Question

Josh is doing an experiment with water. He starts with a plastic cup of water that has a mass of 12 grams. He places the cup in a sealed container and puts it in the freezer overnight. When he takes the container out of the freezer, he notices that the cup has cracked on its sides and the water is now frozen solid. Which sentence BEST describes what Josh discovers about the cup of frozen water?
A) The mass of the cup of frozen water is 12 grams which means the mass did not change.

B) The mass of the cup of frozen water is greater than 12 grams because it changed to a solid.

C) The mass of the cup of frozen water is less than 12 grams because the frozen water is lighter than liquid water.

D) The mass of the cup of frozen water is greater than 12 grams because the water expanded in the cup when it froze.

Answer 1

A) The mass of the cup of frozen water is 12 grams which means the mass did not change.

Answer 2

Answer: A) The mass of the cup of frozen water is 12 grams which means the mass did not change

Explanation:

The mass of water did not change, because the amount of matter is the same: by the law of mass conservation you can assure that the mass of water (12 grams) is the same at the begining than the end.

Both in the physical changes (as in the case of the changes in the phases of the matter) and in the chemical changes, the law of conservation of the mass is obeyed. Therefore, assuming that the liquid water did not spill from the sealed container during the night, the mass remained constant (12 grams).

Regarding the fact that Josh noticed the cup had cracked on its sides, you must observe that water is an exception to the common behavior of the matter: most substances shrink when pass from liquid state to solid state due to the fact that the particles (atoms, molecules or ions) lose kinetic energy and they the approach each other occupying less space.

Being the density inversely relatated to the volume (density = mass / volume), this increase on volume means that the substances will increase its density when pass from liquid to solid.

But water is a remarkable exception. Water density reaches its maximum at about 4°C, which means that solid water (0°C) is less dense than liquid water.

Hence, ice (solid water) will ocuppy more space (more volume) than the same amount of liquid water, which is the reason why the liquid water in the plastic cup expanded when it froze, cracking the cup.