Water is unique due to its remarkable ability to stick to surfaces and materials other than itself. This property, known as adhesion, allows water to wet objects, explaining observations like a spilled drink soaking into a paper towel or dew clinging to a windowpane. This “stickiness” is not a physical glue but a manifestation of the complex electrical structure within the water molecule. Understanding adhesion requires examining the atomic foundation that governs how water interacts with its surroundings.
The Molecular Foundation of Water’s Properties
The water molecule (H₂O) consists of one oxygen atom bonded to two hydrogen atoms. This bent, V-shaped structure is the root of water’s unusual properties. Oxygen is significantly more electronegative than hydrogen, pulling more strongly on the shared electrons in the covalent bonds.
This uneven sharing creates a polar molecule: the oxygen end develops a slight negative charge, and the hydrogen ends acquire slight positive charges. These partial charges turn the water molecule into an electrical dipole, similar to a tiny magnet.
The polarity allows water molecules to form weak, yet numerous, electrical attractions called hydrogen bonds with other charged or polar substances. This powerful electrical attraction between water and a different material is the underlying mechanism of water’s adhesiveness.
The Distinction Between Adhesion and Cohesion
The polarity of the water molecule generates two distinct forces: adhesion and cohesion. Cohesion is the attractive force between molecules of the same substance, resulting from the network of hydrogen bonds between adjacent water molecules.
Adhesion is the attractive force between water molecules and the molecules of a different substance. For water to adhere to a surface, that surface must be polar or electrically charged, allowing hydrogen bonds to form. Water adheres strongly to glass, for instance, because the silicon dioxide molecules are highly polar.
The balance between these two forces dictates how water behaves when contacting another material. If adhesive forces are stronger than cohesive forces, the water will spread out and “wet” the surface, such as a cotton towel. If cohesive forces are stronger, the water pulls itself into a compact droplet, minimizing contact, as seen on a waxed car.
Observable Phenomena Driven by Water’s Adhesiveness
The interactions between adhesion and cohesion are responsible for several visible phenomena. One common example is the formation of the meniscus, the curved surface a liquid forms when confined in a container like a test tube.
In a glass container, water molecules are more strongly attracted to the polar glass walls (adhesion) than to each other (cohesion). This stronger attraction causes the water to “climb” slightly up the sides, creating a characteristic concave curve.
The same interplay of forces drives capillary action, the ability of water to flow upward against gravity through a narrow tube or porous material. Adhesion pulls the water column upward along the tube walls. Cohesive forces then link the water molecules, allowing those pulled up by adhesion to drag the rest of the column along.
This mechanism allows water to be absorbed by paper towels. It is also a fundamental process in the transport of water from the roots to the leaves of plants.