Why does a substance dissolve in one solvent but not another, and how do temperature and pressure affect solubility?
Topic 3.10 Solubility: explain solubility in terms of the intermolecular forces between solute and solvent (like dissolves like), and describe how temperature and pressure affect the solubility of solids and gases.
A focused answer to AP Chemistry Topic 3.10, covering the like dissolves like principle, solute-solvent intermolecular forces, the role of ion-dipole and hydrogen bonding, and how temperature and pressure shift solubility, with full worked examples.
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What this topic is asking
The College Board (Topic 3.10) wants you to explain solubility, why a substance dissolves in one solvent but not another, in terms of the intermolecular forces between solute and solvent. The guiding rule is "like dissolves like": a solute dissolves well when the solute-solvent attractions are comparable in kind and strength to the forces being broken. You should also describe how temperature and pressure shift the solubility of solids and gases.
Like dissolves like
Dissolving is a competition of forces. To dissolve, a solute particle must be pulled away from its neighbors and surrounded by solvent. That only happens readily when the solvent can form attractions with the solute that are about as strong as the ones being broken. A polar solvent surrounds and stabilizes polar or ionic solutes; a nonpolar solvent does the same for nonpolar solutes.
Why ionic and polar solutes dissolve in water
This is why salt and sugar dissolve in water but oil does not. The oil molecules cannot form attractions with water strong enough to break into water's hydrogen-bonded structure, so the two stay separate. Conversely, oil dissolves in nonpolar solvents such as hexane, where dispersion forces on both sides match.
Temperature and pressure effects
The solubility of most solids in water increases with temperature, because the dissolving process is usually favored by added energy. The solubility of a gas in a liquid decreases with temperature (warm water holds less dissolved gas, which is why a warm soft drink goes flat faster) and increases with pressure. The pressure effect on gases is Henry's law: doubling the partial pressure of a gas above a liquid roughly doubles the amount that dissolves, which is how carbonated drinks are made under high pressure.
Try this
Q1. Predict whether ammonia () is soluble in water and justify. [2 points]
- Cue. Yes; ammonia has N-H bonds and can hydrogen bond with water (like dissolves like).
Q2. Explain why opening a warm bottle of soda releases gas faster than a cold one. [1 point]
- Cue. Gas solubility decreases as temperature rises, so warm soda holds less dissolved and loses it more readily.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2023 (style)3 marksSection II (short FRQ). Consider whether each solute dissolves well in water: sodium chloride (), oil (a nonpolar hydrocarbon mixture), and ethanol (). (a) Predict the water solubility of each. (b) Justify each prediction in terms of solute-solvent intermolecular forces. (c) Explain why a carbonated drink goes flat faster when warm.Show worked answer →
A 3-point FRQ on solubility.
(a) Predictions (1 point): is highly soluble; oil is insoluble; ethanol is highly soluble (miscible).
(b) Justify (1 point): water molecules form strong ion-dipole attractions with and , so dissolves; oil is nonpolar and can only form weak dispersion forces with water, which cannot replace water's hydrogen bonds, so it does not dissolve; ethanol's O-H lets it hydrogen bond with water, so it is miscible.
(c) Carbonation (1 point): the solubility of a gas () in a liquid decreases as temperature rises, so a warm drink holds less dissolved and loses it faster.
Markers reward correct solubility predictions, justifications based on matching intermolecular forces, and the temperature-solubility relationship for a gas.
AP 2021 (style)1 marksSection I (multiple choice). Which substance is expected to be most soluble in water? (A) (B) (C) (D) . Justify your choice.Show worked answer →
A 1-point conceptual MCQ. The answer is (C).
Methanol () has an O-H group, so it can hydrogen bond with water and is highly soluble (like dissolves like). , and are all nonpolar and interact with water only through weak dispersion forces, so they are poorly soluble.
Related dot points
- Topic 3.7 Solutions and Mixtures: define solute, solvent and solution, and calculate and use molarity to relate moles, volume and concentration, including dilutions.
A focused answer to AP Chemistry Topic 3.7, covering solute and solvent, the molarity concentration formula, preparing solutions, and dilution calculations with the M1V1 equals M2V2 relationship, with full worked examples.
- Topic 3.8 Representations of Solutions: use particulate-level diagrams to represent the species present in a solution, distinguishing strong electrolytes, weak electrolytes and nonelectrolytes.
A focused answer to AP Chemistry Topic 3.8, covering how to draw and interpret particulate diagrams of solutions, the difference between strong and weak electrolytes and nonelectrolytes, and how dissociation determines the species present, with full worked examples.
- Topic 3.1 Intermolecular Forces: identify and rank the intermolecular forces (London dispersion, dipole-dipole, hydrogen bonding, ion-dipole) present in a substance and relate their strength to properties such as boiling point and vapor pressure.
A focused answer to AP Chemistry Topic 3.1, covering London dispersion, dipole-dipole, hydrogen bonding and ion-dipole forces, how to rank their strength, and how intermolecular forces set boiling point, viscosity and vapor pressure, with full worked examples.
- Topic 3.9 Separation of Solutions and Mixtures (Chromatography): explain how chromatography, distillation and filtration separate the components of a mixture by exploiting differences in their interactions and properties.
A focused answer to AP Chemistry Topic 3.9, covering chromatography (stationary and mobile phases, relative affinities), distillation by boiling point and filtration by particle size, all explained through intermolecular forces, with full worked examples.
- Topic 3.2 Properties of Solids: relate the macroscopic properties of a solid (melting point, hardness, conductivity) to its type (ionic, metallic, covalent network, molecular) and the forces holding its particles together.
A focused answer to AP Chemistry Topic 3.2, covering the four types of solid (ionic, metallic, covalent network, molecular), the forces in each, and how those forces explain melting point, hardness, brittleness and conductivity, with full worked examples.
Sources & how we know this
- AP Chemistry Course and Exam Description — College Board (2020)