Mixtures and Solutions Overview

Mixtures and Solutions

• Solutions, colloids and suspensions are mixtures between a solute and a solvent.
• The solute and solvent can be a gas, liquid or solid.
• Mixtures can be homogenous or heterogenous.
• Homogenous mixtures (true solutions) are evenly distributed and the solute and solvent cannot be distinguished by normal physical means.
• Heterogenous mixtures (colloids and suspensions) can distinguish the solute and solvent by visible means.

Comparing Solutions, Colloids and Suspensions

• Solute particle size:
  • True solutions: < 10 angstroms
  • Colloids: 10 - 1000 angstroms
  • Suspensions: Visible to the naked eye
• Visibility:
  • True solutions: Not visible
  • Colloids: Heterogenous at the microscopic level, visible by an ultramicroscope.
  • Suspensions: Particles settle down with time ("precipitated")
• Filterability:
  • True solutions: Pass through ordinary filter paper
  • Colloids: Pass through ordinary filter paper but retained by parchment membrane
  • Suspensions: Retained by ordinary paper
• Osmotic Pressure:
  • True solutions: High
  • Colloids: Very low
  • Suspensions: None
• Light Scattering:
  • True solutions: Do not reflect light
  • Colloids: Light is dispersed by colloidal particles, known as the Tyndall Effect
  • Suspensions: Particles visible, light is reflected
• Surface Phenomenon:
  • True solutions: None
  • Colloids: Remarkable property of adsorption
  • Suspensions: Much less adsorption
• Electrical Properties:
  • True solutions: None
  • Colloids: Electroosmosis, cataphoresis and coagulation
  • Suspensions: None

Classification of Colloids

Based on physical states of dispersed phase and dispersion medium

• Solid Dispersed Phase:
  • Solid Dispersion Medium: Solid Sol - Colored glass,
  • Liquid Dispersion Medium: Sol - Paints, inks, white of egg,
  • Gas Dispersion Medium: Aerosol - Smoke, dust,
• Liquid Dispersed Phase:
  • Gas Dispersion Medium: Aerosol - Mist, fog, clouds,
  • Solid Dispersion Medium: Gel - Curds, Cheese, Jelly,
  • Liquid Dispersion Medium: Emulsion - Milk, butter, oil in water
• Gas Dispersed Phase:
  • Liquid Dispersion Medium: Foam - Soap lather, aerated water,
  • Solid Dispersion Medium: Solid foam - Cake, bread, pumice stone,

Based on affinity between dispersed phase and dispersion medium

• Lyophobic (Solvent Hating):
  • Hydrophobic (Solvent is water)
  • Prepared by special techniques, less stable, irreversible
  • Small quantities of electrolytes can cause precipitation
  • Charged particles migrate to one direction in an electric field e.g. ferric hydroxide in water sols
• Lyophilic (Solvent Loving):
  • Hydrophilic (Solvent is water)
  • Easily prepared, stable, reversible
  • Small quantities of electrolytes do not affect precipitation
  • Little or no charged particles may or may not migrate in an electric field e.g. gelatin, gum, starch and protein

Based on nature of dispersed phase

• Multimolecular: Aggregates of atoms or molecules forming particles of colloidal size range e.g. gold sol, sulphur sol
• Macromolecular or molecular colloids: Synthetic polymers such as polyethylene, polypropylene
• Association colloids (micelles formation in soap solutions): Behave like true solutions at low concentrations but shows colloidal properties at higher concentrations due to the formation of aggregated particles.

Critical Micellization Concentration (CMC)

• CMC = 10^-3 mol / L