Half reaction/ ion electron method

[Cromptu]

I have recently learned how to balance chemical equations using the ion electron method.
However, I have some doubts..
Suppose the reaction takes place in an acidic medium; now how can we say that we are allowed to add an H+ or H2O molecule?
I mean, where will these elements come from when these reactions take place in real life? from the environment or is this just a method of simplifying the equations and these added elements do not play a role when these reactions take place in real life?

Please help me out here..

 

[Borek]

If the reaction takes place in water assumption that here is plenty of water molecules (together with H⁺and OH^-) around is usually correct, isn't it?

It won't work for reactions taking place in anhydrous environment, but most reactions you are going to balance will be taking place in water solutions.