[KudoAnastasia]

When balancing the charges for half equations in a redox reaction, why do we add H20 or H? I understand that it is to balance the charges but why those elements, in particular, why not any other atom that contains hydrogen or oxygen like hydroxide? I do not understand why those elements are used specifically.

[chenbeier]

In acidic  envirement H⁺ will be added on educt side and water on product side for reduction.
For oxidation vise versa.

In alcaline to neutral condition water will be added on educt side and OH^-  on product side.
For oxidation  cases  vise versa.

This is valid if oxygen is involved an the reaction takes place in aquaeous solution.