Chem 1170: Chapter 12 - Solutions

A solution is a homogeneous mixture composed of two or more substances that are uniformly dispersed at the molecular or ionic level, resulting in a consistent composition throughout. Solutions can consist of ions, molecules, or a combination of both, depending on the nature of the solute and solvent involved.

Types of solutions:

1. Gaseous solution:

   • Example: Air, which is a mixture of gases such as nitrogen, oxygen, carbon dioxide, and trace gases, forms a gaseous solution by blending these components without any visible boundaries.

2. Liquid solution:

   • Example: Saltwater, where salt (NaCl) or sugar dissolves in water to create a homogeneous mixture. The solute (salt or sugar) dissolves at the molecular level, leading to properties distinct from those of pure water.

3. Solid solution:

   • Example: Gold jewelry or alloys like brass (made from copper and zinc) and bronze (composed of copper and tin) are solid solutions. These mixtures are created when different types of metals are melted together and allow for the integration of their molecular structures.

Key Concepts:

• Solute: The substance that is dissolved in a solution. It can exist in solid, liquid, or gaseous states. For example, in a sugar-water solution, sugar is the solute.

• Solvent: The medium that dissolves the solute. It is usually present in a larger quantity. In the case of saltwater, water acts as the solvent, and it dissolves the solute (salt).

• In liquid mixtures, the liquid that is present in the larger quantity typically acts as the solvent, while the liquid in lesser quantity functions as the solute. Understanding this distinction is essential in solution chemistry.

Miscibility

• Miscible: Two liquids that can dissolve completely in each other, forming a homogeneous solution. A common example is ethanol and water.

• Immiscible: Two liquids that do not mix well and remain separate, such as oil and water. This occurs due to differences in polarity, where the polar solvent does not interact favorably with the nonpolar solute.

Solubility

• Solubility: Refers to the maximum amount of solute that can dissolve in a specific amount of solvent at a given temperature. It is a vital property that affects how solutions behave under various conditions.

• Saturated solution: A solution that has reached its maximum capacity for solute. Beyond this point, any additional solute will not dissolve and may precipitate out of the solution.

• Unsaturated solution: A solution that contains less solute than it can potentially dissolve, allowing more solute to be added and dissolved if introduced.

• Supersaturated solution: A state where a solution contains more dissolved solute than typically possible at a given temperature, which is often unstable. Supersaturation can lead to crystallization and occurs in processes such as making rock candy, where the solution cools below the saturation point after being heated.

Formation of Solutions

The principle of "like dissolves in like" is fundamental in understanding solution formation.

• Polar solutes (such as sodium chloride or sugar) dissolve well in polar solvents (like water) due to favorable interactions, including dipole-dipole interactions and hydrogen bonding.

  • This process involves polarity matching between solute and solvent molecules, allowing them to separate and disperse.

• Nonpolar solutes (like oils) dissolve in nonpolar solvents through dispersion forces. These solutes tend to avoid polar solvents like water due to their different physical properties and intermolecular forces.

Effects of Temperature and Pressure on Solubility

1. For most solids:

   • As temperature increases, solubility tends to increase, allowing more solid solute to dissolve in the liquid phase.

2. For gases:

   • As temperature increases, the solubility of gases in liquids decreases, meaning less gas can be dissolved as temperature rises, leading to bubbles forming out of solution.

3. Pressure:

   • Pressure affects the solubility of gases significantly but has little effect on solids. Increasing pressure will increase gas solubility in liquids, as described by Henry's Law.

Henry's Law

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. The law can be expressed mathematically as:
\frac{S2}{S1} = \frac{P2}{P1}
Where S represents solubility and P represents pressure.

• Example of Diving:

  • At sea level, where pressure is at 1 atmosphere, the solubility of helium in blood is 0.9 grams per 100 mL.

  • If a diver ascends to a depth of 1500 feet (approximately 46 atmospheres), you can use Henry’s Law to analyze the increased solubility:
    \frac{S_2}{0.9} = \frac{46}{1}

  • By solving for S_2, you gain insight into the risks of rapid ascent during diving, particularly the dangers of decompression sickness caused by excess helium accumulating in the bloodstream.

Summary of Important Relationships

• The solubility of solids generally increases as temperature rises, allowing more solute to dissolve. In contrast, the solubility of gases tends to decrease with higher temperatures.

• According to Henry's Law, an increase in pressure leads to greater solubility of gases in liquids, emphasizing the importance of pressure in aquatic and diving applications.