Chemistry help me

Question

A student mixes 20.0g of white KCI crystals with distilled water in a beaker. After the mixture was stirred, no crystals are visible and the solution is clear. After several days, all of the water evaporates and white crystals are found in the beaker. Which of following pieces of experimental evidence would best help the student to confirm that a new compound had not been made and that only a physical change occurred?

A. The solution does not change color after stirring

B The KCI crystals are no longer visible after mixing with water

C there is a temperature change in the solution during the dissolving process.

D after the water had evaporated the white crystals in the beaker have a mass of 20g.

Answer 1

In answering this question, we must first understand the difference between chemical and physical changes. During a physical change, a substance changes its appearance but NOT its composition (think about ice melting forming liquid water). In a chemical change, a substance is changed into a chemically different substance (think wood burning to make ash). In the case of ice, the water can be cooled again to form ice; in the case of wood, the ash cannot be turned back into wood.

When the question asks us which piece of evidence would help us confirm that only a physical change occurred, it is basically asking us to choose the evidence that tells us that nothing has changed, with respect to the KCl.

Though color change can be an indicator of a chemical change, the lack of color change is not conclusive enough to indicate that no chemical change has occurred (eliminate A). The crystals dissolving may be an indication of a chemical change (eliminate B). If there is a temperature change during the dissolving process, this often indicates that a chemical change occurred (eliminate C).

Thus, the best answer would be D. The amount of KCl recovered is the exact same amount that we started with. Thinking back to our previous example, upon evaporation of the water, the KCl was able to fully recover its crystal form. Having 20g of the KCl remain in the beaker would indicate that none of our KCl was used in any chemical reactions allowing us to conclude that our change was purely physical.