Acids were first recognized as a distinct class of compounds because of the common properties of their aqueous solutions. Aqueous solutions have a sour taste (many are corrosive and poisons). Acids change the color of acid-base indicators. Some acids react with active metals to release hydrogen gas, H 2.

Acids react with bases to produce salts and water. Some acids conduct an electric current. Used in petroleum refining, automobile batteries, and used as a water-removing agent (can cause serious burns).

Used in making explosives (stains skin and causes serious burns). Used for manufacturing fertilizers and animal feed and for flavoring beverages.

Used in food processing. Used in the manufacture of plastics and as a fungicide.

Aqueous solutions of bases have a bitter taste. Bases change the color of acid-base indicators. Dilute aqueous solutions of bases feel slippery. Bases react with acids to produce salts and water. Bases conduct electric current.

An Arrhenius acid is a chemical compound that increases the concentration of hydrogen ions, H +, in aqueous solution. Examples: HCl, H 2 SO 4, HI, HF

An Arrhenius base is a substance that increases the concentration of hydroxide, OH -, in aqueous solution. Examples: NaOH, LiOH, Ca(OH) 2

A Brønsted-Lowry acid is a molecule or ion that is a proton donor (H + ). A Brønsted-Lowry base is a molecule or ion, that is a proton acceptor. In a Brønsted-Lowry acid-base reaction, protons are transferred from one reactant (the acid) to another (the base). HCO 3 + HOH → H 2 CO 3 + OH - Base Acid

A Lewis acid is an atom, ion, or molecule that accepts an electron pair to form a covalent bond. A Lewis base is an atom, ion, or molecule that donates an electron pair to form a covalent bond. A Lewis acid-base reaction is the formation of one or more covalent bonds between an electron-pair donor and an electron-pair acceptor.

Pure water is a weak electrolyte and undergoes self-ionization as seen below. Remember ionization is the process of adding or removing electrons from an atom or molecule, which gives the atom or molecule a net charge. In the self-ionization of water, two water molecules produce a hydronium ion and a hydroxide ion by transfer of a proton. hydronium ionhydroxide ion (Acid)(Base)

Concentrations of hydronium ion (H 3 O + ) and hydroxide ion (OH - ) are 1.0 x mol/L in water at 25 o C. The product of [H 3 O + ][OH - ] = 1.0 x and the product is called the ionization constant of water, K w. Note: [ ] means molar concentration (mol/L).

Any solution in which [H 3 O + ] = [OH - ] is neutral. Any solution in which [H 3 O + ] is greater than [OH - ] is acidic. Remember acids increase the concentration of H 3 O + in aqueous solutions. Any solution in which [OH - ] is greater than [H 3 O + ] is basic. Remember bases increase the concentration of OH - in aqueous solutions.

Instead of expressing acidity or basicity in terms of the concentration of H 3 O + or OH -, a quantity called pH is used.

The pH scale commonly has values from 0 to 14 with a pH value of 7 considered neutral. pH values less than 7 are acidic and pH value greater than 7 are basic.

pH indicates the hydronium ion (H 3 O + ) concentration of a solution. The pH of a solution is defined as the negative of the common logarithm of the hydronium ion concentration. pH = -log[H 3 O + ] or pH = [H + ] pOH (hydroxide ion concentration) pOH = -log[OH - ] pH + pOH = 14.0