Given a phase change graph for a pure substance, at which temperature does the boiling point occur?

• A. At the temperature where the plateau occurs during the solid-to-liquid transition
• B. At the temperature where the plateau occurs during the liquid-to-gas transition
• C. At the lowest temperature shown on the graph
• D. At the temperature where the slope of the graph is steepest

Answer: (B)

Verified step by step guidance

1. Understand that a phase change graph typically plots temperature versus heat added or time, showing plateaus where phase changes occur because the temperature remains constant during the transition.
2. Identify the different plateaus on the graph: the first plateau usually corresponds to the melting (solid-to-liquid) point, and the second plateau corresponds to the boiling (liquid-to-gas) point.
3. Recall that the boiling point is the temperature at which the substance changes from liquid to gas, so it corresponds to the plateau during the liquid-to-gas transition.
4. Locate the plateau on the graph where the temperature remains constant during the liquid-to-gas phase change; this temperature is the boiling point.
5. Note that the slope of the graph is zero during plateaus (phase changes), so the steepest slope does not indicate a boiling point, and the lowest temperature corresponds to the solid phase, not boiling.