Why some substances dissolve while others remain stubbornly separate
In short: We often think of solutions as liquids, but air and brass are solutions too. The secret to how substances mix lies in the invisible dance of electrical charges and molecular polarity, a mechanism that governs everything from your breathing to industrial chemical manufacturing.
In chemistry, a solution is a homogeneous mixture where the proportions of substances can be varied continuously until they reach the limit of solubility. While we typically associate solutions with liquids, they can exist in gas and solid states as well. For instance, the air we breathe is a gaseous solution primarily composed of nitrogen and oxygen, while brass serves as a solid solution of copper and zinc.
The ability of a substance to dissolve depends heavily on the principle of 'like dissolves like.' Polar solvents, such as water, methyl alcohol, or liquid ammonia, are effective at dissolving polar solutes. This process occurs because the electric dipoles in the solvent molecules can overcome the electrostatic attraction between the oppositely charged ions in a crystal lattice. When these ions are detached and surrounded by solvent molecules, they can migrate to electrodes, creating an electrolyte solution capable of conducting electricity.
Conversely, nonpolar solvents are required to dissolve nonpolar solutes. The physics governing such interactions differs greatly: for basic, nonpolar molecules, attractive potential energy falls off sharply, roughly following the seventh power of their separation distance. In contrast, the energy of attraction and repulsion for electrolytes drops only as the first power of the distance. This fundamental difference in molecular mechanics ensures that electrolytes and nonelectrolytes possess vastly different physical properties.
These chemical principles are vital to life and industry. In the human body, oxygen dissolves into blood plasma to reach tissues, and digestive products are transported via solutions throughout the body. In manufacturing, chemists exploit differences in solubility to purify materials, while engineers must select equipment capable of resisting the solvent action of the chemicals being processed.
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Sources
- Summary by Intellect · source: Solutions: Crash Course Chemistry #27
- Also: Solution | Definition & Examples | Britannica
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