What Are Adhesion and Cohesion Forces?
- When matter is in a liquid state, the molecules attract each other with cohesive forces. Cohesive forces between molecules hold the liquid together. These attractive forces are also known as van der Waals forces. These electrostatic forces bond water molecules together because the molecules are polar; one end of the molecule has a positive charge and the other end has a negative charge. This separation of charges within a molecule is called a dipole moment.
- The cohesive forces that hold together molecules of a liquid are stronger between the molecules on the surface. Surface molecules do have molecules above them to bond with. The bonds of cohesion between a surface molecule and its neighbors are stronger than the forces that hold the molecules together below the surface. The stronger cohesive forces create a film on the surface of the liquid that makes it more difficult to pass an object liquid than an object that is already submerged in the liquid.
- Adhesion is the force between the molecules in a liquid and the container that hold it. Because water molecules are polar, the positive end can form a bond with a negatively charged surface. In a glass tube, adhesive forces pull a liquid upward on the walls of the tube while gravity pulls the liquid downward. These opposing forces cause a crescent-shaped meniscus, or upward curve, to form on the surface of the liquid.
- Capillary action occurs when adhesive forces between a liquid and another surface are stronger than the cohesive forces between the molecules of the liquid. Adhesion causes the liquid to move upward along the surface of a tube. The height that the adhesive forces can pull water upward depends on the circumference of the tube. The thinner the tube, the higher the water will climb.
