Question

Define surface tension. What is the relationship between the intermolecular forces that exist in a liquid and its surface tension?

Short answer

Surface tension is the property of a liquid that arises due to cohesive forces between the liquid molecules and resists force. Stronger intermolecular forces yield higher surface tension as the forces unbalance at the liquid's surface resulting in the liquid behaving as if its surface were covered by a stretched elastic membrane.

Step-by-step solution

Define surface tension

Surface tension is a physical phenomenon that occurs at the surface of a liquid due to the cohesive forces between the liquid molecules. It resists force and causes liquid to take the shape of a droplet, as it tries to minimize its surface area. In terms of energy, it is the energy required to increase the surface area of a liquid by a unit of area.

Describe the relationship between intermolecular forces and surface tension.

The intermolecular forces in a liquid play a major role in determining its surface tension. Stronger intermolecular forces lead to higher surface tension. This is because the molecules at the surface of the liquid are connected to other molecules in the liquid below them by attractive forces. The molecules in the middle of the liquid are surrounded by other molecules and are equally attracted in all directions. However, the molecules at the surface are not surrounded by molecules above them, as those molecules in the middle. This imbalance of forces pulls the molecules at the surface towards the interior of the liquid. This results in the liquid behaving as if its surface were covered by a stretched elastic membrane - which is described as surface tension.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction and repulsion between particles, which include ions, atoms, and molecules. They vary in strength from relatively weak van der Waals forces to the much stronger hydrogen bonds and ionic bonds.

Understanding intermolecular forces allows us to comprehend a number of physical properties of substances, such as boiling and melting points, vapor pressure, and surface tension. In the context of liquids, these forces influence how molecules interact with one another. When molecules are closer together and the intermolecular forces between them are strong, such as in the case of water due to hydrogen bonding, the liquid has a higher surface tension.

Cohesive Forces in Liquids

Cohesive forces are the intermolecular forces that bind similar molecules together. In liquids, these forces are what enable it to resist external forces and maintain a definite volume, yet allowing it to flow. The more pronounced these cohesive forces, the greater the viscosity and surface tension of the liquid.

Cohesive forces are chiefly responsible for phenomena such as water forming into droplets that hold together rather than dispersing into a thin sheen. These droplets are a result of the liquid molecules pulling toward each other, minimizing the surface area, and creating a spherical shape which has the least surface area for a given volume.

Physical Phenomena in Liquids

Liquids exhibit several physical phenomena that are a direct result of intermolecular and cohesive forces. Surface tension is one such phenomenon, representing the elastic tendency of a liquid's surface. Other phenomena include capillary action, where liquid rises or falls in a narrow tube against the force of gravity, and viscosity, which is the measure of a liquid's resistance to flow.

The physical behavior of liquids under different conditions is a fascinating study. When heated, the kinetic energy of the molecules increases, reducing the effects of cohesive forces and thereby lowering the surface tension. Similarly, the addition of surfactants, which are substances that disrupt intermolecular forces, can significantly decrease the surface tension of liquids.