In basic chemical reaction terminology, what are reactants in a chemical reaction?

Introduction / Context:
Learning the basic language of chemistry is essential for understanding equations, laboratory procedures, and exam questions. Two of the most important terms are reactants and products. Reactants appear on the left side of a chemical equation and are transformed during the reaction, while products appear on the right side and are formed as a result. This question asks you to select the best definition of reactants in a chemical reaction.

Given Data / Assumptions:
- We are dealing with ordinary chemical reactions, represented by balanced chemical equations. - Reactants are written on the left side of the equation. - Products are written on the right side of the equation. - Catalysts, if present, are not consumed and are not considered reactants or products in the usual sense.

Concept / Approach:
A reactant is any substance that takes part in a chemical reaction and is consumed or changed during the process. In symbolic form, a reaction is written as Reactants -> Products, where the arrow symbol means yields or gives. Reactants provide the atoms that are rearranged to form new substances. They may be elements or compounds, and they are often listed with plus signs between them when more than one substance is involved. Understanding this definition helps you correctly interpret reaction equations and identify what is being used up in a reaction.

Step-by-Step Solution:
Step 1: Recall that in a chemical equation such as H2 + O2 -> H2O, the substances before the arrow are H2 and O2, and these are called reactants. Step 2: Note that these reactants undergo a chemical change: hydrogen and oxygen molecules react to form water molecules. Step 3: Recognise that the substances after the arrow, in this example H2O, are called products because they are produced by the reaction. Step 4: Compare this understanding with the answer choices. The definition that reactants are starting substances that undergo chemical change fits the usual description. Step 5: Confirm that catalysts and solvents, although present, are not usually treated as reactants if they are not consumed during the reaction.

Verification / Alternative check:
To verify, look at another reaction such as Zn + 2HCl -> ZnCl2 + H2. Here, zinc and hydrochloric acid are on the left side and are consumed as the reaction proceeds, so they are reactants. Zinc chloride and hydrogen gas appear on the right and are formed, so they are products. In almost every textbook example, reactants are defined and illustrated in this way. This consistent usage confirms that reactants are indeed the starting substances that undergo change during the reaction.

Why Other Options Are Wrong:
Option B describes products, not reactants, because products are the final substances formed after the reaction. Option C refers to catalysts, which may influence the rate of reaction but are not consumed and therefore are not reactants in the usual sense. Option D incorrectly claims that only the solvent is a reactant. While solvents participate physically, they may not be chemically changed, and many reactions occur in the absence of a solvent.

Common Pitfalls:
Learners sometimes mix up reactants and products because they focus only on the direction of the arrow instead of the position in the equation. Another common mistake is to think that anything present in the beaker is a reactant, including catalysts and solvents. To avoid confusion, always remember the general pattern: substances before the arrow are reactants, and substances after the arrow are products. Reactants are consumed or changed, whereas catalysts are not used up and products are formed.

Final Answer:
The correct answer is: The starting substances that undergo chemical change during the reaction.