How to tell if a reaction is oxidation or reduction

Which reactions are oxidation or reduction?

1. Na -> Na^+ + 1e^-
2. Zn^2+ + 2e^- -> Zn
3. O2 + 4H^+ + 4e^- -> 2H2O
4. Fe^2+ -> Fe^3+ + 1e^-

I'm not sure how to solve these, any help would be greatly appreciated. :)

Oxidation means losing electrons while reduction means gaining electrons.
In example one Sodium moved from its normal state, which is 0 electrons to Na^+. Na^+ means that sodium has lost an electron. Therefore it is undergoing oxidation.
In example two, zinc went from losing two electrons (+2) to a normal state i.e. it gained two electrons. So this is reduction.
In example 3, hydrogen went from losing one electron (+) to its combined state with water which is just like a normal state so this is reduction too.
In example 4,iron went from losing 2 electrons to losing 3 so it lost an electron which makes it reduction.
An easy method is noting which side of the equation has the e^- sign. Left means reduction, right means oxidation.
http://www.chemguide.co.uk/inorganic/red… could help you understand further. It’s also useful for any other topic in chemistry. Hope this helps!

Oxidation is when oxygen is gained, or when an electron is lost. Reduction is the opposite - when oxygen is reduced (lost) or when an electron is gained.
You can remember it like: OIL RIG = oxidation is loss, reduction is gain