Which segment shows the substance changing from a gas to a liquid

Y-X

Explanation:

- For any substance, solid is the state with least temperature (because the particles have less energy: in fact, they are bond together, so they are not free to move), gas is the state with highest temperature (because particles have more energy, since they are totally free to move), while liquid state lies in the middle.

- In the graph, we can therefore associate U with the solid state (least temperature), W with the liquid state, and Y with the gas state (largest temperature).

- The section labelled with V corresponds to the solid-liquid transition, where the temperature remains constant (during phase transitions, temperature does not change). Similarly, the section labelled with X corresponds to the liquid-gas transition.

- The problem is asking us for the segment representing the change from gas to liquid. According to all the things we have written above, we can say that this is represented by the Y-X segments. In fact, in Y the substance is still a gas: by removing heat, the temperature decreases (we are going towards the left in the graph), until we reach segment X, which represents the transition from the gas to liquid state.

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