What do scientists mean when they use the term, phase change?

A phase change is a change in the state of matter of a sample. Examples: An example of a phase change is water changing from a liquid to vapor.

Explanation:

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When scientist talk about a phase change they are talking about  is when matter changes to from one state liquid, solid, gas, or plasma to another.

Explanation:

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.