Arrhenius acid-base definition
/ uh-RAY-nee-us /
Long before anyone could see molecules, chemists noticed two opposite kinds of solutions. Some, like lemon juice or vinegar, tasted sour, turned blue litmus red, and dissolved metals. Others, like soap or washing soda, felt slippery, turned red litmus blue, and could neutralize the sour ones. The first modern attempt to say what an acid and a base actually ARE came from Svante Arrhenius in the 1880s, and it tied everything to water.
In the Arrhenius picture, an acid is a substance that increases the concentration of hydrogen ions, H+ (really H3O+, the hydronium ion, since a bare proton never floats around alone in water), when dissolved in water. HCl is an acid because HCl + water gives H+ and Cl-. A base is a substance that increases the concentration of hydroxide ions, OH-. NaOH is a base because it dissolves to give Na+ and OH-. Neutralization is then simply H+ meeting OH- to make water: H+ + OH- gives H2O. Strong acids and bases dissociate almost completely; weak ones only partly.
This definition is clean and quantitative and still underlies the everyday pH scale, but it is narrow. It only works in water, it cannot explain why ammonia (NH3, which has no OH- to give) is a base, and it cannot describe acid-base chemistry in liquid ammonia, molten salts, or the gas phase. Those limits are exactly why broader definitions (Bronsted-Lowry, then Lewis) were invented. Arrhenius is best seen as the correct special case for aqueous solutions, not the whole story.
HCl(g) dissolving in water: HCl + H2O gives H3O+ + Cl-. The solution now has extra hydronium ions, so it is acidic. Add NaOH and the OH- it releases pairs with H3O+ to reform water, neutralizing the acid.
Acid raises H3O+, base raises OH-, and the two combine to water — the whole Arrhenius story in one line.
It is a common shorthand to write H+, but a bare proton does not exist free in water — it is always bound as H3O+ (or larger clusters like H5O2+). The Arrhenius definition is also strictly aqueous; it says nothing about acidity in other solvents.