There are several different definitions for acids and bases. The most narrow is the Arrhenius definition. An Arrhenius acid is a molecule that dissociates in water to form hydrogen (H+) ions and an Arrhenius base is a molecule that dissociates in water to form hydroxide (OH-) ions. Hydrochloric acid (HCl) is a chlorine atom covalently bonded to a hydrogen atom. But in water, the covalent bond breaks and an ionic bond is formed instead.

Looking at the Lewis structure for the chlorine atom, you can see that it needs one additional electron to achieve a stable electron configuration. When paired with a hydrogen atom, it can achieve that stable configuration by either sharing an electron with the hydrogen atom and forming a covalent bond, or taking an electron from the hydrogen atom and forming an ionic bond.

In a pure hydrochloric acid state, the covalently bonded configuration is more stable. However, the covalent bond will be extremely polar because the shared electrons will be pulled toward the chlorine atom and spend most of their time around the chlorine atom. But mixed in water and surrounded by polar water molecules, the ionically bonded configuration becomes more stable, and the hydrochloric acid molecules will dissolve and form hydrogen (H+) and chlorine (Cl-) ions.

Acids are corrosive to many substances, including metals and organic molecules. The free hydrogen ions in acids corrode metals by breaking metallic bonds and transforming metal atoms into metal ions. These metal ions then dissolve in water, forming metal oxide (e.g., FeO), metal hydroxide (e.g., Fe(OH)2), and (in the case of hydrochloric acid) metal chloride (e.g., FeCl2) molecules. These reactions all proceed through the transfer of electrons from metal atoms to hydrogen (H+) ions.

Acids and bases are also highly corrosive to organic molecules because they catalyze the breaking of chemical bonds in esters (C-O-C) and amides (O=C-N). Ester and amide functional groups are common in fats and proteins.

Sodium hydroxide (NaOH) is an example of an Arrhenius base. In water, sodium hydroxide dissolves into sodium (Na+) and hydroxide (OH-) ions. Arrhenius acids and bases “neutralize” each other. This means that if you combined hydrochloric acid with sodium hydroxide, they would react to produce a salt and water.

(Salts are substances that are formed in an Arrhenius acid-base reaction. Sodium chloride is simply one example of a salt. We call sodium chloride “salt” just like we call sucrose “sugar”… even though there are many different types of sugar, such as glucose, fructose, and lactose.)

Hydrochloric acid is considered a strong acid because almost all of the hydrochloric acid (HCl) molecules will dissociate into hydrogen (H+) and chlorine (Cl-) ions. Acetic acid (CH3COOH) is considered a weak acid because only a small percentage of the acetic acid molecules will dissociate into hydrogen (H+) and acetate (CH3COO-) ions. In a cup of vinegar, that is less than 0.5% of the acetic acid molecules.

In an acetic acid and water solution, the system is in dynamic equilibrium. The covalent bond between the oxygen and the hydrogen atom in the acetic acid molecule is constantly breaking, forming hydrogen and acetate ions. Meanwhile, hydrogen and acetate ions are constantly colliding, re-forming covalent bonds and acetic acid molecules. When a hydrogen ion and an acetate ion collide, it is over 60,000 times more likely to re-form an acetic acid molecule than an acetic acid molecule is to dissociate into ions. This is because, for weak acids like acetic acid, the covalently bonded configuration is more stable than the ionically bonded configuration, even when surrounded by polar water molecules.

The strength of an acid can be measured by its dissociation constant, Ka. The dissociation constant is the ratio of an acid molecule’s rate of dissociation to its ions’ rate of “re-association.” This ratio is often expressed by the logarithmic constant, pKa, which equals -log10Ka.

The dissociation constant measures the strength of an acid molecule, not the strength of an acid solution. 1 mL of HCl mixed in one liter of water is a weaker acid solution than 50 mL of HCl mixed in one liter of water, even though both are hydrochloric acid solutions. The strength of an acid solution is determined by the concentration of H+ ions in the solution. This is often expressed by the logarithmic constant, pH, which equals -log10[H+].

Solutions with a pH lower than 7 are considered acidic, while solutions with a pH higher than 7 are considered basic. Pure water has a pH of 7. This means that the concentration of H+ ions in pure water is 1 × 10-7 mol/L. Pure water contains H+ ions because water itself is an Arrhenius acid. Similar to hydrochloric acid, the covalent bond between the oxygen atom and one of the hydrogen atoms in a water molecule can break and form an ionic bond instead.

You should notice that water is also an Arrhenius base. Water molecules dissociate into both H+ and OH- ions.

Some molecules behave like acids and bases even though they do not dissociate into H+ or OH- ions. For example, both ammonia (NH3) and sodium bicarbonate (NaHCO3) behave like bases by neutralizing acids and forming salts.

And carbon dioxide reacts with water to produce H+ ions like an acid, but does not contain any hydrogen atoms itself. The acidity of carbon dioxide is what causes the oceans to acidify as carbon dioxide builds up in the atmosphere and dissolves in ocean water.

Brønsted-Lowry acids and bases are defined by their ability to donate and accept protons (H+ ions), respectively. According to this definition, acid-base reactions no longer have to occur in water, and acids and bases are paired with conjugate bases and conjugate acids. Once an acid has donated a proton, it becomes a conjugate base capable of accepting a proton.

The definition for Lewis acids and bases is even broader. Instead of basing their definition on the exchange of H+ ions, Lewis acids and bases are defined by their ability to accept and donate electron pairs. In this reaction, the pyridine (C5H5N) molecule is the base and the borane (BH3) molecule is the acid.

This reaction looks nothing look a classic Arrhenius acid-base reaction, but it fits the Lewis definition. The nitrogen atom in the pyridine molecule has a lone pair of electrons that it can donate. Meanwhile, the boron atom in the borane molecule is electron deficient. Even though the borane molecule is somewhat stable, the boron atom only has six electrons in its valence shell. In this reaction, the boron atom accepts the lone pair of electrons from the nitrogen atom, and the two atoms form a dipolar bond. (A dipolar bond is a covalent bond that is formed when the two electrons shared by the two atoms both come from one of the atoms.)