Mistake Master
Solubility
Oil and water refuse to mix, but oil and grease blend happily. Solubility is not a fixed property of a substance — it is a match. When the intermolecular forces of solute and solvent are compatible, one dissolves in the other.
§1
Like dissolves like.
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Whether a solute dissolves in a solvent depends on matching intermolecular forces. The rule of thumb: like dissolves like. Polar and ionic solutes dissolve in polar solvents (water); nonpolar solutes dissolve in nonpolar solvents.
Dissolving works when the new solute-solvent attractions can replace the solute-solute and solvent-solvent attractions that must be broken. A polar solvent can surround and stabilize ions; a nonpolar solvent cannot.
So solubility is a relationship, not a property of the solute alone. Oil does not dissolve in water because its nonpolar forces do not match water's polar, hydrogen-bonding network — but oil dissolves readily in another nonpolar liquid.
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Predicting solubility.
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Compare the polarities of solute and solvent.
- Classify the solute. Is it polar/ionic (interacts by dipole or ionic forces) or nonpolar (dispersion only)?
- Classify the solvent. Polar (like water) or nonpolar (like hexane or oil)?
- Match them. Polar/ionic in polar, nonpolar in nonpolar → dissolves. A mismatch (polar in nonpolar, or nonpolar in polar) → little dissolving.
- Reason from the forces. Dissolving happens when solute-solvent attractions can stand in for the ones broken. Compatible forces make that possible.
§3
The pieces you'll meet.
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Solubility is about matching forces.
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Worked example: will it dissolve in water?
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Question. Predict whether table salt (NaCl) and cooking oil (nonpolar) dissolve in water.
Salt in water. Water is polar and NaCl is ionic. Water molecules surround and stabilize the Na⁺ and Cl⁻ ions (ion-dipole attractions), so salt dissolves.
Oil in water. Oil is nonpolar, with only dispersion forces; water is polar and hydrogen-bonds. Their forces do not match, so oil does not dissolve — it separates into a layer.
Key point. Oil is not 'insoluble' as a fixed trait; it dissolves happily in another nonpolar solvent. Solubility depends on the pairing.
§5
Mistakes that cost real points.
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"Solubility is a fixed property of the solute alone."
Whether something dissolves depends on both the solute and the solvent. Oil will not dissolve in water but dissolves easily in a nonpolar solvent. Calling a substance simply 'soluble' or 'insoluble' ignores what it is being dissolved in.
Fix. Always state solubility as a pairing: soluble in what? Match the solute's forces to the solvent's.
"Any two liquids will mix if you stir hard enough."
Stirring cannot force a nonpolar and a polar liquid to mix if their intermolecular forces are incompatible. Oil and water separate no matter how vigorously you stir, because dissolving requires compatible forces, not mechanical effort.
Fix. Judge miscibility by force compatibility (like dissolves like), not by how much you stir.
"Polar solutes dissolve in nonpolar solvents."
Polar and ionic solutes dissolve in polar solvents, not nonpolar ones. A nonpolar solvent cannot stabilize the charges or dipoles of a polar solute, so little dissolves. The match must be like-with-like.
Fix. Pair polar/ionic solutes with polar solvents and nonpolar solutes with nonpolar solvents.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.