1. Molecular Interactions+ -
Ionic interactions
Dipole-dipole interactions
Induced dipoles
Hydrogen bond + - - +
+ -
+ -
Intermolecular forces play very significant role in the solubility. There are Coulomb forces acting between charged molecules (ions). Intermolecular interactions between uncharged molecules includes dipole-dipole (Keesom forces), dipole - induced dipole (Debye interactions) and induced dipole - induced dipole interactions (London interactions). These interactions are collectively called Van-der-Waals forces. To account for the energy changes due to these interactions solution theories introduce various solubility parameters, which are determined in experiment. More information about solubility parameters is presented in “Physical Pharmacy” by A. Martin (Lea & Fibiger, Philadelphia, London, 1993, pp ).

2. Water Miscibility
Solvents with relatively high polarity are miscible in water (acetic acid, ethanol, dioxane)
Solvent with low polarity or non-polar are water-immiscible (hexane, petroleum, ether)
General miscibility rule is that “similar likes similar”. Solvents with high polarity mix with water, while solvents low polarity or non-polar are water-immiscible. You can get better idea about water miscible and water immiscible solvents by studying the Solvent Miscibility Table.

3. Effects on Solubility Pressure: increases solubility of gases
Temperature: solubility of most gases decreases with increasing T, solubility of liquids and solids may be affected both ways (usually increases with increasing T)
Co-solvents: increase solubility by changing the solvent polarity.
The solubility (miscibility) depend on the pressure, temperature and presence of third compounds such as co-solvents or salts.

4. Properties of Solutions
Solution - homogeneous mixture
Solute - the substance in the mixture present in lesser quantity
Solvent - the substance present in the largest quantity
Aqueous solution - solution where the solvent is water
Solutions can be liquids as well as solids and gases

5. Examples of Solutions
Air - oxygen and several trace gases are dissolved in the gaseous solvent, nitrogen
Alloys - brass and other homogeneous metal mixtures in the solid state
We will focus on liquid solutions as many important chemical reactions take place in liquid solutions

6. General Properties of Liquid Solutions
Transparent, no visible particles
May have color
Electrolytes are formed from solutes that are soluble ionic compounds
Nonelectrolytes do not dissociate
Volumes of solute and solvent are not additive
1 L ethanol + 1 L water does not give 2 L of solution
Why?
the intermolecular forces of a mixture are different than the ones in the pure substances

7. Solutions and Colloids
Colloidal suspension - contains solute particles which are not uniformly distributed
Due to larger size of particles (1nm nm)
Appears identical to solution from the naked eye

8. Colloidal suspension Solution
Tyndall Effect
the ability of a colloidal suspension to scatter light
See a haze when shining light through the mixture
Solutions: light passes right through without scattering
Colloidal suspension Solution
-light as haze, scatters light no haze

9. Degree of Solubility
Solubility - how much of a particular solute can dissolve in a certain solvent at a specified temperature
Factors which affect solubility:
Polarity of solute and solvent
The more different they are, the lower the solubility
Temperature
Increase in temperature usually increases solubility
Pressure
Usually has no effect (solids and liquids)
If solubility is of gas in liquid, directly proportional to applied pressure

10. Saturation
Saturated solution - a solution that contains all the solute that can be dissolved at a particular temperature
Supersaturated solution - contains more solute than can be dissolved at the current temperature
How is this done?
Heat solvent, saturate it with solute then cool slowly
Sometimes the excess will precipitate out
If it doesn’t precipitate, the solution will be supersaturated

11. Water as a Solvent Enthalpy of Solution.
Water is often referred to as the “universal solvent”
Excellent solvent for polar molecules
Enthalpy of Solution.
The enthalpy of solution of a substance is the energy change when one mole of a substance dissolves in water.
endothermic
(absorbs energy)
AgNO3(s) + 22kJ/mol → Ag+(aq) + NO3-(aq)
(NH4)NO3(s) → NH4+(aq) + NO3-(aq) kJ/mol
Exothermic
(releases energy)