AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/3/3-10)
Unit 3 · Topic 3.10
3.10 Solubility
Substances with similar intermolecular forces tend to dissolve in each other, often summed up as 'like dissolves like'. Ionic and polar solutes usually dissolve in polar solvents such as water, and nonpolar solutes dissolve in nonpolar solvents.
Key terms
- solubility
- miscible
- like dissolves like
- polar solvent
- nonpolar solvent
Why like dissolves like
Dissolving means separating solute particles from each other, separating some solvent particles from each other, and then letting the solute and solvent mix and attract each other. Mixing happens readily when the new solute-solvent attractions are comparable to the attractions that had to be broken.
When IMFs are similar, nothing is lost by mixing. When they're very different, the stronger attractions win and keep their own particles together. Two liquids that mix in any proportion are called miscible.
Polar and ionic solutes in water
Water molecules hydrogen bond strongly to each other. A polar solute like ethanol or sugar can form hydrogen bonds or dipole-dipole attractions with water, which replace the water-water hydrogen bonds it disrupts. So it dissolves.
Ionic solids like NaCl and KNO₃ dissolve because ion-dipole attractions between ions and water molecules are strong enough to pull ions out of the lattice. Not every ionic compound dissolves, though: when the attractions in the lattice are especially strong, the compound can be only slightly soluble, as with AgCl.
Ionic compounds generally don't dissolve in nonpolar solvents like hexane. A nonpolar solvent can't form ion-dipole attractions, so there's nothing to pull the ions out of their lattice.
Nonpolar solutes and solvents
A nonpolar substance like oil, hexane or I₂ dissolves well in a nonpolar solvent, since both have only London dispersion forces.
In water, a nonpolar molecule would have to break into the network of hydrogen bonds between water molecules. It can only offer weak dispersion and dipole-induced dipole attractions in return. Water molecules stay attracted to each other instead, and the nonpolar substance is pushed out into its own layer. That's why oil and water don't mix, not because oil and water repel each other.
Molecules with both parts
Many molecules have a polar end and a nonpolar end. Alcohols are a good example. Methanol (CH₃OH) and ethanol (CH₃CH₂OH) mix with water in any proportion. As the nonpolar carbon chain gets longer, the molecule behaves more like a hydrocarbon, and solubility in water drops. 1-Butanol is only partly soluble, and 1-octanol barely dissolves at all.
Soap works this way: its long nonpolar tail dissolves in grease, while its ionic head stays attracted to water.
| Solute | Dissolves well in water? | Dissolves well in hexane? |
|---|---|---|
| NaCl (ionic) | yes | no |
| CH₃OH (polar, H-bonding) | yes | limited |
| I₂ (nonpolar) | very little | yes |
| C₈H₁₈, octane (nonpolar) | no | yes |
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Predicting solubility
Explain why KI dissolves in water but not in carbon tetrachloride (CCl₄).
Show the solutionHide the solution
- Step 1: KI is ionic. To dissolve, its K⁺ and I⁻ ions must be separated, which requires overcoming strong attractions in the lattice.
- Step 2: In water, polar molecules form strong ion-dipole attractions with K⁺ (oxygen side) and I⁻ (hydrogen side), which make up for the attractions lost.
- Step 3: CCl₄ is nonpolar. It can't form ion-dipole attractions, so nothing makes up for breaking the lattice, and KI stays solid.
Answer: Water forms strong ion-dipole attractions with K⁺ and I⁻; nonpolar CCl₄ can't, so KI dissolves only in water.
- Example 2
The chain-length effect (classic trap)
Methanol (CH₃OH) is miscible with water, but 1-octanol (CH₃(CH₂)₇OH) barely dissolves. Both have an O–H group. Explain.
Show the solutionHide the solution
- Step 1: Both molecules can hydrogen bond with water through their O–H group.
- Step 2: In methanol, the O–H group is a large part of the molecule, so its interactions with water are similar to water's own.
- Step 3: In 1-octanol, a long nonpolar chain of 8 carbons dominates. That chain interacts with water only through weak dispersion forces and disrupts water's hydrogen bonding.
- Step 4: Having an O–H group isn't enough on its own; you have to consider the whole molecule.
Answer: 1-Octanol's long nonpolar chain can't form strong attractions with water and dominates the molecule, so it is barely soluble, while methanol's small size lets its O–H group dominate.
Common mistakes
- Saying oil and water repel each other. They're slightly attracted; water molecules are just much more attracted to each other.
- Assuming every ionic compound dissolves in water.
- Deciding polarity from one functional group while ignoring a long nonpolar chain.
- Explaining solubility with 'like dissolves like' and nothing more. Name the specific IMFs.
On the exam
- Free-response questions often ask why one substance is more soluble than another. Identify the IMFs between solute and solvent in each case and compare them. 'Like dissolves like' alone usually doesn't earn the point.
- Expect to apply solubility reasoning to chromatography and separations (topic 3.9).
Connected topics
Videos
Check yourself
4 questions on 3.10 Solubility. Pick an answer to see if you got it, and why.
Solid iodine, I₂, is only slightly soluble in water but dissolves readily in hexane, C₆H₁₄. Which of the following best explains this observation?
Which of the following liquids is most likely to be miscible with water in all proportions?
| Alcohol | Formula | Solubility in water at 25 °C (g per 100 mL) |
|---|---|---|
| Methanol | CH₃OH | Miscible |
| 1-Propanol | CH₃CH₂CH₂OH | Miscible |
| 1-Butanol | CH₃(CH₂)₃OH | 7.3 |
| 1-Hexanol | CH₃(CH₂)₅OH | 0.59 |
Approximate solubility data for straight-chain alcohols
Which of the following best explains the trend in solubility shown in the data?
1-Pentanol, CH₃(CH₂)₄OH, has one more carbon than 1-butanol. Which of the following is the best prediction for its solubility in water at 25 °C?
0 of 4 answered