Acids and Bases

Acids and bases are two complementary classes of chemical compounds whose reactions are among the most common in chemistry and biology. In the simplest aqueous view, an acid is a substance that releases hydrogen ions into water, while a base accepts them or releases hydroxide ions; a broader definition treats acids as hydrogen-ion donors and bases as hydrogen-ion acceptors. When an acid and a base react, they neutralise each other, typically forming a salt and water.

The acidity of a solution is expressed by the pH scale, which measures the concentration of hydrogen ions. Lower pH values indicate acidic solutions, higher values indicate basic or alkaline solutions, and a neutral solution such as pure water sits in the middle of the scale. Strong acids and bases dissociate almost completely in water, releasing or accepting many ions, whereas weak acids and bases only partly dissociate, leaving most of their molecules intact. This difference governs how vigorously they react and how their pH responds to dilution.

Acid-base chemistry is central to living systems. Amino acids, the building blocks of proteins, are a special case: each molecule contains both an acidic group and a basic group, which is why they are called amino acids. The set of amino acids found in proteins includes alanine, cysteine, aspartic acid, glutamic acid, phenylalanine, histidine, isoleucine, lysine, leucine and methionine, among others. Their acid and base groups allow them to link into chains and to respond to the pH of their surroundings, which is why the body carefully regulates the pH of its fluids.

Frequently asked questions

What does the pH scale measure?

The pH scale measures the concentration of hydrogen ions in a solution; lower values indicate acidic conditions, higher values indicate basic conditions, and the middle is neutral.

What is the difference between a strong and a weak acid?

A strong acid dissociates almost completely in water, releasing nearly all its hydrogen ions, while a weak acid only partly dissociates, leaving most of its molecules intact.

Why are amino acids called acids?

Each amino acid contains an acidic carboxyl group alongside a basic amino group, so it has acid-base character, which is reflected in the name and lets the molecules respond to pH.