As heat flows into a substance, its temperature changes according to the graph in the diagram. For what sections of the graph is the substance undergoing a phase change?

Question

BlackTom AI · Accepted Answer

First, let me restate the question and the options to be crystal clear about what we’re evaluating.
Question: As heat flows into a substance, its temperature changes according to the graph in the diagram. For what sections of the graph is the substance undergoing a phase change?
Answer options: AB and EF; CD and EF; BC and DE; AB and CD

Now, I’ll analyze each option one by one, describing what each segment represents on the temperature vs. heat input graph and whether it corresponds to a phase change.

Option AB and EF:
- AB is the initial upward slope where temperature increases with heat input. This indicates heating without a phase change.
- EF is the final upward slope near the end, again showing temperature rise with added heat and no phase change.
- Conclusion: Neither AB nor EF corresponds to a phase change; both are heating regions with rising temperature.

Option CD and EF:
- CD is the segment where temperature rises with heat input, not a flat plateau.
- EF, as noted, is also a rising segment rather than a plateau.
- Conclusion: Both CD and EF show increasing temperature, not phase changes, so this pair does not identify phase-change regions.

Option BC and DE:
- BC is a horizontal segment on the graph, indicating heat input while temperature remains constant. This is the classic signature of a phase change (e.g., melting or boiling), where energy goes into changing the phase rather than raising temperature.
- DE is another horizontal segment, again showing constant temperature while heat is added, representing a second phase-change interval.
- Conclusion: BC and DE are the sections where the substance is undergoing phase changes.

Option AB and CD:
- Both AB and CD are rising-temperature segments, meaning heating without a phase change.
- Conclusion: This pair does not correspond to phase-change intervals.

Summary: Only the horizontal portions BC and DE represent phase changes, while the other segments correspond to heating with increasing temperature. Therefore, the correct identification is BC and DE.

As heat flows into a substance, its temperature changes according to the graph in the diagram. For what sections of the graph is the substance undergoing a phase change? Single choice

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