How do you know if it is an oxidation reduction reaction?

Oxidation numbers are theoretical constructs, and have marginal physical significance. Formally, the oxidation number of an element in a compound is the charge the element would have if all the bonding electrons were broken, with the charge assigned to the most electronegative atom.

For the molecule `H-F`, we break the bond, and the 2 electrons are assigned to the fluorine centre. I reiterate that this is a theoretical exercise:

`H-F rarrH^+ + F^-`

Thus fluorine is conceived to have an oxidation number of `-I`, and hydrogen an oxidation number of `+I`; Roman numerals are used.

So if oxidation numbers change upon reaction, electron transfer and redox has occurred. Typically powerful oxidants such as elemental fluorine, `F_2`, and oxygen `O_2`, after reaction (after oxidizing something) assume oxidation states of `-I` and `-II` in the product compound.

Oxidation of hydrocarbons underpin our civilization. If we look at the combustion of coal, we can write the equation:

`C(s) + O_2(g) rarr CO_2(g) + Delta`

The oxidation number of an element is formally `0`. The oxidation number of `C` in `CO_2` is `+IV` and of oxygen `-II`. Electron transfer has taken place between oxygen and carbon when the reaction occurs, and thus hydrocarbon combustion is formally redox.