Difference Between Acids and Bases
Just what is an acid? To go beyond describing an acid’s properties to understanding its chemistry, we need to briefly review the basic structure of the atom and its binding forces.
The hydrogen atom, with only one proton and one electron, is the simplest of all the atoms. Suppose now that it loses its electron. All that is left is a single proton. No longer a neutral element, a hydrogen atom stripped of its electron has become an ion—an electrically charged particle. The hydrogen ion bears a single positive charge. We represent this with the symbol H+. Since the hydrogen ion consists of a single proton, chemists use the terms “hydrogen ion” and “proton” interchangeably.
Now that we know what a hydrogen ion is, we can understand one of the most widely accepted definitions of an acid: Acids are any substances that produce hydrogen ions, or protons—that is, a molecule that can lose one or more hydrogen ions from its structure. In fact, it is the hydrogen ion derived from acids that gives acidic foods their sour taste.
Let us see what happens when hydrochloric acid (hydrogen chloride, HCl) dissolves in water (H2O). Theoretically, the HCl could split into two ions—a hydrogen ion (H+) and a chlorine ion (Cl−).
In actual fact, the hydrogen ion (H+) never exists by itself in a solution of water. In such a situation, the ion immediately combines with a water molecule, H2O, as follows:
H+ + H2O → H3O+
This gives us the overall chemical equation for hydrochloric acid dissolved in water:
HCl + H2O → H3O+ + Cl−
H3O+ is called a hydronium ion—a hydrogen ion combined with a molecule of water. It is not a particularly stable molecule, however, and it easily splits back into a molecule of water and a hydrogen ion. Likewise, a hydronium ion will freely give up its “extra” hydrogen to any substance that shows an attraction for it—namely, a negative ion.
The end result of all this chemical give-and-take is a multitude of readily available hydrogen ions. So we see that HCl fits our definition of an acid.
Like acids, bases ionize, or split into ions, when dissolved in water. But while acids split to produce hydrogen ions (H+), most bases produce hydroxyl ions (OH−) when dissolved in water. As you can see from its formula, the hydroxyl ion is made up of an oxygen and a hydrogen atom that together bear a single negative charge.
To illustrate, let us look at what happens when we dissolve the familiar base lye (sodium hydroxide, NaOH) in water. When NaOH ionizes, the chemical split takes place between the sodium and the oxygen atom. The resulting products are a positive sodium ion (Na+) and the negative hydroxyl ion (OH−). So we write the equation for the ionization of sodium hydroxide as follows:
NaOH → Na+ + OH−
Just as the hydrogen ions (H+) give acidic foods their sour taste, hydroxyl ions give basic solutions their soapy feel.
For many years, chemists defined a base as any substance that yields hydroxyl ions in water. But in modern times, chemists found that this definition was insufficient to encompass all bases. Today, the most widely accepted definition of a base can be stated as follows: a base is a chemical substance that can gain hydrogen ions, or protons, in a chemical reaction. So just as an acid is a proton producer, a base is a proton acceptor, a concept illustrated in the following reaction:
H+ + OH− → H2O
Not surprisingly, the negatively charged hydroxyl ion exhibits a strong attraction to a positively charged hydrogen ion. As you can see in the equation shown above, the resulting reaction forms water.
But as you will recall, in a water solution, the hydrogen ion is always carried on a water molecule in the form of a hydronium ion (H3O+). So the following balanced reaction is considerably more accurate:
H3O+ + OH− → 2 H2O
(two water molecules)
When such a reaction occurs, it is said that the acid and base have neutralized one another. When this “cancelling out” process occurs, a solution loses any acidic properties such as sour taste or any basic properties such as soapiness. The end product is simply water.