Dissociation

In an aqueous or gaseous environment, when reactants spontaneously 'come apart,' we say they dissociate into products. The dissociation reaction can be written as

where reactants (a moles of A and b moles of B) dissociate into products  c moles of C and d moles of D). The dissociation of A and B is continuous, as is the recombination of C and D.
A better way to write the dissociation/recombination reaction is:

with a doubly sided arrow. The double arrow shows that reactants and products are in a dynamic equilibrium of dissociation and recombination.