The phrase like dissolves like is a quick way to predict whether a solute will dissolve in a solvent. Polar solvents, such as water, dissolve polar or ionic solutes because their charged regions can attract and surround the particles. Nonpolar solvents, such as hexane, dissolve nonpolar solutes because their molecules interact through similar weak attractions.
This idea matters in everyday examples like salt dissolving in water, oil separating from water, and grease dissolving in nonpolar cleaners.
Dissolving happens when attractions between solute and solvent particles are strong enough to overcome attractions within the solute and within the solvent. In a polar solution, partial positive and partial negative charges line up to stabilize separated particles, such as water molecules surrounding Na+ and Cl- ions. In a nonpolar solution, molecules mix well when their London dispersion forces are similar, so oil and hexane can blend.
If polarity does not match, the particles do not form favorable interactions, so little or no dissolving occurs.
Understanding Chemistry: Polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes
A useful way to think about dissolving is as an energy balance. Before mixing, solute particles attract one another, and solvent molecules attract one another. Separating either group takes energy.
New attractions form when solute particles become surrounded by solvent molecules. Mixing is more likely when these new attractions return enough energy to make up for the separation. This is why a substance can be polar yet still dissolve only a little.
Its particles may attract each other too strongly for the solvent to pull them apart. Calcium carbonate, for example, has ionic particles but is only slightly soluble in water because its crystal structure is very hard to separate.
Water has an unusual effect on nonpolar substances. Water molecules form a changing network of attractions with nearby water molecules. A nonpolar molecule interrupts that network without providing strong replacement attractions.
Water then tends to gather around the nonpolar surface in an organized arrangement. When oil droplets join into a larger drop, less water surface must surround the oil. This effect helps explain why oil forms droplets or a layer instead of spreading evenly through water.
It is often called the hydrophobic effect. It matters in biology because cell membranes are built from molecules with water avoiding tails.
Some substances contain both polar and nonpolar parts. Soap is a familiar example. One end of a soap molecule is attracted to water, while its long hydrocarbon tail is attracted to oils and grease.
In water, many soap molecules can surround tiny grease droplets. Their tails point inward toward the grease, and their polar ends face outward toward the water. This structure is called a micelle.
It keeps the grease spread through the water long enough to be rinsed away. Detergents work in a similar way. They do not make oil truly behave like water, but they help form a stable mixture called an emulsion.
Solubility is not controlled by polarity alone. Temperature often changes how much solid can dissolve. Stirring speeds up dissolving because fresh solvent reaches the solute surface, but it does not usually change the final amount that can dissolve at a fixed temperature.
Crushing a solid has a similar effect because it creates more surface area. Pressure has little effect on most solids and liquids, yet it strongly affects gases. Carbon dioxide stays dissolved in a sealed fizzy drink because of high pressure.
When the container opens, pressure falls and gas escapes as bubbles. When studying solubility, separate the ideas of how fast dissolving happens from how much can dissolve. A clear solution can still be saturated, meaning it holds the maximum amount possible under those conditions.
Key Facts
- Like dissolves like: polar dissolves polar, and nonpolar dissolves nonpolar.
- Water is polar, so it dissolves ionic and polar solutes such as NaCl and sugar.
- Hexane is nonpolar, so it dissolves nonpolar substances such as grease, oil, and wax.
- A bond is often considered polar when the electronegativity difference is about 0.4 to 1.7.
- Ionic compounds dissolve in polar solvents when ion-dipole attractions overcome the ionic crystal attractions.
- Nonpolar solutes do not dissolve well in water because they cannot form strong attractions with water molecules.
Vocabulary
- Solvent
- The substance that does the dissolving and is usually present in the larger amount.
- Solute
- The substance that is dissolved in a solvent to form a solution.
- Polar molecule
- A molecule with an uneven distribution of charge that creates a partial positive end and a partial negative end.
- Nonpolar molecule
- A molecule with an even charge distribution or a symmetrical shape that has no permanent dipole.
- Ion-dipole attraction
- An attraction between an ion and the partial charge on a polar molecule.
Common Mistakes to Avoid
- Assuming any liquid will dissolve any solid is wrong because dissolving depends on particle attractions, especially polarity.
- Calling water nonpolar because it has no full charge is wrong because its bent shape gives it partial positive and partial negative regions.
- Expecting oil to dissolve in water is wrong because oil is nonpolar and cannot make strong favorable attractions with polar water molecules.
- Using only bond polarity and ignoring molecular shape is wrong because a molecule with polar bonds can be nonpolar if the bond dipoles cancel.
Practice Questions
- 1 A C-H bond has an electronegativity difference of 0.4, while an O-H bond has an electronegativity difference of 1.4. Which bond is more polar, and which type of solvent would better dissolve a molecule with many O-H groups?
- 2 A student mixes 5.0 g of NaCl with 100 mL of water and 5.0 g of NaCl with 100 mL of hexane. Predict which mixture forms a solution and explain using solute-solvent attractions.
- 3 A stain contains mostly cooking oil. Explain whether water or hexane would be the better solvent for removing it, and connect your answer to the mnemonic like dissolves like.