25%of a first order reaction is completed in 30 mint. find the time required for completion of 50% of reaction

it's c. elements are placed in rows based on how may valencies electrons have.

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.