When dissolved in water, some chemical substances split up as unequal fragments with electrical charges, positive and negative. Each charged atom or group is called an ion. Opposite charges attract and like charges repel, just like the North and South poles of a magnet. Positive ions are called cations because they migrate toward the cathode in an electric field, and negative ions are called anions because they go toward the anode. (These are pronounced CAT-ions and AN-ions.) As a generalization, inorganic chemicals are most usually “ionic” compounds that ionize when dissolved and organic chemicals are usually non-ionic, but there are many exceptions. Three chemical types are always ionic, whether organic or inorganic, and those are acids, bases, and salts:

Acids: chemicals that liberate a hydrogen ion, H+, when dissolved in water. The H+ is one charged fragment of the original molecule, and the remainder becomes negatively-charged as a result. Example: citric acid:

C6H8O7

<=>

C6H7O7

+

H+

citric acid
molecule

citrate ion

Hydrogen ion

Base: Chemicals that liberate a hydroxide ion, OH-, when dissolved in water the OH-, when dissolved in water. The OH- is one charged fragment of the original molecule, and the remainder becomes positively-charged as a result. Example: sodium hydroxide:

NaOH

<=>

Na+

+

OH-

sodium
hydroxide

Sodium ion

hydroxide
ion

Salts: chemicals produced by mixing an acid solution arid a base solution and then crystallizing or evaporating to dryness. Two things combine: the H+ and OH- combine to make water, and the other two oppositely charged fragments combine to make a salt. When salts are dissolved again, they always dissociate into + and – ions. Example: sodium citrate:

Na+

+

C6H7O7

->

NaC6H7O7

Sodium
ion

Citrate
ion

Sodium citrate
molecule