According to particle models, what must happen for a chemical reaction to occur?

• A. Reactants must be cooled first.
• B. A catalyst must be present.
• C. The particles of the reactants have to collide with one another.
• D. Only a solvent is needed.

Answer: (C)

In the particle model, reactions happen because the tiny particles that make up the reactants are always moving and colliding with each other. For a collision to lead to a chemical change, the colliding particles must have enough energy to break existing bonds and form new ones, and they must strike in a way that allows the new bonds to form. If the energy is too low, the particles just bounce apart and no reaction occurs. That’s why factors like temperature, concentration, and surface area influence how fast reactions happen—they change how often and how energetically collisions occur. A catalyst can speed things up by providing an easier pathway with lower activation energy, but a reaction can still occur without it. Merely having a solvent present won’t automatically cause a reaction; the actual collision with sufficient energy is the essential step.