Solutions: solubility and "like dissolves like"

A solution is a homogeneous mixture -- meaning it looks and behaves the same throughout, with no visible boundaries between its parts -- formed when a solute (the substance being dissolved, usually the smaller amount) spreads evenly through a solvent (the substance doing the dissolving, usually the larger amount). Salt water is a solution: salt is the solute, water is the solvent, and once dissolved, you cannot see individual salt particles anywhere in the mixture.

Whether one substance will dissolve in another follows a simple guiding rule: "like dissolves like." Polar substances (and ionic compounds) tend to dissolve well in polar solvents like water, since the attractions between the polar solvent and the dissolving particles are strong enough to pull the solute apart and surround each piece. Nonpolar substances tend to dissolve well only in nonpolar solvents, like oil dissolving in another oil, since there is no strong polar attraction available to separate them from a polar solvent like water -- which is exactly why oil and water famously do not mix.

A solution can be unsaturated (able to dissolve more solute at that temperature), saturated (holding the maximum amount of solute possible at that temperature, with any excess solute simply settling undissolved), or, unusually, supersaturated (holding MORE dissolved solute than should normally be possible, an unstable state usually created by dissolving solute in hot solvent and then very carefully cooling it without disturbance). Temperature affects solubility differently depending on the state of the solute: most solids become MORE soluble as water gets hotter, while gases become LESS soluble as water gets hotter -- which is why a warm soda goes flat faster than a cold one, and why fish struggle in warm water that holds less dissolved oxygen.