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Colligative Properties

• Freezing Point Depression: The freezing point of a solution is lower than that of the pure solvent.

• Boiling Point Elevation: The boiling point of a solution is higher than that of the pure solvent.

• These changes occur irrespective of the type of solvent used (e.g., water, ethanol).

Conceptual Understanding

• Pure Solvents:

  • Have higher freezing points and lower boiling points.

• Impurity Interaction with Solvents:

  • Adding impurities (like salt) interferes with the freezing and boiling processes.

• Freezing Point Explanation:

  • For water, molecules are held together by intermolecular forces.

  • Cooling lowers kinetic energy, causing molecules to get closer and form ice.

  • With added impurities, some water molecules become busy dissolving these impurities, preventing them from forming the solid state easily, hence requiring a stronger decrease in temperature to freeze.

• Practical Application:

  • In snowy states, salt is used to prevent the formation of ice on roads by lowering the freezing point.

Boiling Point Explanation

• Boiling Process:

  • Boiling occurs when the vapor pressure of a liquid matches atmospheric pressure, allowing molecules to escape into the gas phase.

• Effect of Impurities:

  • Introduction of ionic or non-ionic compounds creates additional intermolecular forces.

  • These additional forces (like ion-dipole interactions) increase the energy required to generate sufficient vapor pressure, leading to a higher boiling point.

Phase Diagrams

• Changes in phase diagrams of solutions:

  • Depression in freezing point shifts left.

  • Elevation in boiling point shifts right.

Calculation of Colligative Properties

• Formulas for freezing point depression and boiling point elevation:

  • Using Kf (freezing point constant) and Kb (boiling point constant) based on the type of solvent.

  • M (molality) is used to find the concentration of the solution.

• Understanding dissociation in solutions helps predict colligative property changes:

  • Van't Hoff factor (i): accounts for the number of particles produced by a solute in solution.

Comparison of Solutions

• Colligative property example calculation:

  • Aqueous Urea:

    • 0.01 molarity results in a freezing point depression of -0.0186 °C.

  • Sodium Chloride (electrolyte):

    • Same molarity but dissociates into ions, causing a greater freezing point depression to -0.0361 °C.

• General Rule: Ionic compounds typically cause a greater change in colligative properties compared to non-electrolytes due to their ability to dissociate into multiple particles.

Determining Van't Hoff Factor

• To improve accuracy in boiling point elevation and freezing point depression calculations, the Van't Hoff factor can be used, which often requires experimental data.

Types of Mixtures: Solutions vs Colloids

• Solutions:

  • Homogeneous mixtures where particle size is less than 1 nm.

• Colloids:

  • Heterogeneous mixtures with larger particles (between 1 and 1000 nm) which can be visually identified.

  • Examples include emulsions (liquid-liquid) like oil and water, and aerosols (solid or liquid in gas).

Tyndall Effect

• Definition: Scattering of light by particles in a colloid results in the Tyndall effect.

• This effect is absent in true solutions due to their small particle size.

Summary of Colloidal Mixtures

• Suspensions have large solid particles in a liquid.

• Emulsions consist of liquid in liquid (e.g., oil and water, milk).

• Understanding the characteristics of each mixture type is critical for practical applications.