Does a redox reaction always involve oxygen?

No. The name "oxidation" comes from early chemistry where oxygen was the most common electron-stealer, but any element can be involved. For instance, reacting sodium with chlorine gas is a redox reaction with no oxygen in sight.

Can an atom be oxidized and reduced at the same time?

Yes, this is called a disproportionation reaction. For example, when hydrogen peroxide ($H_2O_2$) breaks down into water ($H_2O$) and oxygen gas ($O_2$), the oxygen atoms in the peroxide are both oxidized and reduced.

What is an oxidizing agent?

The oxidizing agent is the chemical that causes oxidation by stealing electrons from another substance. Because it gains those stolen electrons, the oxidizing agent itself gets reduced. It sounds backward, but just remember it describes what the chemical does to the other reactant.

How do you identify a redox reaction?

A redox (oxidation-reduction) reaction is any chemical reaction where electrons are transferred from one atom to another. If you can spot that an atom's charge or "oxidation state" has changed from the reactant side to the product side, you have found a redox reaction.

Think of it like a financial transaction. If one person gives five dollars to another, one person's bank balance goes down (oxidation) and the other's goes up (reduction). In chemistry, the currency is electrons. If no electrons change hands, it is not a redox reaction.

What are oxidation states?

Oxidation states (or oxidation numbers) are bookkeeping numbers we assign to atoms to keep track of their electrons. In an element like pure sodium ( $Na$ ), the oxidation state is 0. In a monatomic ion like $Na^+$ , it is +1. By assigning these numbers to every atom in a compound, we can see exactly where the electrons are residing.

Oxidation vs. Reduction

"OIL RIG" is the classic way to remember what happens to the electrons: Oxidation Is Loss, Reduction Is Gain. Because electrons have a negative charge, gaining electrons actually reduces an atom's oxidation state (the number goes down). Losing electrons increases its oxidation state (the number goes up).

How to spot the transfer

To identify a redox reaction, assign oxidation numbers to every atom on the left side of the equation, and then do the same for the right side. Compare the two sides. If any of the numbers change, electrons have been transferred, and it is a redox reaction. If they all stay exactly the same, it is not.

Where students slip

A common mistake is assuming that any reaction involving charged ions is automatically a redox reaction. Acid-base (neutralization) and precipitation reactions almost always involve ions, but the electrons do not actually change hands; the ions just trade partners. Always check the oxidation numbers rather than just looking for charges.

Worked through

Is the reaction $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$ a redox reaction? If so, what is oxidized and what is reduced?

Yes, it is a redox reaction. First, assign oxidation states to everything. On the left side, pure solid $Zn$ is 0. In $HCl$ , $H$ is +1 and $Cl$ is -1. On the right side, in the $ZnCl_2$ compound, $Zn$ is +2 and $Cl$ is -1. Pure $H_2$ gas is 0. The zinc went from 0 to +2, meaning it lost two negative electrons, so zinc is oxidized. The hydrogen went from +1 to 0, meaning it gained an electron, so hydrogen is reduced. The chlorine stayed at -1 on both sides, so it did not participate in the electron transfer.

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Where this comes from: OpenStax Chemistry 2e, Chapter 4.2: Classifying Chemical Reactions · Khan Academy: Oxidation-reduction (redox) reactions