“Like dissolves like” is a rule in chemistry that predicts what will dissolve in what: substances with similar molecular properties mix well together, while substances with very different molecular properties don’t. Polar substances dissolve in polar solvents, and nonpolar substances dissolve in nonpolar solvents. It’s the reason oil and water don’t mix, sugar dissolves in tea, and nail polish requires a special remover instead of plain water.
What “Polar” and “Nonpolar” Actually Mean
To make sense of this rule, you need to understand one key concept: polarity. Molecules are made of atoms that share electrons. When the sharing is uneven, one end of the molecule carries a slight positive charge and the other a slight negative charge. That uneven distribution makes the molecule polar. Water is the classic example. Its oxygen atom hogs electrons from the two hydrogen atoms, creating a lopsided charge that makes water molecules strongly attract each other.
Nonpolar molecules share their electrons more evenly, so they don’t have that charge imbalance. Fats, oils, and waxes fall into this category. Their molecules interact through weaker, more temporary attractions that arise from the constant motion of electrons.
The rule, then, is straightforward: polar molecules form strong attractions with other polar molecules, so they dissolve in each other. Nonpolar molecules form their own kind of attractions with other nonpolar molecules, so they dissolve in each other. But polar and nonpolar molecules don’t have enough in common to attract each other strongly, so they tend to separate.
Why Oil and Water Won’t Mix
The oil-and-water example is so familiar it almost seems obvious, but the molecular explanation is more interesting than most people realize. Water molecules form a tight network of hydrogen bonds, where each molecule can participate in up to four connections with its neighbors. When you introduce oil (a nonpolar substance), the water molecules near the oil can still maintain their hydrogen bonds, but doing so requires them to rearrange into a much more constrained, ordered structure around the oil molecules. This ordering is an entropic penalty: the system loses randomness, which nature resists.
The result is that water essentially squeezes the oil out. Oil droplets clump together not because they’re attracted to each other especially strongly, but because grouping together minimizes the surface area that water has to reorganize around. This is why a puddle of oil on water forms a single slick rather than scattering into tiny droplets.
Everyday Examples of the Rule
You encounter “like dissolves like” constantly, even if you’ve never thought about it in chemical terms.
- Sugar in water: Sugar molecules have many polar groups that form hydrogen bonds with water. That’s why sugar dissolves easily in coffee or tea but won’t dissolve in cooking oil.
- Grease on your hands: Water alone doesn’t cut through grease because grease is nonpolar. You need soap, which bridges the gap (more on that below).
- Nail polish remover: Nail polish is made of nonpolar or moderately polar compounds. Acetone, the active ingredient in most removers, is polar enough to mix with water but also has a nonpolar region that interacts with the polish. Plain water can’t touch it.
- Dry cleaning: Traditional dry cleaning uses nonpolar solvents instead of water. These solvents are effective at dissolving greases, oils, and waxes that water-based washing can’t remove, while also being gentler on delicate fibers that water would swell or damage. Liquid carbon dioxide, one newer alternative, works the same way: as a nonpolar solvent, it pulls out oily stains that water would leave behind.
How Soap Breaks the Rule
Soap is the most common workaround for “like dissolves like.” Each soap molecule has a split personality: one end is polar and attracted to water, while the other end is a long nonpolar tail attracted to fats and oils. Chemists call molecules like this amphiphilic, meaning they have an affinity for both types.
When you wash greasy dishes, the nonpolar tails of soap molecules burrow into the grease. The polar heads stay oriented toward the water. Enough soap molecules surround each tiny droplet of grease to form a sphere called a micelle, with the grease trapped inside and the water-friendly heads on the outside. The water can now carry the whole package away down the drain. The soap doesn’t actually change the rule. It exploits it by acting as a molecular translator between two substances that otherwise wouldn’t interact.
Why It Matters for Vitamins and Nutrition
The “like dissolves like” principle directly affects how your body handles vitamins. Vitamins fall into two groups based on their solubility, and the distinction has real consequences for your health.
Vitamins A, D, E, and K are fat-soluble. Their molecular structures are nonpolar, so they dissolve in fats rather than water. Your body absorbs them more easily when you eat them alongside dietary fat, and it stores them in the liver, fatty tissue, and muscles. Because they accumulate rather than wash out, taking large amounts of fat-soluble vitamin supplements can cause them to build up to harmful levels.
Vitamin C and the eight B vitamins are water-soluble. Their polar structures dissolve readily in water, which means your body doesn’t store them well. Excess amounts pass out through urine. You need to consume these vitamins regularly to avoid deficiency. The one exception is B12, which your liver can store for years.
This is “like dissolves like” playing out inside your body. Fat-soluble vitamins dissolve into your fat stores (nonpolar dissolving in nonpolar). Water-soluble vitamins dissolve into your blood and other water-based body fluids (polar dissolving in polar) and get filtered out by your kidneys.
When the Rule Doesn’t Quite Work
The rule is useful, but it’s not perfect. Polarity isn’t a simple on-off switch. It’s a spectrum, and some substances fall in the middle. Ethanol (drinking alcohol), for instance, has a polar end that hydrogen-bonds with water and a short nonpolar carbon chain. It mixes completely with water, but it can also dissolve some nonpolar substances that water can’t. That’s why vanilla extract uses alcohol as a solvent: the flavor compounds from vanilla beans are not water-soluble enough on their own.
Temperature and pressure also matter. Gases become less soluble in water as temperature rises (which is why a warm soda goes flat faster), while many solid substances become more soluble at higher temperatures. These factors operate alongside polarity, not in place of it.
Researchers have worked to move beyond the vague notion of “polarity” and quantify the rule more precisely. A 2021 study used the dielectric constant (a measurable property related to how strongly a substance’s molecules interact electrically) and molar volume of each liquid to build a miscibility map that predicts whether two liquids will mix. The predictions matched known experimental data remarkably well, confirming that “like dissolves like” is more than just a rough guideline: it reflects real, measurable molecular forces.
A Quick Way to Predict Solubility
If you’re trying to figure out whether one substance will dissolve in another, ask yourself a simple question: are both substances polar, or are both nonpolar? If yes, they’ll likely dissolve in each other. If one is polar and the other isn’t, they probably won’t mix well. For substances that fall somewhere in the middle of the polarity spectrum, expect partial solubility. It’s not a guarantee in every case, but it’s the single most reliable shortcut in all of chemistry for predicting what mixes with what.