III. Factors Affecting Solubility

Polar Solvents

Dissolve ionic and polar solutes well by forming hydrogen bonds and hydrating ions; water is a prime example.

Hydrocarbon Chain Effect on Solubility

Longer carbon chains decrease water solubility, while increased branching increases water solubility.

Nonpolar Solvents

Dissolve nonpolar solutes (oils, fats, waxes) primarily due to induced dipole interactions; examples include benzene, CCl4, and mineral oil.

Semipolar Solvents

Act as intermediate solvents to increase miscibility between polar and nonpolar substances by inducing polarity; examples include ketones and alcohols.

Acetone Co-solvent Action

Increases the solubility of ether in water.

Alcohol Co-solvent Action

Increases the miscibility of a water and castor oil mixture.

Propylene Glycol Co-solvent Action

Increases the miscibility of water with peppermint oil or benzyl benzoate.

Dielectric Constant

Measures how well a solvent can reduce the attractive forces between charged particles; higher constants mean higher polarity.

Endothermic Substances

Substances that absorb heat during dissolution; increasing temperature increases their solubility (e.g., KNO3, KCl).

Exothermic Substances

Substances that release heat during dissolution; increasing temperature decreases their solubility (e.g., Ca(OH)2).

Gas Solubility and Temperature

Increasing the temperature decreases the solubility of gases in liquids.

pH Effect on Acidic Drugs

Acidic drugs in low pH (acidic) environments precipitate, showing decreased solubility.

pH Effect on Basic Drugs

Basic drugs in high pH (basic) environments precipitate, showing decreased solubility.

Phenobarbital pH Behavior

A weak acid that becomes ionized and highly soluble at a high pH.

Atropine pH Behavior

A weak base that becomes ionized and highly soluble at a low pH.

Critical pH (pHc / pHp) for Weak Acids

The pH below which a weak acid precipitates from solution in its undissociated form.

Critical pH (pHc / pHp) for Weak Bases

The pH above which a weak base precipitates from solution in its undissociated form.

Particle Size Effect on Solubility

Smaller particle sizes create a larger surface area, leading to greater interaction with the solvent and increased solubility.

Salting-in

The addition of a small concentration of salt that increases the hydrophilicity of the solution and enhances solute solubility.

Salting-out

The addition of a high concentration of salt that competes for water molecules, reducing available solvent and causing the solute to precipitate.