Which of these is an indication that a chemical reaction has occured? formation of a precipitate melting of a substance boiling of a substance freezing of a substance

The following transition occurs at a molecular level for a substance. what transition corresponds to this change in microscopic structure? the carbon dioxide molecules on the left are in a regular, tightly packed pattern. after heating, it becomes much lower density. a. melting b. boiling c. sublimation d. freezing