Understanding the chemical nature of substances like hydroxide can often lead to confusion regarding their classification as an acid or a base. Correctly identifying whether a compound behaves as an acid or a base requires a clear understanding of fundamental chemical definitions and how these substances interact.
What Defines an Acid?
Acids are typically characterized by their ability to increase the concentration of hydrogen ions (H⁺) in a solution. According to the Arrhenius definition, an acid is a compound that dissociates in water to produce H⁺ ions. For example, hydrochloric acid (HCl) releases H⁺ and Cl⁻ ions when dissolved in water, making it an Arrhenius acid. These H⁺ ions readily combine with water molecules to form hydronium ions (H₃O⁺), which are often used interchangeably with H⁺.
A broader definition, the Brønsted-Lowry theory, identifies an acid as any substance that can donate a proton (an H⁺ ion) to another molecule. This definition expands on the Arrhenius concept by not requiring the presence of water. Acids typically have a sour taste and can change blue litmus paper to red. They also react with certain metals to produce hydrogen gas and can conduct electricity in aqueous solutions.
What Defines a Base?
Bases are substances that generally increase the concentration of hydroxide ions (OH⁻) in an aqueous solution. Under the Arrhenius definition, a base is a compound that dissociates in water to release OH⁻ ions. For instance, sodium hydroxide (NaOH) dissolves in water to produce Na⁺ and OH⁻ ions, classifying it as an Arrhenius base.
The Brønsted-Lowry theory provides a more encompassing definition, characterizing a base as any substance that can accept a proton (H⁺ ion) from another molecule. Bases often feel slippery to the touch, can taste bitter, and turn red litmus paper blue. They also conduct electricity when dissolved in water due to the presence of mobile ions.
Hydroxide: A Fundamental Base
Hydroxide (OH⁻) is classified as a base, not an acid. It is a diatomic anion consisting of one oxygen and one hydrogen atom, carrying a negative electric charge. This enables it to behave as a base under both the Arrhenius and Brønsted-Lowry definitions.
As an Arrhenius base, the hydroxide ion increases the concentration of OH⁻ in water when compounds like sodium hydroxide (NaOH) dissolve. From the Brønsted-Lowry perspective, the hydroxide ion acts as a proton acceptor. It readily accepts a hydrogen ion (H⁺) to form a water molecule (H₂O), thereby neutralizing an acid. Many metal hydroxides, such as lithium hydroxide and calcium hydroxide, are strong bases because they readily liberate hydroxide ions in solution.