7. Structure of liquids. Molecular pressure and surface tension. Additional (Laplace) pressure. Embolism.

Introduction

• Molecular structure of bodies.
• Molecular structure refers to the arrangement of atoms within a molecule and the chemical bonds holding them together. This structure significantly influences how molecules interact with each other.
• Field of Molecular Action:
• It's a conceptual sphere surrounding a molecule that represents the effective range of intermolecular forces (IMFs). These forces are weak compared to the intramolecular forces (bonds) within a molecule, but they play a crucial role in determining various physical properties like boiling point, surface tension, and viscosity.
• Types of Intermolecular Forces:
• There are two main types of IMFs that act within the field of molecular action:
• Cohesive Forces: These attractive forces act between identical molecules. They are responsible for holding molecules together in a liquid or solid state. The strength of cohesive forces determines properties like boiling point – the higher the cohesive forces, the more energy it takes to overcome them and transition from liquid to gas.
• Adhesive Forces: These attractive forces act between different molecules. They are responsible for the adhesion between unlike molecules, such as glue sticking to paper. While weaker than cohesive forces, they still play a role in various phenomena like wetting and capillary action.
• Cohesion Pressure:
• The concept of cohesion pressure (Pc) you mentioned relates to the strength of cohesive forces. It's a theoretical pressure that arises due to the attractive forces between molecules within a liquid. The equation Pc = Fk/A represents this pressure, where:
• Pc: Cohesion pressure
• F: Intermolecular force between two molecules
• k: Number of nearest neighbors a molecule interacts with (coordination number)
• A: Area of contact
• Surface tension of liquids
• Surface tension is a property of liquids arising from the intermolecular forces between their molecules. It creates a kind of "invisible skin" on the surface that tends to minimize its area.
• Force per Unit Length:
• The equation F = σ/L, accurately describes surface tension.
• F: Force acting on the surface (in Newtons)
• σ (sigma): Coefficient of surface tension, representing the force per unit length (in N/m) acting along the surface
• L: Length of the line along which the force acts (in meters)
• Surface Tension in Action:
• Floating Objects: Objects less dense than the liquid can rest on the surface due to the upward buoyant force counterbalanced by the downward force of gravity. The surface tension acts like a trampoline, supporting the object's weight.