Mega Mack

Mega Mack, also known by the name megamalatricarboSonichloride, is the name given in some continuities to the pink liquid found during the Chemical Plant Zone in Sonic the Hedgehog 2. According to the Sonic 2 US Manual, Robotnik flooded the factory with a pool of the poisonous liquid to poison Sonic in just a few moments time. In this game, it has the properties of water (apart from its colour), meaning that Sonic or Tails can drown after being submerged for extended periods of time. The Mega Mack functions like regular water so it doesn't poison Sonic in game, however there are a lack of air bubbles in the liquid so its easier to drown in.

A similar pink liquid, possibly also intended to be Mega Mack, appears in Act 3 of the Scrap Brain Zone in Sonic 1. The water in Sonic CD's Palmtree Panic and Tidal Tempest's respective Bad Futures are pink as well.

Sonic the Comic
In Sonic the Comic by Fleetway Editions, Mega Mack is revealed to actually be a poisonous substance. It was designed by Doctor Robotnik and another scientist specifically for the purpose of killing Sonic. Sonic can not only drown in it, but prolonged exposure will actually poison him (and, indeed, he admits to feeling slightly dizzy after standing in it for a short time).

The second scientist who worked with Robotnik to invent the Mega Mack was caught in a chamber that flooded with Mega Mack, but instead of dying he was transformed into the villain Megatox, whose body is made up entirely of the substance.

Adventures of Sonic the Hedgehog
MegamalatricarboSonichloride appeared in the episode "The Mobius 5000" where Sonic and the others got stuck in the liquid, but they got out thanks to Sonic's speed.