Question

a substance is changing its state (phase) while its temperature is not changing. what was the movement of its molecules before changing phase? what was the movement of its molecules after changing phase? (from a liquid to a solid)
options:
a. before changing phase, the molecules were moving around each other. after changing phase, they were moving slowly from each other.
b. before changing phase, the molecules were moving around each other. after changing phase, they were moving in place.
c. before changing phase, the molecules were moving away from each other. after changing phase, they were moving around each other.
d. before changing phase, the molecules were moving in place. after changing phase, they were moving around each other.

Explanation:

To determine the correct option, we analyze the molecular motion during phase changes:

1. Understand Phase Change and Molecular Motion: In a solid, molecules vibrate (move in place). When melting (solid to liquid), molecules gain energy and move around each other. When freezing (liquid to solid), molecules lose energy and move less freely (but the question here is about a phase change, likely melting or similar where initial state is more restricted and final is more free - or vice versa? Wait, the options describe "Before changing phase" and "After changing phase". Let's check each option:

• Option A: Before: moving around each other; After: moving away from each other. This doesn't fit typical phase changes (e.g., liquid to gas: molecules move away, but liquid molecules move around each other. But if before is liquid (moving around) and after is gas (moving away), but the other options... Wait, maybe the initial phase is solid (molecules in place or vibrating) and after is liquid (moving around each other). Wait, let's re - examine the options:

• Option B: Before: moving in place; After: moving around each other. This fits a phase change from solid (molecules vibrate in place) to liquid (molecules move around each other).

• Option C: Before: moving around each other; After: moving away from each other. This would be liquid to gas, but let's check the other options.

• Option D: Before: moving in place; After: moving around each other? Wait, no, the original image's option D (wait, the labels are a bit unclear, but based on the text:

Looking at the text in the image (after OCR correction):

• Option a: "Before changing phase, the molecules were moving around each other. After changing phase, they were moving away from each other."

• Option b: "Before changing phase, the molecules were moving in place. After changing phase, they were moving around each other."

• Option c: "Before changing phase, the molecules were moving around each other. After changing phase, they were moving away from each other." (Wait, maybe a typo, but likely option b is the correct one as it represents solid to liquid phase change: in solid, molecules are relatively fixed (moving in place - vibrating), in liquid, they move around each other.

To solve this, we analyze molecular motion during phase changes. In a solid, molecules vibrate (move in place). When changing to a liquid (e.g., melting), molecules gain energy and move around each other. Option B describes this: before phase change (solid, molecules move in place), after phase change (liquid, molecules move around each other). Other options do not match typical molecular motion in phase changes (e.g., Option A's after - motion "moving away" is more gas - like, Option C's before - motion "moving around" and after "moving away" is liquid to gas but the first part of the option may not match the "before" for a common phase change scenario as well as Option B for solid to liquid).

Answer:

B. Before changing phase, the molecules were moving in place. After changing phase, they were moving around each other.