Answer:-  Not all collisions of reactant molecules result in a chemical reaction because several conditions must be met for a reaction to occur. These conditions include:

1. Activation Energy (Ea): Colliding molecules need to possess sufficient energy, typically greater than the activation energy (Ea) of the reaction. The activation energy represents the minimum energy required for the reactant molecules to overcome energy barriers and transition into products.

2. Proper Orientation: In addition to having enough energy, the colliding molecules must also be in the correct orientation for effective collision. Not all molecular collisions result in the formation of products; only those collisions with the proper molecular orientation can lead to a successful reaction.