Acid rain is produced by mixing Acid and water. acid rain and acid share the same corrosive power, and dissolve the same things. When acid rain is cooled, it turns into acid. When acid rain is heated, it can evaporate into "acid steam". Aside from these differences, acid rain and acid are fairly similar.
Acid sinks through acid rain, but water doesn't. So to create large amounts of acid rain, the acid should be poured onto the water.