Acid is seen in the game Commando 2 as a small lake in a cave in Mission 2 and it also appears in Commando 1 on the fourth stage as an obstacle in the airship. It is highly dangerous, and whenever a player falls into the acid they will die instantly.
In Commando 1, the only way of passing the pools of acid is by swinging on the platforms in between the acid pools.
The only method of traversing the acid in Commando 2 is by floating on a metal barrel. Oddly, metal reacts with acid, so it is almost impossible to pass through an acid lake with metalic materials.