Question

mrs. buckley made an omelet. in a pan, she mixed three 50-gram eggs with 75 grams of milk and 25 grams of cheese for a total of 250 grams. she covered the pan with a lid and put it on a hot stove.
after a few minutes, mrs. buckley saw that the omelet was cooked. it had a mass of 225 grams. there were many water drops on the inside of the pan’s lid.
which statement explains the change in mass?
there was some water vapor in the air outside the pan. this water vapor condensed to form water drops inside the pan’s lid.
the eggs, milk, and cheese gained thermal energy as they were heated in the pan. so, they gained mass.
there was some water in the eggs, milk, and cheese. when the omelet cooked, the water vaporized. then, it condensed on the pan’s lid.

Explanation:

1. Analyze the first option: The water vapor outside the pan condensing inside the lid wouldn't affect the mass of the omelet (the mixture in the pan), as the vapor is from outside. So this is incorrect.
2. Analyze the second option: Thermal energy doesn't add mass. Mass is conserved in a closed system (the pan with lid, mostly closed), and gaining thermal energy (heat) doesn't create matter. So this is incorrect.
3. Analyze the third option: The eggs, milk, and cheese contain water. When heated, water vaporizes (turns to gas) and then condenses on the cool lid. The mass of the omelet decreases because some water (originally part of the 250g mixture) left as vapor (though it condensed on the lid, it's no longer part of the omelet's mass). This explains the mass change from 250g to 225g.

Answer:

There was some water in the eggs, milk, and cheese. When the omelet cooked, the water vaporized. Then, it condensed on the pan's lid.