* Temperature: A number written over the arrow represents the temperature in degrees Celsius (°C) at which the reaction occurs. For example, Δ, which means heat is required.
* Pressure: A number or symbol indicating the pressure at which the reaction takes place. For instance, "P" or "High P" might be used.
* Catalyst: The chemical formula of a catalyst that speeds up the reaction without being consumed itself. Catalysts are usually written above the arrow with a symbol like "Pt" (platinum) or "Ni" (nickel).
* Light: "hv" indicates that light energy is required for the reaction to proceed.
* Other reagents: Occasionally, a reagent that is not a primary reactant in the main equation might be written above the arrow if it plays a crucial role in the reaction.
Example:
```
2H₂O(l) → 2H₂(g) + O₂(g)
Δ
```
This equation indicates that liquid water (H₂O) decomposes into hydrogen gas (H₂) and oxygen gas (O₂) when heated (Δ).
It's important to remember that the chemical formula above the arrow is not a reactant or product in the chemical reaction itself. It simply describes the conditions needed for the reaction to occur.
