Acid

Acid is an uncommon, dangerous, and often lethal hazard appearing as an effervescent fluid to distinguish it from water and to indicate that it can do significant damage to anything that touches it. Its color varies between games, with it being green in some games and clear in others. Most Pikmin, except for Green Pikmin and other acid-resistant Pikmin types, die instantly upon contact with acid with no chance of saving them, although other games can be a bit more forgiving.

In Pikmin Rebirth
Acid is a hazard appearing in Pikmin Rebirth. It can be identified by its faint green tint and fizzling appearance. Acid acts as a secondary fluid, being less common than water but more so than sludge. Acid is one of the more lethal hazards as it's the only one that can instantly kill all Pikmin regardless of source. Additionally, unlike water, acid can heavily damage any captain that touches it, draining health at an exponential rate the longer a captain remains in contact. Acid is resisted by the Green Pikmin and their abilities allow them to form a shield around the captain they are following, allowing for the mitigation of the heavy damage from the fluid, with a larger army of Green Pikmin allowing for higher resistance to acid. Additionally, Green Pikmin can become infused with acid to become Corrosive Pikmin, allowing them to melt stone, metal, and similar materials while also ignoring the defense of enemies, most notably those with high damage resistance such as the Silver Bulborb. Alongside forming pools, acid can also come from geysers and jets from surfaces, similar to water, fire, poison, and ice, as well as bubbles that will pop at the slightest touch.

In Pikmin: Lair of the Umbrax Dweevil
In Pikmin: Lair of the Umbrax Dweevil, Mushroom Pikmin are resistant to acid. If a Pikmin touches acid, it will run around in little circles, melting, and then die unless a leader whistles them or a Mushroom Pikmin helps out.

In Pikmin: Wide World
Acid appears in Pikmin: Wide World as a relatively infrequent hazard that is found first in Ancient Metalworks and later in Endless Sea, both as enemies that utilize it and as pools of acidic water. Acid appears as a clear, fizzy liquid. Bodies of acidic water have small bubbles rising to the surface, with a noticeable haze lingering above, and are always extremely shallow. A distinct hissing sound is heard whenever acid splashes anywhere. Green Pikmin are immune to acid and its effects, and can freely walk in it. Pikmin besides Green Pikmin that touch acid will have a droplet of it on the end of their stem and begin running around in panic, and Pikmin that walk into a pool of acidic water will begin flailing around in it and begin dissolving. The time frame in which Pikmin can be saved before they succumb is half of that of water. Unprotected leaders that touch acid will continue taking significant damage so long as they remain in contact with it unless they have the Antacid Polish upgrade, which grants them immunity to acid and its effects. Leaders without the Antacid Polish will also continue taking damage from acid for a short time after touching it. The Antacid Polish also allows leaders to punch acid-covered enemies without fear of taking damage by landing a hit.

Acid Beetles are always found wandering around a small area on dry land, and occasionally open their wings to squirt acid from their hindquarters. Widemouthed Acid Beetles function much the same way as Acid Beetles, but are extremely hostile and will eat Pikmin in addition to spraying them with acid. Both creatures can be defeated without Green Pikmin, but Green Pikmin are recommended for use against them regardless. Acid Dumples, although they don't directly utilize acid to attack, are almost always found in pools of acidic water and must be taken out using Green Pikmin if they can't be lured onto dry land. Acidic Jellyfloats are always found above or nearby pools of acidic water, and are themselves covered in acidic slime that drips off of them, requiring Green Pikmin to take them out no matter where they are found. Chemical Bulblaxes leave trails of acid in their wake and are constantly protected by a veil of acid, requiring Green Pikmin to kill them. Both Lesser Bubblimps and Greater Bubblimps may sometimes be protected by a bubble of acidic water, meaning Green Pikmin are needed to kill the frail creatures, although a few punches from a leader with the Antacid Polish can do the job just as well. Mouthy Mimitators can spew from their mouth a toxic substance that acts as both acid and poison, meaning no Pikmin is immune to it. Sulfuric Dweevils unleash a shower of acid from their body in self-defense, but can be killed using any Pikmin type. Bullwogs can rest in pools of acidic water but do not use it directly to attack Pikmin, and can be attacked by any Pikmin type. Chemical Goolixes are composed entirely of acid except for their nuclei. The Grimacing Cremblub has acid-filled pustules covering its back that burst open when even slightly damaged, causing acid to splatter in all directions as well as dealing a fair amount of damage to the behemoth. The Hydroe Host, as it is a heavily-mutated Mouthy Mimitator under the control of a Leech Hydroe, is capable of regurgitating a toxic substance from its mouth. The Monster Dumple is a gargantuan creature that lurks in a large pool of acidic water but does not use acid directly. The Mossy Long Legs periodically opens the two halves of its main body to drizzle acid onto anything below it. Lastly, the Tombcarver Mawdad can spew acid from its mouth, an attack that covers a wide area.