Asample of a substance has a high density, yet a low particle motion. this sample must be a

Solid  

Explanation:

The particles in a solid are packed together in a crystal lattice. Since there are so many particles in a small volume, the substance has a high density.

The particles are locked in position in the solid. They cannot move past each other. All they can do is vibrate about their equilibrium position

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.