By Claire Gillespie Updated Mar 24, 2022
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Water is often called the “universal solvent” because it can dissolve a vast array of substances. When a solute fully dissolves in a solvent, the resulting homogeneous mixture is called a solution. Despite water’s remarkable solvency, some materials simply will not dissolve in it.
Oil, paraffin wax, and sand are classic examples of substances that remain undissolved in water. Even highly soluble compounds reach a saturation limit beyond which additional solute stays as solid residue.
Solubility depends on the relative strengths of the attractive forces between solute particles, solvent particles, and between solute and solvent. Glucose, for instance, dissolves readily because the hydrogen‑bonding interactions with water outweigh both glucose–glucose and water–water interactions.
When two liquids fully mix, they are miscible; otherwise, they are immiscible. Oil (hydrocarbons) and water exemplify immiscibility. Oil’s lower density causes it to float, and the oil droplets never integrate into the aqueous phase.
Water’s polarity—partial positive charge on hydrogen atoms and partial negative charge on oxygen—makes it highly selective. Polar or ionic solutes are attracted to water, while non‑polar substances like paraffin wax (long chains of C and H) are repelled, adhering to the “like dissolves like” rule.
It’s essential to distinguish dissolution from erosion and suspension. Sand, for example, does not dissolve because water’s internal attractions dominate over water–sand interactions. Stirring suspends the sand, producing a cloudy mixture; when stirring stops, the sand settles and the water above becomes clear. Long‑term water contact can erode rock surfaces, transporting fine particles downstream, but this is a physical removal rather than chemical dissolution.
Even solutes that are highly soluble—such as sugar or sodium chloride—exhibit a saturation point. At equilibrium, the rate of dissolution equals the rate of recrystallization, so adding more solute does not increase concentration; instead, excess remains as undissolved solid.
These principles explain why certain everyday materials resist dissolving in water and underscore the nuanced interplay of molecular forces in aqueous chemistry.
