1. A sealed flask initially contains pure nitrogen dioxide gas (NO2). Over time, the nitrogen dioxide forms dinitrogen tetroxide gas (N204). The graph below shows the relative amounts of (NO2) and (N204) over time. What is true about the time indicated by the blue arrow?

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.