Why Does Water Dissolve Salt and Not Oil? Like Dissolves Like
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Whether one substance dissolves in another comes down to whether the attractions between their molecules are comparable. That single principle explains cleaning, extraction, paint, cooking and why oil and water separate.
What dissolving requires
Dissolving means separating the particles of one substance and dispersing them among those of another, which requires breaking the attractions holding each substance together and forming new ones between them. The process happens when the energy released by the new attractions is comparable to the energy required to break the old ones, and it fails when it is not. Water molecules attract each other strongly through hydrogen bonding, so a substance dissolves in water only if it can form comparable attractions, which polar and charged substances can and non-polar ones cannot. Forcing a non-polar substance among water molecules would require breaking their attractions without replacing them, which does not happen spontaneously, so the two separate.
The categories that matter
Solvents are grouped by the attractions their molecules can form:
- •Polar protic, including water and alcohols, which form hydrogen bonds
- •Polar aprotic, which are polar and cannot donate hydrogen bonds
- •Non-polar, including hydrocarbons and oils, which attract each other only weakly
- •Ionic substances dissolve in polar protic solvents and not in non-polar ones
- •Non-polar substances dissolve in each other and not in water
- •Molecules with both a polar and a non-polar part bridge the two, which is what soap does
Why soap works
The whole cleaning industry rests on molecules built to be two things at once. A soap or detergent molecule has a long non-polar tail that associates happily with grease and a charged or strongly polar head that associates happily with water. In water, such molecules cluster with the tails inward and the heads outward, forming small spheres, and a droplet of grease is taken up inside one of those spheres with its surface surrounded by the tails and its exterior presenting the heads to the water. The grease is therefore carried away in water it could never dissolve in, and the process is mechanical rather than chemical. The same arrangement explains emulsifiers in food, which keep oil and water mixed in sauces and dressings.
What raises and lowers solubility
Several factors shift how much dissolves and each has familiar consequences. Temperature usually increases the solubility of solids, which is why more sugar dissolves in hot tea, and it decreases the solubility of gases, which is why a warm fizzy drink loses its gas faster and why warming water drives out dissolved oxygen that fish need. Pressure strongly affects gases and barely affects solids, which is why a sealed carbonated drink holds its gas and releases it when opened. Particle size affects the rate of dissolving without changing how much eventually dissolves. Stirring does the same. And the presence of other dissolved substances can reduce solubility, which is why salt added to water reduces how much of certain other things will dissolve in it.
Where the choice matters
Selecting a solvent is a routine decision with substantial consequences. Extraction of a compound from a mixture works by choosing a solvent that dissolves what is wanted and not what is not, which is how caffeine is removed from coffee, how essential oils are obtained and how a great deal of chemical purification proceeds. Reaction rates and outcomes depend on the solvent, since it interacts with the reacting species and can stabilise one arrangement over another. Paint and adhesive formulation depends on a solvent that holds the material in solution and then evaporates. And solvent choice has become a major environmental and health question, since many effective organic solvents are toxic, flammable or persistent, which has driven substantial work on replacing them with water, with supercritical carbon dioxide and with less harmful alternatives.
The takeaway
Dissolving requires breaking the attractions within each substance and replacing them with comparable ones between them, which is why polar dissolves polar and non-polar dissolves non-polar. Soap molecules have a non-polar tail and a polar head, so they surround grease and carry it away in water. Solvent choice governs extraction, reaction outcomes and a substantial environmental problem.