What are efflorescent acids?

Because it is safer: when you add a bit of water to a full container of acid, the acid can react violently and explode in your face. If you add a little bit of acid to water, the reaction would be...

Not exactly. Acid rain does contain abnormally high amounts of nitric acid and sulfuric acid, but the concentrations are still very dilute.

Hydrochloric acid.

It is weak, all carboxylic acids are weak.

acetic acid is vinegar