Acid & Alkaline Reactions
The combination of acidic compounds and basic (also known as alkaline) compounds constitutes one of the major reaction types in chemistry. Acid-base chemistry is a rich topic, and includes acid-base titrations (in which acids and bases are analyzed), pH and buffered solutions.
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Acids
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Acids increase the amount of hydrogen ions (H⁺) in a solution through a process called dissociation. Muriatic acid (also known as hydrochloric acid, HCl), for example, dissociates into hydrogen ions and chloride ions:
HCl ' H⁺ + Cl⁻
Acids are further characterized as "strong" or "weak." Strong acids such as HCl fully dissociate; that is, every molecule of HCl breaks apart into its respective ions. Weak acids do not fully dissociate. An example is acetic acid (the active ingredient in vinegar, CH₃COOH). In this case, some of the acetic acid molecules dissociate and some do not. The exact proportion of molecules that undergo dissociation varies from one acid to another, and also depends on temperature.
Bases
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Bases increase the amount of hydroxide ions (OH⁻) in a solution. Like strong acids, strong bases fully dissociate when dissolved. Sodium hydroxide (NaOH), for instance, breaks into sodium ions (Na⁺) and hydroxide ions. As with acids, bases may also be "weak." A weak base, such as carbonate ion (CO₃²⁻), does not directly release hydroxide ions, but rather increases the amount of hydroxide ions in the solution by removing H+ from water:
CO₃²⁻ + H₂O ' HCO3⁻ + OH⁻
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Acid + Base
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The general rule in acid-base chemistry is acid + base = salt + water.
This process is called neutralization. Keep in mind that "salt" refers to a water-soluble compound comprised of a metal and a nonmetal. It does not necessarily refer to table salt (sodium chloride).
As an example, consider the reaction when hydrochloric acid (HCl) is mixed with potassium hydroxide (KOH):
HCl + KOH ' KCl + H₂O
Uses
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Any compound that is acidic or basic and soluble in water can be analyzed by acid-base titration. A chemist, for example, might use an acid-base titration to analyze the acid content of wine or vinegar. In this case, a dilute solution of strong base (most likely sodium hydroxide), whose concentration is known to high precision, is added dropwise to the wine or vinegar until the acid and base are in equal proportion. The chemist can then calculate the exact amount of acid in the wine or vinegar.
Over-the-counter antacid tablets contain basic compounds (usually calcium carbonate or magnesium hydroxide) that neutralize excess stomach acid.
Acid-base chemistry is also the underlying principle behind pH buffers. Buffered solutions resist changes in pH when acids or bases are added to the solution. Nature makes extensive use of buffers in natural bodies of water such as lakes and streams.
Fun Fact
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Svente Arrhenius received the 1903 Nobel Prize in Chemistry for his theories on the dissociation of acids.
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