A double arrow (⇌) in a chemical equation indicates a reversible reaction. This means the reaction can proceed in both directions, with reactants forming products and products reforming reactants simultaneously.

Here's what it means:

* Forward reaction: Reactants on the left side of the arrow transform into products on the right side.

* Reverse reaction: Products on the right side of the arrow transform back into reactants on the left side.

The equilibrium of the reaction is determined by the relative rates of the forward and reverse reactions. If the forward rate is faster, the reaction will favor the formation of products. If the reverse rate is faster, the reaction will favor the formation of reactants.

Example:

The reaction between hydrogen (H₂) and iodine (I₂) to form hydrogen iodide (HI) is a reversible reaction:

H₂(g) + I₂(g) ⇌ 2HI(g)

This means:

* Forward reaction: Hydrogen and iodine react to form hydrogen iodide.

* Reverse reaction: Hydrogen iodide decomposes back into hydrogen and iodine.

The position of the equilibrium depends on the conditions (temperature, pressure, concentration) of the reaction.