Cohesion, Adhesion, Surface Tension, and Capillary Action: Study Notes

Cohesion and Adhesion

Introduction to Cohesion and Adhesion

Cohesion and adhesion are fundamental concepts in chemistry that describe how molecules interact with each other and with surfaces. Cohesion refers to the attraction between molecules of the same substance, while adhesion refers to the attraction between molecules of different substances.

• Cohesion: Responsible for phenomena such as surface tension in water.

• Adhesion: Explains why water can "stick" to glass or other surfaces.

• Example: Water droplets forming beads on a waxed car due to cohesion; water climbing up a paper towel due to adhesion.

Intermolecular Forces

Intermolecular forces are the forces that mediate interaction between molecules, including hydrogen bonds, dipole-dipole interactions, and London dispersion forces.

• Hydrogen Bond: Strong attraction between molecules containing hydrogen bonded to electronegative atoms (e.g., O, N, F).

• Dipole-Dipole Interaction: Attraction between polar molecules.

• London Dispersion Forces: Weak attractions present in all molecules, especially nonpolar ones.

• Example: Water molecules exhibit hydrogen bonding, leading to high cohesion.

Surface Tension and Capillary Action

Surface Tension

Surface tension is the energy required to increase the surface area of a liquid due to cohesive forces among liquid molecules. It allows liquids to resist external force and form droplets.

• Definition: The force per unit length acting along the surface of a liquid.

• Formula: $\gamma = \frac{F}{L}$, where $\gamma$ is surface tension, $F$ is force, and $L$ is length.

• Example: Water has high surface tension due to strong hydrogen bonding.

Capillary Action

Capillary action is the ability of a liquid to flow in narrow spaces without external forces, resulting from the combination of cohesive and adhesive forces.

• Definition: The movement of liquid up a thin tube due to adhesion to the tube walls and cohesion among liquid molecules.

• Formula: $h = \frac{2\gamma \cos \theta}{\rho g r}$, where $h$ is height, $\gamma$ is surface tension, $\theta$ is contact angle, $\rho$ is density, $g$ is gravity, and $r$ is tube radius.

• Example: Water rising in a glass capillary tube.

Comparing Polar and Nonpolar Liquids

Properties of Polar vs. Nonpolar Liquids

Polar liquids, such as water and glycerin, have molecules with partial charges, leading to strong intermolecular forces. Nonpolar liquids, such as hexane and mineral oil, lack these charges and have weaker forces.

Property	Polar Liquids	Nonpolar Liquids
Intermolecular Forces	Hydrogen bonds, dipole-dipole	London dispersion
Surface Tension	High	Low
Capillary Action	Strong	Weak
Adhesion to Glass	Strong	Weak

Applications and Examples

Real-World Applications

• Biology: Capillary action helps plants transport water from roots to leaves.

• Chemistry: Surface tension affects reactions at liquid surfaces.

• Everyday Life: Water droplets forming beads, cleaning with paper towels, ink movement in pens.

Summary of Key Concepts

• Cohesion and adhesion are responsible for many observable phenomena in liquids.

• Surface tension is higher in polar liquids due to strong intermolecular forces.

• Capillary action depends on both cohesion and adhesion, and is strongest in polar liquids.

• Polar substances share strong intermolecular attractions; nonpolar substances rely on weaker forces.