Every solution splits into two roles. One component does the dissolving and the other gets dissolved, and two simple rules decide which is which. When both components are in the same physical state, such as two liquids poured together, the solvent is whichever one is present in the greater amount.
When they start in different states, the solvent is the component whose state the finished solution keeps. Work through the numbers. Mix 25.0 g of ethanol with 175.0 g of water and you have 200.0 g of solution.
Water accounts for 175.0 / 200.0 = 87.5 percent by mass, so water is the solvent and ethanol is the solute. Flip the proportions and the labels flip with them. If a 100.0 g sample of rubbing alcohol holds 70.0 g of isopropyl alcohol and 30.0 g of water, the alcohol is the solvent at 70.0 percent by mass, and the water is now the solute.
The second rule covers the awkward cases. Roughly 200 g of table sugar will dissolve in 100 g of water near room temperature, so the sugar outweighs the water by a wide margin. Chemists still call the water the solvent, because the syrup is a liquid and the sugar arrived as a solid.
The same reasoning makes water the solvent in a carbonated drink, where the dissolved carbon dioxide started out as a gas.
Why water dissolves so much
Water’s nickname, the universal solvent, comes straight from its shape. Oxygen carries two lone pairs of electrons that push the two hydrogen atoms into a bent arrangement with a bond angle near 104.5 degrees. Oxygen also pulls shared electrons much harder than hydrogen does, so each O-H bond is polarized.
Because the molecule is bent instead of straight, those two bond polarities do not cancel, and the molecule ends up with a negative oxygen end and a positive hydrogen end. That permanent dipole buys water two abilities. It can form hydrogen bonds, unusually strong attractions between the hydrogen of one molecule and a lone pair on the next.
It also gives water a dielectric constant near 80, which means the electrical pull between two opposite ions sitting in water drops to roughly one eightieth of its strength in a vacuum. Sodium and chloride ions in a salt crystal cannot keep hold of each other under those conditions, so the crystal comes apart. No solvent dissolves everything, though, so universal is a nickname rather than a fact.
What dissolving looks like up close
Freed ions never drift around bare. Solvent molecules swarm each one and lock into a cage called a solvation shell, or a hydration shell when the solvent is water. Around a sodium ion, roughly six water molecules point their negative oxygen ends inward.
Around a chloride ion they turn about and point their positive hydrogen ends inward instead. That wrapping is what keeps the ions apart and the solution clear.
Like dissolves like, and the reason behind it
Dissolving is a trade, not magic. Solute particles have to be pulled away from each other, solvent molecules have to be pushed aside to make room, and new solute-solvent attractions have to form in their place. The trade works out when the new attractions are about as strong as the ones that were broken.
Polar solvents therefore suit polar and ionic solutes, and nonpolar solvents suit nonpolar solutes. Oil fails in water because water molecules would have to give up hydrogen bonds to surround a hydrocarbon chain that offers nothing comparable in return.
The three solvent families
Polar protic: polar molecules carrying an O-H or N-H bond, so they can donate hydrogen bonds. Water, ethanol, and methanol are the everyday members, and they dissolve salts, sugars, and other polar solutes well. Polar aprotic: polar molecules with no O-H or N-H to donate.
Acetone and dimethyl sulfoxide are typical. They dissolve many ionic compounds while leaving negative ions loosely wrapped and unusually reactive. Nonpolar: molecules with almost no charge separation and dielectric constants near 2, such as hexane and toluene.
These dissolve fats, oils, waxes, greases, and most hydrocarbons. Solvent choice changes outcomes, not just convenience. Solubility can differ by many orders of magnitude between families, and reaction speed shifts as well.
A substitution reaction that crawls in ethanol often runs far faster in acetone, because ethanol hydrogen-bonds tightly to the attacking ion and holds it back while acetone leaves it comparatively free. Whenever water is the solvent the solution is called aqueous, and every dissolved species in the equation carries the label (aq).
Solvents fill household and lab shelves alike. Acetone strips nail polish, mineral spirits thin oil-based paint, isopropyl alcohol lifts grease off electronics, and water carries nearly everything in a biology lab. Most organic solvents are also volatile and flammable.
Acetone has a flash point near minus 20 degrees C, so it gives off an ignitable vapor well below room temperature. That property is the reason organic solvents are handled inside a fume hood, which draws vapor away from the bench and vents it out of the building.