Reversible reactions: A reaction that can occur in both directions (the products can react to produce the original reactants again).
Chemical equations for reversible reactions:
When writing chemical equations for reversible reactions, two arrows are used, each with just half an arrowhead – the top one point right, and the bottom one pointing left:
Dehydration of Hydrated Copper (II) Sulfate:
Hydrous Copper (II) Sulfate ⇌ Anhydrous Copper (II) Sulfate + Water
Thermal Decomposition of Ammonium Chloride:
Ammonium Chloride ⇌ Ammonia + Hydrogen Chloride