In a chemical reaction, the change in concentration of a reactant with time is

Step 1: Analyze the question The question asks for the term that describes how the concentration of a reactant changes with time.

Step 2: Explain the concept Look at what "change in something with time" means. It is a rate. In chemical kinetics:

Rate of reaction = change in concentration of a reactant (or product) ÷ time taken

Rate = −Δ[Reactant] / Δt, or Rate = +Δ[Product] / Δt

The units are usually mol dm⁻³ s⁻¹.

Step 3: Check each option

• A. Entropy of reaction: a measure of disorder (randomness) in a system. It has nothing to do with time.
• B. Enthalpy of reaction: the heat change in a reaction (ΔH). It is not related to time.
• C. Rate of reaction: the change in concentration per unit time. This matches the definition.
• D. Order of reaction: the sum of the powers of the concentration terms in the rate law. It tells how the rate depends on concentration, but it is not the change in concentration with time.

Step 4: Answer The correct option is C. rate of reaction.

Tip: Whenever you see "change with time" in a chemistry question, think rate. Remember that entropy is about disorder, enthalpy is about heat, and order is about the powers in the rate equation.