↵ Learning Objective Identify an Arrhenius acid and an Arrhenius base. Write the chemical reaction between an Arrhenius acid and an Arrhenius base. Historically, the first chemical definition of an acid and a base was put forward by Svante Arrhenius, a Swedish chemist, in 1884. An Arrhenius acid is a compound that increases the H + ion concentration in an aqueous solution. The H + ion is just a bare proton, and it is rather clear that bare protons are not floating around in an aqueous solution. Instead, chemistry has defined the hydronium ion (H 3 O + ) as the actual chemical species that represents an H + ion. H + ions and H 3 O + ions are often considered interchangeable when writing chemical equations (although a properly balanced chemical equation should also include the additional H 2 O). Classic Arrhenius acids can be considered ionic compounds in which H + is the cation. Table \(\PageIndex{1}\) lists some Arrhenius acids and their names. Formula Name HC2H3O2 (also written CH3COOH) acetic acid HClO3 chloric acid HCl hydrochloric acid HBr hydrobromic acid HI hydriodic acid HF hydrofluoric acid HNO3 nitric acid H2C2O4 oxalic acid HClO4 perchloric acid H3PO4 phosphoric acid H2SO4 sulfuric acid H2SO3 sulfurous acid An Arrhenius base is a compound that increases the OH − ion concentration in aqueous solution. Ionic compounds of the OH − ion are classic Arrhenius bases. Example \(\PageIndex{1}\) Identify each compound as an Arrhenius acid, an Arrhenius base, or neither. HNO 3 CH 3 OH Mg(OH) 2 Solution This compound is an ionic compound between H + ions and NO 3 − ions, so it is an Arrhenius acid. Although this formula has an OH in it, we do not recognize the remaining part of the molecule as a cation. It is neither an acid nor a base. (In fact, it is the formula for methanol, an organic compound.) This formula also has an OH in it, but this time we recognize that the magnesium is present as Mg 2 + cations. As such, this is an ionic compound of the OH − ion and is an Arrhenius base. Exercise \(\PageIndex{1}\) Identify each compound as an Arrhenius acid, an Arrhenius base, or neither. KOH H 2 SO 4 C 2 H 6 Answer Arrhenius base Arrhenius acid neither Acids have some properties in common. They turn litmus, a plant extract, red. They react with some metals to give off H 2 gas. They react with carbonate and hydrogen carbonate salts to give off CO 2 gas. Acids that are ingested typically have a sour, sharp taste. (The name acid comes from the Latin word acidus , meaning "sour.") Bases also have some properties in common. They are slippery to the touch, turn litmus blue, and have a bitter flavor if ingested. Acids and bases have another property: they react with each other to make water and an ionic compound called a salt. A salt , in chemistry, is any ionic compound made by combining an acid with a base. A reaction between an acid and a base is called a neutralization reaction and can be represented as: acid + base → H 2 O + salt The stoichiometry of the balanced chemical equation depends on the number of H + ions in the acid and the number of OH − ions in the base. Example \(\PageIndex{2}\) Write the balanced chemical equation for the neutralization reaction between H 2 SO 4 and KOH. What is the name of the salt that is formed? Solution The general reaction is as follows: H 2 SO 4 + KOH → H 2 O + salt Because the acid has two H + ions in its formula, we need two OH − ions to react with it, making two H 2 O molecules as product. The remaining ions, K + and SO 4 2 − , make the salt potassium sulfate (K 2 SO 4 ). The balanced chemical reaction is as follows: H 2 SO 4 + 2KOH → 2H 2 O + K 2 SO 4 Exercise \(\PageIndex{2}\) Write the balanced chemical equation for the neutralization reaction between HCl and Mg(OH) 2 . What is the name of the salt that is formed? Answer 2HCl + Mg(OH) 2 → 2H 2 O + MgCl 2 ; magnesium chloride Key Takeaways An Arrhenius acid is a compound that increases the H + ion concentration in aqueous solution. An Arrhenius base is a compound that increases the OH − ion concentration in aqueous solution. The reaction between an Arrhenius acid and an Arrhenius base is called neutralization and results in the formation of water and a salt.