Asubstance with a definite shape and volume is a of matter.

solid

Explanation:

Solid  involves the state in which matter has a definite shape and volume. In that respect, solids can be crystalline, which means that the atoms or molecules are arranged in a repeating pattern of rows, like iron, diamond and ice. Solids can also be amaorphous, which are constituted by atoms or molecules in no organized pattern, such as rubber and wax.

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.