In a chemical equation representing a chemical reaction what do the numbers in front of the molecules of compound and elements indicate? Do they signify number of molecules or moles or volume?
Example : $$\ce{ 2H_2 +O_2 -> 2H_2O}$$
Here does the "2" before $\ce{H2}$ and $\ce{H2O}$ indicate number of molecules or moles or volume? In a particular chapter in my textbook it was written that they indicate number of molecules(atoms) but in a later chapter the numbers were being treated as if they are number of moles or volume.