How do you identify the oxidising and reducing agents in a reaction?

The oxidising agent is the substance that is reduced, while the reducing agent is the substance that is oxidised.

In a chemical reaction, the oxidising agent and the reducing agent can be identified by looking at the changes in oxidation states of the elements involved. The oxidising agent is the substance that gains electrons and is reduced in the process. This is because it causes another substance to lose electrons, hence it is 'oxidising' that substance. On the other hand, the reducing agent is the substance that loses electrons and is oxidised in the process. It causes another substance to gain electrons, hence it is 'reducing' that substance.

To identify these agents, you need to understand the concept of oxidation states. The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as the charge an atom might carry if all bonds were 100% ionic. An increase in oxidation state indicates oxidation, while a decrease indicates reduction.

For example, in the reaction between hydrogen and fluorine to form hydrogen fluoride: H2 + F2 → 2HF, hydrogen is oxidised from an oxidation state of 0 in H2 to +1 in HF. Therefore, hydrogen is the reducing agent. Fluorine is reduced from an oxidation state of 0 in F2 to -1 in HF, so fluorine is the oxidising agent.

Remember, the terms 'oxidising agent' and 'reducing agent' are based on what the substances do to other substances. The oxidising agent oxidises another substance by taking electrons from it, and in the process, it itself gets reduced. The reducing agent reduces another substance by giving it electrons, and in the process, it itself gets oxidised.