Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- Soluble: a noticeable amount dissolves. Miscible: two liquids mix in all proportions. Immiscible: they form two layers.
- Dissolving overcomes solute–solute and solvent–solvent attractions and forms solute–solvent attractions.
- It happens readily when the new attractions are comparable to those lost, which is when solute and solvent have similar intermolecular interactions: “like dissolves like”.
- Nonpolar substances stay out of water because water molecules hold each other tightly by hydrogen bonds, not because water repels them.
Key relationships
| Solute type | Dissolves well in | Main solute–solvent interaction |
|---|---|---|
| Ionic (e.g. NaCl) | Water (often) | Ion–dipole |
| Molecules with O–H or N–H (e.g. ethanol, glucose) | Water | Hydrogen bonding |
| Nonpolar (e.g. I₂, alkanes, wax) | Hexane and other nonpolar solvents | London dispersion |
| Polar head + long nonpolar chain (e.g. 1-hexanol) | Less in water, more in hexane as the chain grows | Both, in competition |
Water around ions: δ− oxygen ends face cations; δ+ hydrogen ends face anions.
Always soluble in water (needed in Unit 4): all sodium, potassium, ammonium and nitrate salts.
Assumptions and limits of the model
- “Like dissolves like” compares types of interaction. It ignores their strengths, so it can fail: CaCO₃ and AgCl are ionic but barely dissolve.
- It is qualitative: it predicts which solvent is better, not how much dissolves.
- Mixing also spreads particles out, which favours dissolving; that idea is developed in a later unit.
Mistakes to avoid
- “Water repels hexane.” It attracts hexane weakly; it attracts itself far more strongly.
- “Ionic = soluble.” Check the strength of the ion–ion attractions, and learn the always-soluble list.
- Drawing water backwards around an ion. Opposite partial and full charges face each other.
- “Dissolving breaks covalent bonds.” Molecular solutes such as glucose stay intact.
- Answering “like dissolves like” with nothing else. Name the forces in the solute, in the solvent and between them.
- Confusing density with mixing. Density decides which layer is on top, not whether liquids mix.
Quick self-check
- Which way do water molecules face a Mg²⁺ ion? (Oxygen ends, δ−, toward the ion.)
- Is I₂ more soluble in water or in hexane? Why? (Hexane: both nonpolar, so dispersion forces replace dispersion forces.)
- Put in order of increasing solubility in water: 1-hexanol, ethanol, 1-butanol. (1-hexanol < 1-butanol < ethanol)
- Why do ethanol and water mix in all proportions? (Ethanol–water hydrogen bonds replace the ethanol–ethanol and water–water hydrogen bonds that are lost.)
Next: practice questions.