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A chemical reaction is a process where substances, known as reactants, transform into different substances, called products. During a reaction, the chemical bonds in the reactants are broken, and new bonds are formed to create the products. These reactions can happen for various reasons, such as to release energy, form new compounds, or change the state of matter. For example, cooking food involves chemical reactions that change the raw ingredients into cooked meals by breaking and forming new bonds. Additionally, rust forming on iron is another everyday example of a chemical reaction.

In any chemical reaction, we always start with reactants and end with products. Reactants are the substances that undergo a chemical change during the reaction. Products are the new substances formed as a result of this chemical change. For example, in the reaction \[ \text{2Na}(s) + \text{Cl}_2(g) \rightarrow \text{2NaCl}(s) \], sodium (Na) and chlorine (Cl_2) are the reactants, and sodium chloride (NaCl) is the product. Understanding what the reactants and products are is essential because you need this information to write and balance a chemical equation accurately.

Stoichiometry is a branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. It allows us to determine how much of a reactant is needed to produce a certain amount of product. When balancing chemical equations, stoichiometry plays a crucial role. For instance, in the reaction \[ \text{S}_8(s) + 24\text{F}_2(g) \rightarrow 8\text{SF}_6(g) \], stoichiometry helps us understand that 1 molecule of \text{S}_8 reacts with 24 molecules of \text{F}_2 to produce 8 molecules of \text{SF}_6. This balance ensures that no atoms are lost or gained during the reaction, following the principle of the conservation of mass.

Oxidation states (or oxidation numbers) indicate the degree of oxidation (loss of electrons) or reduction (gain of electrons) an atom undergoes in a chemical reaction. They are essential for understanding redox reactions where oxidation and reduction occur simultaneously. For instance, consider the reaction \[ \text{FeS}(s) + 2\text{HCl}(aq) \rightarrow \text{FeCl}_2(aq) + \text{H}_2\text{S}(g) \]. Here, \text{Fe} in \text{FeS} has an oxidation state of +2, and in \text{FeCl}_2, it remains +2, illustrating no change in oxidation state for iron. However, understanding changes in oxidation states is crucial in reactions where electron transfer occurs, as it helps in balancing and understanding the nature of the reactions.