Acids and Bases

Acids and bases are chemical compounds that have certain specific properties in aqueous solutions. In most chemical circumstances, acids are chemicals that produce positively-charged hydrogen ions, H⁺, in water, while bases are chemicals that produce negativelycharged hydroxide ions, OH^-, in water. Bases are sometimes called alkalis. Acids and bases react with each other in a reaction called neutralization. In a neutralization reaction, the hydrogen ion and the hydroxide ion react to form a molecule of water:

Chemically, acids and bases may be considered opposites of each other. The concept of acids and bases is so important in chemistry that there are several useful definitions of "acid" and "base" that pertain to different chemical environments, although the definition above is the most common one.

Acids and bases have some general properties. Many acids have a sour taste. Citric acid, found in oranges and lemons, is one example where the sour taste is related to the fact that the chemical is an acid. Molecules that are bases usually have a bitter taste, like caffeine. Bases make solutions that are slippery. Many acids will react with metals to dissolve the metal and at the same time generate hydrogen gas, H₂. Perhaps the most obvious behavior of acids and bases is their abilities to change colors of certain other chemicals. Historically, an extract of lichens (V. lecanora and V. rocella) called litmus has been used since it turns blue in the presence of bases and red in the presence of acids. Litmus paper is still commonly used to indicate whether a compound is an acid or a base. Extracts made from red onions, red cabbage, and many other fruits and vegetables change colors in the presence of acids and bases. Such materials are called indicators.