Intermolecular forces are fundamental attractions between molecules that govern how matter interacts. These attractions dictate how a substance behaves in bulk, such as a liquid forming a bead or a solid sticking to a surface. Cohesion and adhesion are the two primary ways these molecular forces manifest, determining how molecules interact with either like or unlike neighbors. Understanding these forces is essential for explaining natural phenomena, from the shape of a water drop to the growth of the tallest trees.
Cohesion: Attraction Between Identical Molecules
Cohesion is defined as the attractive force that exists between molecules of the same substance, causing a material to hold itself together. In the case of water, this powerful attraction is due to hydrogen bonds, which form between the partially positive hydrogen atom of one water molecule and the partially negative oxygen atom of an adjacent one. This dense network of intermolecular bonds gives water its remarkable internal strength.
This self-attraction is most visibly demonstrated in surface tension, which causes the liquid surface to behave like a stretched elastic film. Cohesive forces pull the surface molecules inward, minimizing the total surface area and making the liquid resistant to external force. This effect is why water forms nearly spherical droplets on non-stick surfaces and allows small insects, such as water striders, to walk across the surface. The stronger the cohesive forces, the more the liquid tends to maintain its own shape rather than spread out.
Adhesion: Attraction Between Different Molecules
Adhesion describes the attractive force between the molecules of one substance and the molecules of a different substance. This force is responsible for things “sticking” to one another, occurring when a liquid’s molecules are more attracted to an adjacent surface than they are to each other. The mechanism often involves polar water molecules being attracted to charged or polar regions on the surface of another material, such as glass or cellulose.
A common demonstration of adhesion is the concave meniscus that forms when water is placed in a glass cylinder. Here, the water molecules’ adhesive attraction to the polar glass walls is stronger than their cohesive attraction to each other, causing the water level to climb slightly up the sides. Adhesion also enables substances like paint to adhere to a wall or glue to bond two different materials together. If the adhesive forces are stronger than the cohesive forces, the liquid is said to “wet” the surface, spreading out instead of beading up.
Synergistic Effects: How Both Forces Operate in Nature
Many natural processes require the simultaneous action of both adhesive and cohesive forces working together. This combined effect is clearly seen in capillary action, which is the ability of a liquid to flow in narrow spaces against the force of gravity. Capillary action is dependent on the diameter of the tube or space; the narrower the opening, the higher the liquid will rise.
This process is fundamental to the transport of water within plants, moving water from the roots up through the xylem vessels to the leaves. Adhesion first pulls the water molecules upward along the inner walls of the narrow xylem tubes. As these leading molecules move up, their strong cohesive bonds pull the entire column of water behind them, maintaining an unbroken stream. The cohesive forces hold the continuous water column together under the tension created by the evaporation of water from the leaves, a process called transpiration.