This cheat sheet explains the memory aid that polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes. Students need this idea to predict whether substances will mix, dissolve, or separate into layers. It is especially useful for understanding water, oils, alcohols, salts, and many lab observations.
The main rule is often called like dissolves like.
Polar substances have uneven charge distribution and can attract other polar substances through dipole-dipole forces or hydrogen bonding. Nonpolar substances have more even charge distribution and attract other nonpolar substances mainly through London dispersion forces. Ionic solutes often dissolve well in very polar solvents because ion-dipole attractions can pull ions apart.
A helpful guide is that large electronegativity differences, especially , often suggest polar bonds, but whole-molecule shape also matters.
Key Facts
- The main memory aid is like dissolves like, meaning polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes.
- Water, , is a polar solvent because its bent shape gives the molecule a partial negative oxygen end and partial positive hydrogen ends.
- Oil and hexane, , are nonpolar, so they mix well with other nonpolar substances but not with water.
- A polar bond often forms when the electronegativity difference is about , but molecular geometry determines overall polarity.
- Ionic compounds such as often dissolve in water because ion-dipole attractions between , , and can overcome parts of the ionic lattice.
- Hydrogen bonding occurs when is bonded to , , or , and it helps explain why water dissolves many polar substances.
- Nonpolar solutes are not usually soluble in water because water-water attractions are stronger than water-nonpolar attractions.
- A substance is more likely to dissolve when the attractions formed between solute and solvent are strong enough to replace solute-solute and solvent-solvent attractions.
Vocabulary
- Polar solvent
- A solvent made of molecules with uneven charge distribution that can attract ions or other polar molecules.
- Nonpolar solvent
- A solvent made of molecules with mostly even charge distribution that dissolves nonpolar substances well.
- Solute
- The substance being dissolved in a solution.
- Solvent
- The substance that does the dissolving and is usually present in the greater amount.
- Dipole
- A separation of charge in a molecule or bond that creates a partial positive end and a partial negative end.
- Ion-dipole force
- An attraction between an ion and the partial charge on a polar molecule.
Common Mistakes to Avoid
- Assuming every molecule with polar bonds is polar is wrong because a symmetrical molecular shape can cancel bond dipoles.
- Thinking water dissolves everything is wrong because water is polar and does not form strong attractions with most nonpolar substances.
- Calling oil hydrophobic because it repels water is misleading because oil mainly fails to mix with water due to weak oil-water attractions compared with water-water attractions.
- Forgetting the role of intermolecular forces is wrong because dissolving depends on replacing solute-solute and solvent-solvent attractions with solute-solvent attractions.
- Predicting solubility from formula alone is unreliable because both bond polarity and molecular shape affect whether a molecule is polar or nonpolar.
Practice Questions
- 1 Would dissolve better in water, , or hexane, ? Explain using polarity and ion-dipole forces.
- 2 A bond has electronegativity values and . Calculate and decide whether the bond is likely polar.
- 3 Ethanol, , has a polar group and a nonpolar carbon chain. Predict whether ethanol mixes better with water than hexane does, and explain.
- 4 Two liquids form separate layers after shaking. What does this suggest about their polarities and the strength of their intermolecular attractions?
Understanding Polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes Memory Aid
Dissolving is a competition between attractions. Before mixing, solute particles attract each other and solvent molecules attract each other. To make a solution, some of those original attractions must be pulled apart.
New attractions then form between solute and solvent particles. Mixing is favorable when the new attractions make up for much of the energy needed to separate the original particles.
This is why a simple label such as polar or nonpolar is a useful starting point, not a complete calculation. Solubility can be low when the new attractions are too weak, even if the substances have some similar features.
Water gives a clear example of this energy balance. Water molecules form a connected network of hydrogen bonds. A small nonpolar molecule cannot form strong attractions with that network.
Water tends to remain close to water, pushing the nonpolar particles together. This effect helps oil form droplets or a separate layer in water. Soap works because one end of a soap molecule interacts with water while its long hydrocarbon end interacts with oil or grease.
Many soap molecules surround oily material and form tiny structures called micelles. The oil is held in the middle, while the water-friendly ends face outward into the water.
Ionic solids add another step. In a crystal, positive and negative ions are held in an ordered lattice by strong electrical attraction. Water molecules can surround exposed ions with their oppositely charged ends facing inward.
This process is called hydration. If hydration provides enough attraction, ions leave the crystal and spread through the water. Some ionic compounds still dissolve only a little because their lattices are especially hard to separate.
Temperature can change the outcome. Many solids dissolve more as temperature rises because added thermal energy helps separate particles. Gases usually dissolve less in warmer liquids because faster-moving gas particles escape more easily.
Students meet these patterns in cooking, cleaning, medicine, and laboratory work. Sugar spreads through hot tea, but waxy grease needs detergent or a different solvent. Nail polish remover can dissolve some organic coatings because its molecules interact with parts of those materials better than water does.
In separatory funnels, immiscible liquids form layers. The denser layer is usually at the bottom, though density does not decide whether liquids mix. When predicting solubility, first identify ions, possible hydrogen bonding sites, polar groups, and long nonpolar regions.
Then consider the whole structure, temperature, and the amount of each substance. A molecule with both polar and nonpolar parts may have limited solubility rather than fitting neatly into one category.