(refer to attachment)
1. list the windows in order of decreasing density.
2. compare the density of the gas in window (a) to the density of gas in window (b).

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.