In the particle collision model, what condition is necessary for a reaction to occur?

• A. Reactants must be cooled.
• B. A catalyst must be present.
• C. Reactant particles must collide with one another.
• D. The solvent must be water.

Answer: (C)

Collision between reactant particles is the essential condition for a reaction to happen. When particles collide, they must have enough energy to get over the activation energy barrier and be oriented in a way that allows bonds to break and new bonds to form. If they collide with insufficient energy or poor orientation, they simply bounce apart and no reaction occurs. Factors like higher temperature or greater concentrations increase the number of effective collisions, while a catalyst lowers the energy needed for those collisions to be productive. Cooling lowers kinetic energy and slows the reaction, and a solvent (like water) is not required for every reaction, though it can influence how particles move and collide.