Final answer:
When 4.50 grams of sodium chloride is mixed with 1.25 grams of ammonium chloride, a physical mixture is prepared because no chemical reaction occurs between the two compounds.
Step-by-step explanation:
If a student mixes 4.50 grams of sodium chloride with 1.25 grams of ammonium chloride and no chemical reaction occurs, she has prepared a physical mixture. This is because sodium chloride and ammonium chloride, when mixed in their solid forms, do not react with each other to form a new substance. Instead, they simply coexist as a mixture of two different ionic compounds. This can be compared to the behavior of sodium chloride in reactions with compounds like silver nitrate, which does lead to a chemical change, forming a precipitate of silver chloride. In the student's case, what we have is more akin to a mechanical blend and there is no change in the chemical identity of the individual components.