogen bonds. These bonds are responsible for holding the liquid together.
The collective strength of these hydrogen bonds gives water a powerful internal attraction force called cohesion. Cohesion is the force between molecules of the same substance, causing them to stick together. This force gives water its high surface tension, allowing it to form a “skin” on its surface or gather into drops.
The interplay between cohesion and adhesion determines how water behaves when it encounters a solid surface. Cohesion pulls water molecules inward toward the bulk of the liquid, while adhesion pulls them outward toward the surface. Water’s strong cohesive forces mean that its molecules prefer to stick to each other, which is why it beads on hydrophobic materials.
Applying Chemistry to the Paradox
The final answer to the paradox emerges from applying the definition of wetness to water’s molecular mechanics. Wetness is the state of a solid surface that has a liquid adhering to it. Therefore, water is the substance that causes the wetness in an external material.
Water molecules are bound to each other by cohesion, but they cannot adhere to themselves in the way they adhere to a different substance. For water to be wet, it would have to be coated in a layer of another substance, or it would have to adhere to itself, which is a contradiction of the term’s scientific use. The description “wet” is an adjective for a surface, not a noun for the liquid itself.
The paradox exists because we often associate the sensation of being covered in water with the word “wet.” However, when a scientist uses the term, they are referring to the molecular dynamics where the adhesive force between water and a surface is strong enough to overcome water’s internal cohesive forces. Since water is the source of the adhesion, it acts as the wetting agent.
Ultimately, water is not wet; it simply has the ability to make other things wet. It is the molecular force—the hydrogen bonding creating cohesion and the subsequent ability to engage in adhesion—that allows water to transfer the state of wetness to an external object. The liquid is the performer of the action, not the object on which the action is performed.