Acid–base reaction

An acid–base reaction is a fundamental chemical process occurring between an acid and a base. These reactions are essential for understanding chemical behavior across many different environments. Scientists use these interactions to determine pH, which measures how acidic or basic a solution is, often through a method called titration. Because these reactions can happen in gases, liquids, or solids, chemists use several different theoretical frameworks to explain them. These frameworks, known as acid–base theories, complement one another to provide a complete picture of how molecules interact.

One way to understand these reactions is through the Arrhenius theory. Developed by Svante Arrhenius, this model focuses specifically on substances dissolved in water, or aqueous solutions.

An Arrhenius acid is a substance that increases the concentration of hydrogen ions (H+) in water. When these ions enter water, they create hydronium ions (H3O+). An Arrhenius base is a substance that increases the concentration of hydroxide ions (OH-) in a solution. When an Arrhenius acid and base react, they undergo a neutralization reaction. This process typically produces a salt and water as the final products. For example, mixing hydrochloric acid (HCl) with sodium hydroxide (NaOH) produces sodium chloride (NaCl) and water.

While the Arrhenius model is useful, it is quite restrictive because it only applies to water. It cannot explain reactions in non-aqueous solutions, such as substances dissolved in toluene or liquid ammonia. To solve this, the Brønsted–Lowry theory was introduced in 1923 by Johannes Brønsted and Martin Lowry.

This theory defines an acid as a proton donor and a base as a proton acceptor. A proton is another name for a hydrogen cation (H+). In this model, the reaction is viewed as the transfer of a proton from one molecule to another. This process creates what are called conjugate acids and conjugate bases. The acid becomes a conjugate base after losing its proton, while the base becomes a conjugate acid after gaining one.

Because the Brønsted–Lowry theory focuses on proton transfer rather than water, it works in many different environments. It can explain reactions in gases, liquids, and solids. This model even explains why water is amphoteric, meaning it can act as both an acid and a base. In one instance, a water molecule can donate a proton to become a hydroxide ion. In another instance, a water molecule can accept a proton to become a hydronium ion. This flexibility allows chemists to study complex systems, such as acetic acid dissolving in liquid ammonia, which the Arrhenius model cannot fully describe.

Our understanding of these substances has evolved through centuries of discovery. In 1754, Guillaume-François Rouelle first used the term "base" to describe substances that react with acids to form salts. Around 1776, Antoine Lavoisier proposed that acids were defined by the presence of oxygen. This "oxygen theory" lasted for decades until Sir Humphry Davy proved in the 1810s that some acids, like hydrohalic acids, do not contain oxygen. Later, in 1838, Justus von Liebig proposed a hydrogen theory, suggesting acids were compounds where hydrogen could be replaced by a metal. These shifting perspectives allowed scientists to move from simple observations to the sophisticated molecular models used today.

We can see these chemical principles in action in our own homes, specifically in baking powder.

Baking powder is not the same as baking soda, which is sodium bicarbonate (NaHCO3). Instead, baking powder is a mixture of sodium bicarbonate and various acidic salts. When baking powder meets water, the acid and the base react to produce carbon dioxide gas. This gas creates millions of tiny bubbles within dough. These bubbles are responsible for making breads and cakes rise. A common acidic component used in these mixtures is monocalcium phosphate, or calcium dihydrogen phosphate.

Finally, the most inclusive way to view these reactions is through the Lewis definition. Proposed by Gilbert N. Lewis in 1938, this theory removes the requirement for a substance to contain hydrogen. While the Brønsted–Lowry theory is a subset of the Lewis model, the Lewis model is the broadest. It defines acids and bases based on how they handle electron pairs rather than just protons. This allows chemists to classify a much wider range of substances as acids or bases, ensuring that the rules of chemistry apply to almost every molecular interaction encountered in science.