Target: one creature
Attack: Intelligence vs. Reflex
Hit: "2d8 + Intelligence modifier acid damage, and ongoing 5 acid damage (save ends)."
Miss: "Half damage, and ongoing 2 acid damage (save ends)."
Effect: "Make the secondary attack."
Secondary range: area burst 1 centered on the primary target
Secondary target: each creature within the secondary area of effect other than the target
Secondary attack: Intelligence vs. Reflex
The "Class Compendium: The Arcanist" article in Dragon 401 added the evocation keyword to acid arrow, and made its secondary attack an area burst attack that is made even on a miss. Previously, no secondary attack occurred on a miss. The December 2011 update finalized these changes.