Here's a breakdown:

* → This single arrow indicates a unidirectional reaction, meaning the reaction proceeds only from left to right. It signifies that the reactants on the left side are being converted into products on the right side.

* ⇌ This double arrow indicates a reversible reaction, meaning the reaction can proceed in both directions. It signifies that reactants can form products, and products can also react to form reactants. This is often the case in equilibrium reactions.

Example:

* Unidirectional:

2H₂ + O₂ → 2H₂O (Hydrogen gas and oxygen gas react to form water.)

* Reversible:

N₂ + 3H₂ ⇌ 2NH₃ (Nitrogen gas and hydrogen gas react to form ammonia, but ammonia can also decompose back into nitrogen and hydrogen.)

The arrow always points from the reactants to the products.