Final answer:
The correct answer is C. exactly 100g, as the water undergoes a cycle of evaporation and condensation within a closed system, implying no change in mass due to the principle of mass conservation.
Step-by-step explanation:
When a student pours exactly 100g of water into an empty plastic bottle, seals it, and places it in the sun, the water will eventually evaporate, condense, and then drip down the inside of the sealed bottle. Despite undergoing this phase change, the mass of the water inside the bottle remains the same, assuming no water escapes or additional matter enters the bottle. Therefore, the correct answer to how much water is left inside the bottle is C. exactly 100g.
This process is an example of the water cycle occurring within a closed system. Evaporation consists of water in the liquid state transitioning into the vapor phase because of added heat. When this vapor hits the cooler parts of the bottle, it condenses back into liquid. These transformations do not result in mass loss or gain as long the conditions remain a closed system. Hence, the principle of mass conservation applies.