What is the law of solubility?

The "law of solubility" is a general term that encompasses several rules governing how substances dissolve, but it's most accurately represented by specific laws like Henry's Law, which primarily addresses the solubility of gases.

Understanding Solubility

Solubility refers to the ability of a substance (solute) to dissolve in a solvent to form a solution. Several factors influence solubility, including temperature, pressure, and the nature of the solute and solvent. Different "laws" or rules dictate how these factors affect the solubility of different types of substances.

Henry's Law: Gas Solubility and Pressure

Definition

Henry's Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid. This relationship is named after the English chemist William Henry (1774-1836).

Mathematical Representation

The law can be expressed mathematically as:

• S = kP

Where:

• S is the solubility of the gas in the liquid.
• k is Henry's Law constant, which depends on the gas, solvent, and temperature.
• P is the partial pressure of the gas above the liquid.

Implications

This law implies that if you increase the pressure of a gas above a liquid, more of that gas will dissolve in the liquid. Conversely, if you decrease the pressure, the solubility of the gas decreases, and it may come out of the solution.

Example

Think of carbonated beverages:

1. High Pressure: In a sealed soda bottle, carbon dioxide gas (CO₂) is under high pressure. This high pressure forces a large amount of CO₂ to dissolve in the liquid (water).

2. Opening the Bottle: When you open the bottle, you release the pressure.

3. Reduced Solubility: The pressure of CO₂ above the liquid decreases, so the solubility of CO₂ in the liquid also decreases, as per Henry's Law.

4. Bubbles Form: Excess CO₂ comes out of the solution in the form of bubbles, which you see fizzing.

Factors Affecting Solubility

While Henry's Law specifically addresses gas solubility and pressure, other factors and "rules of thumb" govern solubility in general:

• Temperature:

  • Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.
  • However, the solubility of gas solutes in liquid solvents decreases with increasing temperature. (Think of a heated soda going flat faster.)

• "Like Dissolves Like": This principle states that substances with similar intermolecular forces are more likely to dissolve in each other.

  • Polar solvents (like water) tend to dissolve polar solutes (like salt or sugar).
  • Nonpolar solvents (like hexane) tend to dissolve nonpolar solutes (like oil or grease).

• Common Ion Effect: The solubility of a sparingly soluble salt is reduced when a soluble salt containing a common ion is added to the solution.

• Pressure (for Gases): As explained by Henry's law, pressure has a significant impact on the solubility of gases.

Summary Table