Final answer:
b) Reversible reactions have a double arrow, and nonreversible reactions have a single arrow.
A reversible reaction is denoted by a double arrow in a chemical equation, signifying the reaction can proceed in both directions, while a nonreversible reaction uses a single arrow showing progression in only one direction.
Step-by-step explanation:
When writing a chemical equation, a reversible reaction is distinguished from an irreversible (or nonreversible) reaction by the type of arrow used.
Reversible reactions have a double arrow (one arrow pointing in each direction) to indicate that the reaction can proceed in both the forward and reverse directions. In contrast, irreversible reactions have a single arrow pointing from reactants to products, indicating that the reaction proceeds in only one direction under specified conditions.
In reversible reactions, the reactants turn into products, but as the concentration of products increases beyond a certain threshold, some of the products convert back into reactants. This dynamic exchange continues until a relative balance is achieved, a state known as chemical equilibrium. Chemical equations for reversible reactions often depict this balance with a double headed arrow (⇄), while irreversible reactions are represented with a unidirectional arrow (→).