H2o is the chemical formula for water. in order to produce a single molecule of water, how many hydrogen atoms are needed?

Answer is: two atoms of hydrogen are needed to produce a single molecule of water. 
Chemical reaction: 2H₂ + O₂ → 2H₂O. /÷2.
H₂ + 1/2O₂ → H₂O.
Beside two atoms of hydrogen, there is also one atom of oxygen in a single molecule of water (H₂O). Atoms of hydrogen and oxygen are connected by polar covalent bonds.

That result from subduction of oceanic plates beneath lighter continental plates. most of the earth's volcanoes are located around the pacific ring of fire because that the location of most of the earth's subduction zones.

Ithink this is what you meant. the temperature at which a liquid begins to boil is called the boiling point. the boiling point of water is 212°f or 100°c.  and at that point will start to turn into steam (gas)