Which statement describes a physical change?
a. a carton of milk souring
b. a piece of silver flatware tarnishing
c. dry ice subliming into carbon dioxide
d. two clear solutions forming a precipitate when mixed

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.

the ratio of the conjugate base to the acid required to attain the desired ph.