2.1 Acid-base Theory

2.1.1 Arrhenius Theory

Acids are substances that dissociate in water to produce hydrions （H⁺）. An example of a common acid is sulfuric acid，H₂SO₄. In solution，H₂SO₄ dissociates to form hydrogen and sulfate ions according to the following equation:

Additional examples of acids are vinegar，aspirin，and lemon juice. These substances share the following common properties.

（1） Acid solutions taste sour （acid means "sour" in Latin）. Lemons，oranges and other citrus fruits owe their sour taste to the presence of citric acid; the taste of sour milk is due to the presence of lactic acid.

（2） Acids react with many metals to form hydrogen gas. Reactions of this type were studied in connection with the preparation of hydrogen.

（3） Acids turn litmus paper red. Litmus is a dye which has a red color in acid solution and a blue color in basic solution; paper which has been soaked in litmus is defined as litmus paper. Substances of this type，which enable us to determine whether a given solution is acid or base，are called indicators. Methyl orange and phenolphthalein are other indicators frequently used by chemists.

（4） Acid solutions conduct electricity.

（5） Acids react with bases to form salts and water.

（6） Acids react with carbonates to form CO₂（carbon dioxide gas）.

Common strong acids are sulfuric acid （H₂SO₄），nitric acid （HNO₃），hydrochloric acid （HCl），hydrobromic acid （HBr） and hydroiodic acid （HI）. Most other acids are generally only partially ionized and consequently only moderately strong or weak.

Bases are substances that produce hydroxide ions （OH^-） in water solutions. An example of a common base is sodium hydroxide，NaOH. In solution，it dissociates to form sodium ions and hydroxide ions according to the following equation:

Common types of bases are lye，household ammonia，and most soap. The following are four characteristic properties of bases.

（1） Basic solutions taste bitter and feel slippery to the touch.

（2） Bases turn litmus paper blue.

（3） Basic solutions conduct electricity.

（4） Bases neutralize acids.

The common strong bases are sodium hydroxide （also known as caustic soda or lye） （NaOH），potassium hydroxide （KOH），calcium hydroxide [Ca（OH）₂] and barium hydroxide [Ba（OH）₂].

2.1.2 Brönsted-Lowry Theory

In the Brönsted-Lowry theory，an acid is defined as a substance which is capable of donating or giving up a proton and a base is defined as a proton acceptor. In this reaction（2-4），water is acting as a base because it accepts a proton from the HCl molecule. Hydrogen chloride gives up a proton upon dissolving in water，not by a simple dissociation，but by reaction with water. Hydrogen chloride acts as an acid.

It should be noted that the Brönsted-Lowry theory is different from the Arrhenius theory in that the characteristic species in an aqueous acid solution is the hydronium ion，H₃O⁺，not the proton，H⁺.^② The characteristic species in basic solution remain the hydroxylion.

2.1.3 Lewis Concept

A more general concept of acids and bases was introduced by Gilbert N. Lewis. Lewis defined an acid as a molecule or ion that can accept an electron pair from another molecule or ion，and a base as a substance that can share its electron pair with an acid. Thus，an acid is an electron-pair acceptor and a base is an electron-pair donor. When a Lewis acid reacts with a Lewis base，consequently，a coordinate covalent bond is formed. This is essentially the neutralization reaction. In the reaction，H⁺ is a Lewis acid and NH₃ a Lewis base.

There are many reactions which fit the Lewis acid-base concept shown as equation （2-5）.

According to the Lewis theory，substances （for example BF₃ ） other than proton donors behave as acids.^③ In contrast to proton loss or gain in the Brönsted-Lowry concept，the Lewis concept emphasizes the electron pair—a Lewis acid lacks an electron pair in an empty orbit，or has an orbit that can be vacated and a Lewis base has a nonbonding electron pair and can supply this pair to another substance lacking an electron pair.

It is evident that the Lewis concept applies not only to the chemical behavior correlated by the Brönsted-Lowry concept，but also to many chemical reactions that do not involve proton transfer，and for this reason it is most useful.^④ The three theories are summarized in the Table. These the-ories explain how acid-base reactions occur; we will generally use the theory that best explains the chemical reaction under consideration.

Table Acid-base Definitions According to Three Theories