Acid

Acid is a damage type introduced in Bloons TD 3, although recognized as a separate damage type compared to Normal since Bloons TD 6. As with Normal, attacks with Acid can pop any bloon type. The Bloons Wiki represents this damage type with, the upgrade icon for the Larger Potions, depicting the potions that Alchemists have by default. Acid is established to be a separate damage type to Normal to describe primarily chemical-related attacks from Alchemists and Glue Gunners.

Alchemists and Glue Gunners have this by default as part of their main attack. Whether upgrades of certain other towers with alternate chemical-related attacks use either Acid or Normal  is debatable, but normally are categorized as Normal for simplicity's sake. Both Acid or Normal  are interchangeable if the attack is clearly chemical-based.