Why does increasing the concentration of a reactant increase the rate of reaction?

Increasing the concentration of a reactant is the same as increasing the concentration of squash- there are more particles in a solution and in a given space. More particles mean that there is a greater chance of the particles colliding with each other, and when they collide the reaction takes place.The greater probability of collision leads directly to an increased frequency of collision (more collisions per second), which is an increased rate of reaction. This can be thought of as if you have two students running around a classroom then they are less likely to collide than when there are 20 students running around a classroom, where collisions lead to reactions.