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6.3: Solubility- Molecular Compounds

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    542105

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    Learning Objectives
    • Explain the significance of the statement "like dissolves like."
    • Explain why certain substances dissolve in other substances.

    Like Dissolves Like

    Chemists use the phrase "Like Dissolves Like" to describe why some molecular compounds mix, while other do not.

    Sugar, Water, and Vegetable oil are all molecular compounds, but while sugar dissolves in water, vegetable oil separates from water.

    sugar dissolved in water vs oil and water

    Figure \(\PageIndex{1}\): (left) Sugar Dissolves in water, (right) water and oil mix. (CC BY-SA 1.0 Generic; Victor Blacus)

    Molecule Polarity

    Individual Molecules have a property called "polarity" that determines how they will behave when mixed.

    The term "Polar" means that a molecule has a positive and negative end- think like a little bar magnet.

    "Non-Polar" means that the molecule does not have a positive and negative end.

    Molecule Polarity

    Polarity is caused by an imbalance in sharing electrons between atoms in a molecule. Chemists have specific processes to help determine if a molecule is polar and to what extent. For a simple approximation, though, follow the below guidelines.

    Carbon and Hydrogen alone will always be non-polar,

    Oxygen and Nitrogen are the elements most likely to make a molecule polar. The higher percentage of the molecule that is composed of oxygen and nitrogen, the more likely it will be polar.

    When Chemists say "Like Dissolves Like", they mean polar molecules want to mix with other polar molecules.

    Water (H₂O) and sugar (C₁₂H₂₂O₁₁) mix because they are both polar.

    Water (H₂O) and Linoleic Acid (C₁₈H₃₂O₂), one main component of vegetable oil, do not mix because water is polar and linoleic acid is non-polar (composed mostly of carbon and hydrogen).

    Example \(\PageIndex{1}\)

    Water is considered a polar solvent. Which substances should dissolve in water?

    1. methanol (CH3OH)
    2. octane (C8H18)

    Solution

    Because water is polar, substances that are polar or ionic will dissolve in it.

    1. Because of the OH group in methanol, we expect its molecules to be polar. Thus, we expect it to be soluble in water.
    2. Like other hydrocarbons, octane is nonpolar, so we expect that it would not be soluble in water.
    Exercise \(\PageIndex{1}\)

    Toluene (C7H8) is widely used in industry as a nonpolar solvent. Which substances should dissolve in toluene?

    1. water (H2O)
    2. octane (C8H18)
    Answer

    Octane (C8H18) will dissolve. It is also non-polar.

    Summary

    • “Like dissolves like” is a useful rule for deciding if a solute will be soluble in a solvent.

    Contributions & Attributions

    This page was constructed from content via the following contributor(s) and edited (topically or extensively) by the LibreTexts development team to meet platform style, presentation, and quality:

    This page titled 6.3: Solubility- Molecular Compounds was last modified on Wed, 17 Dec 2025 22:13:31 GMT and is shared under a not declared license and was authored, remixed, and/or curated by Seth Yates.

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