Unit 3 · Properties of Substances and Mixtures

AP Chemistry Solubility

Like dissolves like, miscible and immiscible liquids, and the solubility trends the exam asks you to explain.

Topic 3.10 boils solubility down to one idea: substances with similar intermolecular interactions mix. The basics of solutes, solvents, and how dissolving works are in AP Chemistry solutions. This page goes further, into the trends and comparisons that free-response questions ask you to justify, and the wording that earns the point.

The Three Attractions That Decide Solubility

Every solubility question compares three sets of intermolecular forces:

1. Solute–solute attractions that must be overcome to pull the solute apart.
2. Solvent–solvent attractions that must be disrupted to make room.
3. Solute–solvent attractions that form when they mix.

A substance dissolves well when the solute–solvent attractions are similar in strength to the ones broken. That is why polar and ionic substances dissolve in water, whose molecules form hydrogen bonds and ion–dipole attractions, while nonpolar substances dissolve in nonpolar solvents held together only by London dispersion forces.

Miscible and Immiscible Liquids

Two liquids that mix in all proportions are miscible; two that separate into layers are immiscible. Water and ethanol are miscible because ethanol’s O–H group hydrogen bonds with water about as well as water bonds to itself. Water and hexane are immiscible: hexane can offer only dispersion forces, which cannot replace the hydrogen bonds water would have to give up, so the water molecules stay together and hexane, less dense, floats on top. Hexane and octane, both nonpolar, mix completely.

Why Alcohols Get Less Soluble as the Chain Grows

Each alcohol has one O–H group that hydrogen bonds with water. The rest of the molecule is a nonpolar hydrocarbon chain that attracts water only through weak dispersion forces and gets in the way of water’s own hydrogen bonding. With one to three carbons the polar end dominates and the alcohol mixes completely; each added CH₂ makes the nonpolar part larger, so solubility in water falls. The same reasoning explains why longer alcohols dissolve better in nonpolar solvents.

Why Some Ionic Solids Barely Dissolve in Water

Water dissolves an ionic solid by surrounding each ion with oriented water molecules, forming ion–dipole attractions. That works when those attractions can compete with the attractions holding the lattice together. For ions with large charges or small radii, such as those in CaCO₃ and BaSO₄, the lattice wins and very little dissolves. How much does dissolve is measured by K_sp, covered in Ksp and molar solubility. The AP course only expects you to know that all sodium, potassium, ammonium, and nitrate salts are soluble; the fuller list is in the solubility rules reference.

Worked Example 1: Ranking Four Substances by Solubility in Water

Rank hexane (C₆H₁₄), 1-hexanol, 1-butanol, and ethanol from least to most soluble in water, and justify the order.

Hexane < 1-hexanol < 1-butanol < ethanol. Hexane has no polar group, so it cannot hydrogen bond with water. The three alcohols each have one O–H group that hydrogen bonds with water, so they are all more soluble than hexane, but the larger the nonpolar chain, the smaller the share of the molecule that interacts favorably with water. Ethanol, with only two carbons, is miscible.

Worked Example 2: Explaining Why Iodine Dissolves in CCl4

Solid I₂ dissolves readily in carbon tetrachloride but only slightly in water. Explain in terms of intermolecular forces.

I₂ and CCl₄ are both nonpolar, so the attractions between them are London dispersion forces, similar in kind and strength to those within each pure substance; mixing them costs little and they dissolve. To dissolve in water, I₂ would have to break water’s hydrogen bonds but could replace them only with much weaker dispersion attractions, so very little dissolves.

“Like dissolves like” on its own rarely earns an explanation point. Name the forces on both sides and compare their strengths.

Worked Example 3: Predicting Which Liquid Pairs Are Miscible

Predict whether each pair is miscible: (a) water and methanol, (b) water and CCl₄, (c) CCl₄ and hexane.

• (a) Miscible. Methanol’s O–H group hydrogen bonds with water.
• (b) Immiscible. CCl₄ is nonpolar and cannot replace water’s hydrogen bonds; the denser CCl₄ forms the bottom layer.
• (c) Miscible. Both are nonpolar, held together by dispersion forces.

Common Solubility Mistakes

• Calling a molecule polar because it has polar bonds. CCl₄ has polar C–Cl bonds but is nonpolar overall, because its shape cancels them; see molecular polarity.
• Saying bonds inside the solute break when it dissolves. Dissolving sugar or ethanol overcomes intermolecular forces; the covalent bonds stay intact.
• Assuming every ionic compound dissolves in water. Many do not, because their lattice attractions are too strong.
• Ignoring the size of the nonpolar part. One O–H group makes methanol miscible but cannot do the same for 1-hexanol.

Differences in solubility and polarity are also how mixtures are separated, the idea behind chromatography. Explanations like Example 2 are typical free-response points; to see how they add up to a 1–5 score, use the AP Chem Score Calculator.

Related Resources

• AP Chem Score Calculator
• Unit 3 Review: Properties of Substances and Mixtures
• AP Chemistry Solutions
• AP Chemistry Intermolecular Forces
• AP Chemistry Hydrogen Bonding
• AP Chemistry Solubility Rules
• AP Chemistry Ksp and Molar Solubility
• AP Chemistry Chromatography

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