Which event must always occur for a chemical reaction to take place? (1) formation of a precipitate (2) formation of a gas (3) effective collisions between reacting particles (4) addition of a catalyst to the reaction system

3 - Effective collisions between reacting particles

For a chemical reaction to occur, reacting particles must collide - however, not all collisions result in a reaction. They must have enough energy in the collision to react (that is, be above the activation energy). The rate of reaction is determined by the rate of successful collisions, that is to say, the more successful collisions there are in a unit time, the faster the reaction rate.

(3) effective collisions between reacting particles is your answer. Without these collisions, the reacting particles cannot trade particles or react with each other to create a chemical reaction.