Volcanic eruptions and emissions from vegetation are the natural causes of which phenomenon

Acid rain

The volcanic eruption is a natural phenomena that results in the liberation of the lava from the heap of the earth along with the gases. The volcanic eruption causes major damages to the vegetation lying above the erupted crater of the volcano. This results in the immense release of gases such as carbon dioxide, carbon monoxide, sulfur dioxide, hydrogen chloride, hydrogen fluoride, methane and ammonia all these gases are major pollutants of the air. These gases reacts with water vapor in the atmosphere and condense in the form of acid rain.  

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.