The overall rate of a reaction between two species doules when:

Explanation:

The overall rate of a reaction between two species doubles when: the order of both species changes or any of the species changes.

The overall order of the reaction is found by adding up the individual orders. For example, if the reaction is first order with respect to both A and B (a = 1 and b = 1), the overall order is 2. Therefore overall order is 2 or second order reaction although order of individual species is 1.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.