Final answer:
The symbol Δ over an arrow in a chemical equation denotes that heat is necessary for the reaction to occur. This Greek letter is used in science to represent a change in quantity and conveys the special condition of heat for a reaction.
Step-by-step explanation:
The symbol Δ placed over the arrow in a chemical equation signifies that heat is required to initiate the chemical reaction. This uppercase Greek letter delta (Δ) is commonly used in mathematics and science to represent a change in quantity, or the difference between a final and initial value. To illustrate, in the chemical equation NH4Cl → NH3 + HCl, the Δ over the arrow indicates that the decomposition of ammonium chloride into ammonia and hydrochloric acid requires heating.
Δ serves as a shorthand to denote special conditions such as heat in a chemical equation. The use of symbols like Δ streamlines the representation of reactions, avoiding the need to write out the full conditions, and concisely instructs chemists on how to carry out the reactions. By employing this notation, we maintain clarity and brevity in the description of chemical processes, such as the rate expression for a reaction where Δ indicates the change in concentration over time.