Acid and base are terms used by chemists to categorize chemicals according to their pH. An acid is generally considered to be any material that gives up a hydrogenion in solution, while a base is any material that creates a hydroxide ion in solution. Many of these acids and bases are familiar in everyday life. The vinegar we use in salad dressings gets its tart flavor from acetic acid, and one of the most common household drugs, aspirin, is a type of acid.Proteins, butter and oils, fruits, and berries all contain a number of natural organic acids. Bases feel slippery when dissolved in water and are used to make soap and other household products. When people get heartburn, they might take baking soda or an antacid tablet, both of which are mild bases. Countless industrial processes use acids and bases as reactants or catalysts to make a variety of consumer goods.
People were probably aware of common acids and bases in prehistoric times, ever since they learned how to make wine. When wine turns sour, it changes to vinegar, or dilute acetic acid. In early times, people roasted limestone to obtain lime (calcium oxide), a base. Gradually, scientists learned to formulate new acidic and basic substances. In the 700s, an Arabian alchemist named Geber (c. 721-815) prepared nitric acid and acetic acid obtained by distilling vinegar. Some time before 1300, sulfuric acid was prepared, and alchemists created aqua regia, a mixture of sulfuric and nitric acids that is capable of dissolving gold, platinum, and many other materials. When strong acids became widely available during the Middle Ages, they launched an experimental revolution. For the first time, alchemists were able to decompose substances without high temperatures and long waiting periods.
During the 1600s, alchemical methods of preparing acids were improved. Johann Rudolf Glauber (1604-1670), a German chemist, set up a small factory for making acids and salts that are formed when acids and bases neutralize each other. Soon chemists became more interested in studying the properties of acids and bases and the neutralization reaction between the two substances. Dutch physician Franciscus Sylvius (1614-1672) diagnosed the human body in terms of its balance between acids and bases. Although Sylvius's ideas were simplistic, it is true that our health depends on a proper balance of acids and bases in our cells and in body fluids such as blood.
During the 1660s, Robert Boyle discovered that certain plant extracts, such as litmus, can be used to distinguish acids from bases. Litmus paper turns red when dipped in an acid, blue when exposed to a base. Since then several other indicator substances have been found that change color at different levels of acidity in a solution. Boyle went on to characterize acids, noting their sour or tart taste and their ability to corrode metals. Scientists speculated that acids were made of sharply pointed particles that literally pricked the tongue or scratched the metal. Neutralization, they theorized, occurs when an acid particle's spikes fit into a basic particle's pores.
During the 1700s, chemists attempted to describe the neutralization process in terms of the affinity, or degree of attraction, between acidic and basic particles. In 1791, German chemist Jeremias Benjamin Richter (1762-1807) demonstrated that a particular acid and base always neutralize each other in the same proportions. The idea that chemicals react in certain fixed proportions is called stoichiometry, upon which quantitative chemistry is based.
A new but erroneous definition of acids was developed by Antoine-Laurent Lavoisier in the 1770s. He mistakenly believed that all acids contain oxygen (named from the Greek words meaning acid-producing) because he observed that acids are formed when oxygen compounds are dissolved in water. Lavoisier's theory was disproved in the early 1800s, when chemists began using the new tool of electricity to break compounds into elements. First, Humphry Davy demonstrated that hydrochloric acid (HCl) contains no oxygen. His finding was supported by Joseph Gay-Lussac, who proved that oxygen is not a component of prussic acid (now known as hydrocyanic acid [HCN]).
Our modern understanding of acids began to take root in the 1830s, when German chemist Justus von Liebig defined an acid as a compound that contains hydrogen in a form that can be displaced by a metal. Bases, however, were understood only in terms of their ability to neutralize acids.
Then in the 1880s, Svante August Arrhenius proposed that when acids and bases dissolve in water, their molecules break up into electrically charged particles called ion s (a term introduced by Michael Faraday). Acids produce positively charged hydrogen ions (H+), while bases produce negatively charged hydroxyl ions (OH- ). Arrhenius's theory also explained, in very simple terms, what happens when an acid and base neutralize each other: the positive and negative ions unite to form water (H2O), which is neutral.
The strength of a particular acid or base depends on its concentration of hydrogen ions that is measured by the pH system on a scale of one (strongest acid) to 14 (strongest base). Strong bases are sometimes called alkalis. Because acidic and basic, or alkaline, solutions both conduct electricity, their strength can also be quantified by measuring their electrical conductivity.
Although Arrhenius's theory represented a giant step in our understanding of acids and bases, it had its limitations. What about solvents other than water, for example? And what about ammonia, which contains no oxygen but produces hydroxyl ions when dissolved in water? Another complication was the fate of the hydrogen ion in water. Instead of floating free, hydrogen ions combine with water molecules to produce a positively charged hydronium ion (H 3O+).
In 1923, Arrhenius's concept was refined by Danish chemist Johannes Nicolaus Bronsted (1879-1947), who broadened our definition of acids and bases. The hydrogen ion, he pointed out, is a proton--a hydrogen atom without its electron, or negatively charged particle. So Bronsted defined acids as proton donors (they release hydrogen ions) and bases as proton acceptors (any substance that will combine with a loose proton). The same idea was proposed simultaneously by British chemist Thomas M. Lowry. The Bronsted-Lowry definition holds up no matter what the solvent is, and it explains why pure acids and dissolved acids behave differently.
The same year that Bronsted's work was published, American chemist Gilbert Newton Lewis suggested a slightly different way of looking at the new definition. Instead of donating protons, acids accept unattached pairs of electrons; conversely, instead of accepting protons, bases supply pairs of electrons. Under Lewis's definition, even substances that do not produce hydroxyl ions can be considered bases.
Despite all of these refinements, most common acids and bases behave just as Arrhenius described. Today, these substances are used in refining oil and sugar and in manufacturing a great variety of products, including fertilizers, explosives, plastics, soap, paper, film, drugs, synthetic fabrics, dyes, solvents, and pesticides.
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