Characteristics of Solutions

• Solute — the substance present in the smaller amount; it gets dissolved (e.g. the salt in salt water).
• Solvent — the substance present in the larger amount; it does the dissolving (e.g. the water in salt water). In aqueous solutions, water is always the solvent.
• Solution — the resulting homogeneous mixture of solute(s) dissolved in solvent.
• Aqueous (aq) — dissolved in water. This is the most common type of solution in high school chemistry.
• Miscible / immiscible — two liquids that mix in all proportions to form one solution are miscible (ethanol and water); liquids that separate into layers are immiscible (oil and water).

At a given temperature, a solvent can only dissolve so much solute before it "fills up." Why is there a limit at all? Dissolved particles are constantly moving, and some of them collide with the undissolved solid and stick back onto it. The more solute that is dissolved, the more often this return trip happens, until the rate of particles leaving the solid equals the rate of particles rejoining it. From then on the concentration stops changing, even though particles are still moving in both directions. This gives us three states worth distinguishing.

• Unsaturated — the solution holds less solute than the maximum possible at that temperature; more solute could still dissolve.
• Saturated — the solution holds exactly the maximum amount of solute possible at that temperature. Any extra solute added will not dissolve and will sit at the bottom as excess undissolved solid — this is a dynamic equilibrium where dissolving and recrystallizing happen at equal rates.
• Supersaturated — the solution holds more solute than the normal maximum at that temperature. This is an unstable, non-equilibrium state usually made by dissolving solute at a high temperature (where solubility is greater) and then carefully cooling the solution without disturbing it. The excess dissolved solute will crystallize out rapidly if the solution is disturbed (e.g. by a seed crystal, a scratch, or a bump).

Pure water is a poor conductor of electricity. But dissolve table salt in it, and the solution conducts well. The difference is whether the dissolved solute produces free-moving charged particles (ions) in solution — electric current in a solution is carried by ions migrating toward the oppositely charged electrode, not by electrons flowing through a wire. Distilled water barely conducts because it contains only a tiny concentration of ions from water itself; tap water conducts somewhat better because it carries dissolved minerals such as ${\text{Ca}}^{2+}$, ${\text{Mg}}^{2+}$ and ${\text{HCO}}_3^{-}$ ions.

• Strong electrolyte — dissolves (or reacts with water) to produce a high concentration of ions; conducts electricity well. Examples: ionic compounds like $\text{NaCl}$, and strong acids/bases like $\text{HCl}$ and $\text{NaOH}$.
• Weak electrolyte — produces only a small concentration of ions; conducts weakly. Example: acetic acid, ${\text{CH}}_3\text{COOH}$, in vinegar.
• Non-electrolyte — dissolves as whole, uncharged molecules; produces no ions and does not conduct electricity. Examples: sugar (sucrose, ${\text{C}}_{12}{\text{H}}_{22}{\text{O}}_{11}$) and ethanol dissolved in water.