Water is a weak acid

The water molecule has a slight but significant tendency to ionize into a hydroxide ion (OH-) and a hydrogen ion (H+). Actually, two water molecules participate in this ionization. One of the two molecules "captures" a hydrogen ion from the other, forming a hydroxide ion and a hydronium ion:

Water molecule (H2O)

Water molecule (H2O)

Hydroxide ion OH-, a base

Water molecule (H2O)

Hydronium ion ^O+, an acid

The hydronium ion is in effect a hydrogen ion bound to a water molecule. For simplicity, biochemists tend to use a modified representation of the ionization of water:

The ionization of water is important to all living creatures. This fact may seem surprising, since only about one water molecule in 500 million is ionized at any given time. But we are less surprised if we focus on the abundance of water in living systems and the reactive nature of the H+ produced by ionization.

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Essentials of Human Physiology

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