Chemical reactions are the processes by which substances interact to form new substances with different compositions. Chemical reactions are constantly occurring in the world around us; everything from the rusting of an iron fence to the metabolic pathways of a human cell are all examples of chemical reactions. Chemistry is an attempt to classify and better understand these reactions.

The symbol (aq) is also commonly used in order to represent an aqueous solution, in which compounds are dissolved in water.

A reaction might take the following form:

A (aq) + B (g) C (s) + D (l)

In the above example, A and B, known as the reactants, reacted to form C and D, the products.

Kinds of chemical reactions

Irreversible reactions

Some chemical reactions can occur in only one direction. These reactions are called irreversible reactions. These reactions are like making a cake. The ingredients of a cake—such as eggs and flour—are the reactants. They are mixed together and baked to form the cake, which is the product.

Combustion reactions are generally irreversible. The chemical equation for a combustion reaction is:

These two products cannot react to reform the fuel and oxygen, so the reaction is irreversible. The one-way arrow in the equation shows that the reaction can go in only one direction.

Reversible reactions

Many chemical reactions can occur in both directions. These reactions are called reversible reactions. Not only can the reactants change to the products, but the products can change back to the reactants, at least under certain conditions.

How would you represent the reversible reaction in which hydrogen and oxygen gases combine to produce water?

H₂ + O₂ H₂O

In a reversible reaction, both forward and reverse directions of the reaction generally occur at the same time. When this happens, there is no overall change in the amount of reactants and products, even though the reactions keep occurring in both directions. This point is called equilibrium. The term equilibrium means “state of balance,” and it is used to refer to a state of balance between any opposing changes.