FYUGP B.Sc Chemistry Semester 1: Unit 8 - Liquid State

This unit explores the structure and physical properties of liquids, including vapour pressure, surface tension, viscosity, and micelle formation. These notes are designed to help students understand the behavior of liquids and their applications in everyday life.

1. Qualitative Treatment of the Structure of the Liquid State

Liquids have a structure intermediate between solids and gases:

• Short-Range Order: Molecules are close together but not in a fixed arrangement.
• Fluidity: Molecules can move past each other, allowing liquids to flow.
• Intermolecular Forces: Stronger than in gases but weaker than in solids.

2. Physical Properties of Liquids

Key physical properties of liquids include:

a. Vapour Pressure

• Definition: The pressure exerted by the vapour in equilibrium with its liquid at a given temperature.
• Factors Affecting Vapour Pressure: Temperature and intermolecular forces.

b. Surface Tension

• Definition: The energy required to increase the surface area of a liquid.
• Measurement: Using a stalagmometer or capillary rise method.

c. Coefficient of Viscosity

• Definition: A measure of a liquid's resistance to flow.
• Measurement: Using a viscometer (e.g., Ostwald viscometer).

Fig 1: Surface Tension in Liquids

3. Temperature Variation of Viscosity

Viscosity of liquids decreases with increasing temperature due to increased molecular motion. In contrast, the viscosity of gases increases with temperature.

Fig 2: Temperature Variation of Viscosity in Liquids and Gases

4. Effect of Solutes on Surface Tension and Viscosity

Adding solutes can alter the surface tension and viscosity of liquids:

• Surface Tension: Solutes like detergents reduce surface tension.
• Viscosity: Solutes generally increase viscosity due to increased intermolecular interactions.

5. Cleansing Action of Detergents

Detergents clean by forming micelles, which trap dirt and oil:

a. Micelle Formation

• Structure: Spherical aggregates of detergent molecules with hydrophobic tails inward and hydrophilic heads outward.
• Function: Encapsulate grease and dirt, allowing them to be washed away.

b. Critical Micelle Concentration (CMC)

• Definition: The minimum concentration of detergent required for micelle formation.
• Importance: Determines the effectiveness of detergents.

Fig 3: Micelle Formation and Cleansing Action

Practical Applications

• Industrial Processes: Understanding viscosity and surface tension is crucial in industries like petroleum, cosmetics, and food processing.
• Environmental Science: Micelle formation is used in environmental cleanup of oil spills.
• Everyday Life: Detergents and soaps rely on micelle formation for cleaning.

These notes provide a comprehensive understanding of the liquid state and its properties. Practice problems and real-world applications will help reinforce these concepts.