"Like Dissolves Like": Solubility and Polarity

Sugar dissolves in tea but not in cooking oil. Grease won’t wash off with water, but comes off easily with washing-up liquid. Nail varnish comes off with acetone, not water. Chemists sum up these observations with a short rule: “like dissolves like”. Substances tend to dissolve in solvents that are chemically similar to them — especially in polarity. This article explains what the rule means, why it works, and where it breaks down.

The rule

• Polar substances and ionic compounds tend to dissolve in polar solvents (water, ethanol, methanol, propanone).
• Non-polar substances tend to dissolve in non-polar solvents (hexane, petrol, oils, cyclohexane).
• Polar and non-polar substances tend not to mix.

A polar molecule has a permanent separation of charge because of polar bonds arranged asymmetrically; a non-polar molecule doesn’t (see how to tell if a molecule is polar).

Some everyday examples

Solute	Solvent	Dissolves?	Why
Table salt (ionic)	Water (polar)	Yes	Ion–dipole attractions
Sugar (polar, many –OH)	Water	Yes	Hydrogen bonding
Ethanol (polar)	Water	Yes, in all proportions	Hydrogen bonding
Cooking oil (non-polar)	Water	No	Can’t hydrogen-bond; water prefers itself
Iodine (non-polar)	Hexane (non-polar)	Yes	Similar London forces
Iodine	Water	Only slightly	Non-polar solute, polar solvent
Wax	Petrol	Yes	Both non-polar
Nail varnish (polymer)	Propanone (acetone)	Yes	Similar intermolecular forces

Why it works: breaking and making attractions

Dissolving involves three sets of attractions (see intermolecular forces):

1. Solute–solute attractions must be overcome to separate solute particles (costs energy).
2. Solvent–solvent attractions must be overcome to make space for them (costs energy).
3. Solute–solvent attractions form when the particles mix (releases energy).

Dissolving happens readily when the new solute–solvent attractions are comparable to or stronger than the ones that were broken. That’s most likely when solute and solvent have similar types of intermolecular force.

Case 1: polar in polar — sugar in water

Sugar molecules are held to each other by hydrogen bonds between their many –OH groups. Water molecules are also held together by hydrogen bonds. When sugar dissolves, sugar–sugar and water–water hydrogen bonds are replaced by sugar–water hydrogen bonds. The trade is roughly even, so dissolving is easy.

Case 2: ionic in polar — salt in water

Water molecules surround ions, with their δ− oxygen ends towards cations and δ+ hydrogen ends towards anions. These ion–dipole attractions release enough energy to pull ions out of the lattice (see ion–dipole forces).

Case 3: non-polar in non-polar — iodine in hexane

Iodine molecules are held by London forces, and so are hexane molecules. When iodine dissolves, I₂–I₂ and hexane–hexane London forces are replaced by I₂–hexane London forces of similar strength. Iodine dissolves readily, giving a purple solution.

Case 4: non-polar in polar — oil in water

Oil molecules are held by London forces. Water molecules are held by strong hydrogen bonds. To mix oil into water, hydrogen bonds between water molecules must be broken — but oil molecules can’t form hydrogen bonds with water to replace them. They can only form weak London forces. So mixing costs energy without enough payback.

There’s also an entropy effect: water molecules next to an oil molecule arrange themselves into more ordered “cages” to keep as many hydrogen bonds as possible, which reduces entropy. This hydrophobic effect drives oil droplets to clump together, minimising their contact with water. It’s why oil forms a separate layer on top of water (oil is also less dense), and why salad dressing separates.

Molecules with both personalities

Many molecules have a polar part and a non-polar part. Their solubility depends on the balance.

Alcohols

Alcohols have a polar –OH group and a non-polar hydrocarbon chain:

Alcohol	Solubility in water at 20 °C
Methanol, CH₃OH	Mixes in all proportions
Ethanol, C₂H₅OH	Mixes in all proportions
Propan-1-ol, C₃H₇OH	Mixes in all proportions
Butan-1-ol, C₄H₉OH	About 7–8 g per 100 g water
Pentan-1-ol, C₅H₁₁OH	About 2 g per 100 g water
Hexan-1-ol, C₆H₁₃OH	About 0.6 g per 100 g water

As the non-polar chain gets longer, the molecule becomes more “oil-like”, and solubility in water falls. The same pattern appears in carboxylic acids, amines and many other families.

Soaps and detergents

Soap molecules have a long non-polar hydrocarbon tail (about 12–18 carbons) and a polar or ionic head (for example, –COO⁻ Na⁺). In water:

1. The non-polar tails dissolve in grease or oil (“like dissolves like”).
2. The polar heads stay in the water.
3. Soap molecules surround grease droplets, tails inward and heads outward, forming micelles.
4. The micelles are carried away by water, taking the grease with them.

Soap is a bridge between two things that wouldn’t otherwise mix. The same principle lets detergents break open cell membranes in the DNA extraction practical, and it underpins how phospholipids form membranes.

“Like dissolves like” in the real world

• Vitamins: water-soluble vitamins (C and the B vitamins) have many polar groups and are excreted in urine; fat-soluble vitamins (A, D, E, K) are largely non-polar and are stored in fatty tissue (see vitamin myths).
• Drugs: medicines must be soluble enough in water to travel in the blood, but non-polar enough to cross fatty cell membranes (see how drugs are designed).
• Dry cleaning: uses non-polar solvents to remove greasy stains that water can’t.
• Pollution: non-polar pollutants such as some pesticides dissolve in animal fat rather than water, and build up along food chains.
• Chromatography: separates substances according to how strongly they’re attracted to polar or non-polar phases (see chromatography).

Limitations of the rule

“Like dissolves like” is a guide, not a law:

• Not all ionic compounds dissolve in water. Calcium carbonate and barium sulfate are almost insoluble, because their lattice energies are very large (see solubility rules).
• Some polar substances dissolve poorly if they’re very large or pack very tightly in the solid — cellulose is full of –OH groups but is insoluble in water.
• Solubility changes with temperature: most solids become more soluble in hot water, while gases become less soluble.
• Reactions can look like dissolving: HCl gas “dissolves” in water by reacting to form ions.

Common misconceptions

• “Oil doesn’t dissolve in water because oil is lighter.” Density decides which layer is on top, not whether they mix.
• “Water dissolves everything.” It’s a great solvent for polar and ionic substances, but not for non-polar ones.
• “Dissolving is always exothermic.” Many substances dissolve with a small energy cost, helped by the entropy increase.
• “Salt molecules dissolve.” Salt has no molecules; its ions separate.

Key takeaways

• Polar and ionic solutes dissolve in polar solvents; non-polar solutes dissolve in non-polar solvents.
• Dissolving works when solute–solvent attractions can replace solute–solute and solvent–solvent attractions.
• Oil and water don’t mix because oil can’t replace water’s hydrogen bonds (the hydrophobic effect).
• Molecules with polar and non-polar parts, like alcohols and soaps, show intermediate behaviour.
• The rule has exceptions, especially for ionic compounds with large lattice energies.