The question of whether water is wet moves beyond a simple linguistic puzzle and into the fundamental physics and chemistry of liquids. To resolve this debate, one must adopt the precise definitions used by scientists to describe the interaction between a liquid and a solid surface. The answer lies in understanding water’s molecular structure and the forces that govern how it interacts with other materials. Exploring the unique properties of the water molecule reveals why water is the agent of wetness, but cannot be wet itself.
What It Means For Something To Be Wet
Scientifically, wetness is not an inherent property of a liquid, but rather a state or condition that a solid object or surface achieves. For an object to be considered “wet,” it must be coated or covered by a layer of liquid that adheres to its surface. This definition establishes a necessary relationship between two different substances: the liquid and the object it is coating. Wetting is therefore a process that requires the liquid to spread out and maintain contact with a foreign surface, such as a piece of fabric or human skin.
This distinction is important because it means a liquid must be able to stick to something else to cause the state of wetness. A liquid cannot coat or cover itself in the manner required by this definition, just as a hand cannot shake its own hand. When a liquid is placed on a surface, the degree to which that surface becomes wet is determined by molecular forces. This foundational definition restricts the term “wet” to the solid object that has been covered, rather than the liquid that is doing the covering.
The Polarity of Water
The unique behavior of water, including its ability to cause wetness, stems from its molecular structure. A water molecule ($\text{H}_2\text{O}$) consists of two hydrogen atoms bonded to one oxygen atom. Oxygen is highly electronegative, meaning it pulls more strongly on shared electrons than hydrogen. This unequal sharing creates a polar molecule, where the oxygen atom develops a slight negative charge ($\delta^-$) and the hydrogen atoms develop slight positive charges ($\delta^+$).
This polarity turns each water molecule into a dipole with distinct positive and negative ends. The partial positive charge on one molecule is attracted to the partial negative charge on a neighboring molecule, forming a hydrogen bond. Although individually weak, the sheer number of these bonds creates a strong, constantly shifting network that holds liquid water together. This powerful internal attraction directly influences the process of wetting.
Adhesion Versus Cohesion
The process of wetting is governed by a competition between two intermolecular forces: cohesion and adhesion. Cohesion is the attractive force between molecules of the same substance, driven in water by the network of hydrogen bonds. These cohesive forces cause water molecules to cluster together, giving water high surface tension and causing droplets to form a spherical shape.
Adhesion is the attractive force between molecules of different substances, such as water and a solid surface. For a surface to become wet, the adhesive forces between the water molecules and the surface must overcome the cohesive forces holding the water together. If adhesion is stronger than cohesion, the water spreads out and adheres to the surface, resulting in wetting. This occurs on hydrophilic, or water-attracting, surfaces like glass, where water flattens out.
If the cohesive forces are stronger, the water molecules prefer to stick to each other rather than the surface. In this case, the water beads up into distinct droplets and minimizes contact with the surface. This is characteristic of hydrophobic, or water-repellent, materials like wax or certain fabrics. The ratio of adhesion to cohesion determines whether a liquid will coat a surface or simply roll off.
The Agent of Wetness
The scientific definition of wetness requires a liquid to adhere to a separate, solid surface. Water, being the liquid itself, cannot adhere to itself in the necessary manner to satisfy this condition. It is the network of cohesive hydrogen bonds that defines water as a liquid, but these same bonds prevent it from forming the distinct liquid-on-solid coating that defines the state of wetness.
Water is correctly understood as the agent that causes the state of wetness in other materials. It possesses the molecular properties—specifically its strong polarity and capacity for adhesion—that allow it to coat and cling to solid surfaces. Since wetness is a relational property requiring two substances, water acts as the tool that imparts the condition to an object, but it does not possess the condition itself. Therefore, water makes things wet, but by the precise definition of the term, water is not wet.