Question

What is surface tension? What is the relationship between intermolecular forces and surface tension? How does surface tension change with temperature?

Short answer

Surface tension is the force at the surface of a liquid due to intermolecular forces. Stronger intermolecular forces increase surface tension, while higher temperatures decrease it.

Step-by-step solution

Understanding Surface Tension

Surface tension is a physical property that represents the force acting on the surface of a liquid, which makes it behave like a stretched elastic sheet. It arises due to the cohesive forces between liquid molecules, particularly those at the liquid's surface.

Relationship with Intermolecular Forces

The stronger the intermolecular forces between the molecules of a liquid, the greater the surface tension. This is because molecules at the surface of a liquid experience a net inward force, which contributes to the minimization of surface area.

Effect of Temperature on Surface Tension

As temperature increases, the kinetic energy of the molecules also increases, which tends to reduce the effectiveness of intermolecular forces. Consequently, with higher temperatures, the surface tension of a liquid generally decreases.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between molecules in a substance. These forces play a crucial role in dictating many physical properties of substances, including surface tension.

• Types of Intermolecular Forces: There are several types of intermolecular forces which include hydrogen bonding, dipole-dipole interactions, and London dispersion forces. Depending on the type and strength of these forces, the behavior of the liquid can be significantly different.
• Importance in Surface Tension: The molecules at the surface of a liquid do not have other like molecules on all sides of them and as a result, they cohere more strongly to those directly associated with them on the surface. The outcome is that the liquid sort of "contracts" and this creates surface tension.
• Stronger Intermolecular Forces: When these forces are stronger, they result in a higher surface tension because the molecules pull closer together at the surface.

Cohesive Forces

Cohesive forces are the forces that hold molecules of the same substance together. These forces are vital in explaining why liquids act the way they do, especially concerning surface tension.

• Role in Liquids: In a liquid, cohesive forces are responsible for the surface tension that occurs. This is because these forces try to pull the molecules together, causing the surface of the liquid to resist being penetrated.
• Effect on Surface Behavior: On the surface, cohesive forces lead to a net inward attraction of molecules. They make the surface molecules denser compared to those in the bulk of the liquid.
• Impact on Droplets: These cohesive forces are what give rise to the spherical shape of small droplets as the liquid seeks to minimize surface area.

Cohesive forces are just one type of the broader category of intermolecular forces but play a direct role in forming the surface tension of liquids.

Effect of Temperature on Liquids

Temperature has a direct impact on the molecular interactions within a liquid, influencing properties like viscosity and surface tension.

• Impact on Kinetic Energy: As the temperature increases, molecules in the liquid gain kinetic energy. They move more vigorously and this increased movement tends to disrupt the intermolecular forces holding them together.
• Reduction of Surface Tension: With increased temperature, these molecular movements weaken the cohesive forces that contribute to surface tension, usually leading to a reduction in surface tension as the temperature rises.
• Practical Implications: This relationship means that in warm conditions, liquids spread more easily and have lower resistance to forces like stirring or pouring.

By understanding how temperature affects these forces, one can predict changes in a liquid's behavior under different thermal conditions.