Chemistry Lecture: Solubility and Miscibility

Introduction

• Discussing solubility and miscibility in this lecture.
• Both terms often used interchangeably but have technical differences.
  • Soluble: part of a substance can dissolve in another.
  • Miscible: substances can mix in all proportions without separation.

Examples

• Table Salt (NaCl) and Water:
  • Soluble but not miscible.
  • Small amounts dissolve; excessive amounts lead to undissolved salt.
• Isopropanol (Rubbing Alcohol) and Water:
  • Completely miscible in all proportions.

Key Concepts

• Like Dissolves Like:
  • Compounds with similar polarity and structure dissolve in each other.
  • Polar molecules dissolve in polar solvents (e.g., water and ethanol).
  • Nonpolar molecules dissolve in nonpolar solvents (e.g., cyclohexane and hexane).
  • Polar and nonpolar do not mix (e.g., water and hexane).

Polarity and Solubility

• Polarity:
  • Not binary; a gradient exists from very polar (e.g., water) to nonpolar (e.g., alkanes).
• Ethanol:
  • Lies between polar and nonpolar, can dissolve in both water and hexane.

Practical Examples

• Solubility of Alcohols:
  • Methanol vs. Hexanol:
    • Methanol is more soluble in water due to smaller nonpolar region.
    • Hexanol has a larger nonpolar region, reducing solubility in water.
  • In Hexane:
    • Methanol is only partially soluble.
    • Hexanol dissolves completely due to similar structure and larger nonpolar region.

Application in Class

• Focus on understanding the relationship between polarity and solubility.
• Will not require exact predictions but understanding trends and relationships.
  • Example questions might involve comparing solubility of different compounds in water or hexane.