Which of the following is a factor that can affect the rate of a chemical reaction?

• A. Color of the solution
• B. Odor of the mixture
• C. Concentration
• D. Shape of the container

Answer: (C)

The rate of a chemical reaction depends on how often reacting particles collide with enough energy to react. When you increase the concentration, there are more molecules in the same volume, so collisions happen more frequently and there are more productive collisions per unit time. That speeds up the reaction. The color of the solution or the odor of the mixture don’t change how often molecules collide, and the shape of the container doesn’t alter the collision rate in a well-mixed solution, so they don’t systematically affect the rate. Other important factors, like temperature and catalysts, can also influence rate, but among these options, concentration is the one that directly speeds things up.