Which of these would indicate a chemical change has occurred? a) salt is dissolved into water. b) a child colors a picture with a crayon. c) solid iron is heated until it turns into a liquid. d) a metal is added to an acid and hydrogen gas is formed.

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.