AP Chemistry 3.7 Solutions and Mixtures Notes - New Syllabus 2024-2025

Definition and Nature of Solutions vs. Heterogeneous Mixtures

A solution (also called a homogeneous mixture) is a uniform mixture of two or more substances in a single phase, where the macroscopic properties (such as composition, color, or density) are the same throughout the sample. In contrast, a heterogeneous mixture consists of visibly distinct components or phases, and its properties vary from one region to another.

Key Properties:

1. Homogeneous Nature of Solutions:

• Solutions are uniform throughout—the composition is identical in every portion of the sample.
• Particles of the solute are evenly distributed at the molecular or ionic level in the solvent.
• Examples include salt water, air, and metal alloys such as brass.

2. Phases of Solutions:

• Gaseous solutions: Example—air (a mixture of nitrogen, oxygen, and other gases).
• Liquid solutions: Example—salt dissolved in water.
• Solid solutions: Example—alloys such as copper–zinc (brass) or iron–carbon (steel).

3. Macroscopic Uniformity:

• In a homogeneous solution, physical properties (such as density, color, and refractive index) do not vary with location in the sample.
• In a heterogeneous mixture, these properties do vary depending on where the measurement is made (e.g., sand at the bottom of water vs. clear liquid above).

4. Particle Size and Separation:

• In true solutions, the solute particles are at the molecular or ionic scale (diameter < 1 nm).
• They do not scatter light (no Tyndall effect) and cannot be separated by filtration.
• Heterogeneous mixtures, by contrast, have larger particles that can often be separated mechanically (e.g., filtration or decantation).

 A solution is uniform at the molecular level, exhibiting consistent macroscopic properties throughout the sample. Heterogeneous mixtures, on the other hand, contain visibly or microscopically distinct regions with different compositions and properties.