Explanation:
When writing chemical equations for reversible reactions, the usual one-way arrow is not used. Instead, two arrows are used, each with just half an arrowhead - the top one pointing right, and the bottom one pointing left.
For example:
ammonium~chloride \rightleftharpoons ammonia + hydrogen~chloride
NH_{4}Cl(s) \rightleftharpoons NH_{3}(g) + HCl(g)
