1. A sample of a pure solid substance is heated at a constant rate and its temperature
recorded as a function of time. This graph has a long flat portion because
a. heat is rapidly absorbed by the substance between 9 and 18 minutes
b. the substance starts to melt at 9 minutes and is fully melted by 18 minutes
c. the substance rapidly cools at the 9 minute point by radiating away absorbed heat
d. the temperature becomes difficult to measure beyond 50° C
The long flat portion of the temperature–time graph represents the melting of the pure solid; during this interval, heat energy goes into changing the state (solid → liquid) rather than raising temperature, so the correct option is (b).
Detailed explanation of the graph
When a pure solid is heated at a constant rate, its temperature first rises steadily while it is entirely solid, reflecting an increase in kinetic energy of particles. At the melting point, added heat is used to break intermolecular forces and convert the solid into liquid, so temperature remains constant, giving a horizontal or flat region on the heating curve.
Why option (b) is correct
Option (b) states that “the substance starts to melt at 9 minutes and is fully melted by 18 minutes,” which matches the plateau from 9 to 18 minutes where temperature stays constant while the solid–liquid mixture exists. Throughout this phase-change interval, heat absorbed is the latent heat of fusion, and only after all solid has melted does the temperature begin to rise again as a pure liquid is heated.
Why option (a) is incorrect
Option (a) claims that “heat is rapidly absorbed by the substance between 9 and 18 minutes,” but heat is actually absorbed at a constant rate over the entire experiment, not more rapidly in that section. The distinctive feature of the flat portion is not faster heating but a change in how energy is used: potential energy increases to overcome bonding while temperature (kinetic energy) remains unchanged.
Why option (c) is incorrect
Option (c) suggests “the substance rapidly cools at the 9 minute point by radiating away absorbed heat,” which would appear as a downward-sloping line rather than a horizontal plateau on the graph. A cooling process would require removal of heat, whereas in this experiment heat continues to be supplied, so cooling cannot explain a flat segment at constant temperature.
Why option (d) is incorrect
Option (d) argues that “the temperature becomes difficult to measure beyond 50 °C,” but measurement difficulties do not systematically create a long, perfectly horizontal region on a heating curve. The reproducible flat section at a well-defined temperature is a hallmark of a phase change of a pure substance, not an artifact of instrumentation.
Introduction
In chemistry, the heating curve flat portion during melting on a temperature–time graph is one of the most important signatures of a phase change in a pure substance. Understanding this plateau helps students correctly interpret multiple-choice questions about heating curves and explains why temperature can stay constant even while heat is continuously supplied.
What a heating curve shows
A heating curve plots temperature against time (or heat added) as a substance is heated at a constant rate, revealing how its physical state changes from solid to liquid and then to gas. Sloped segments indicate that the substance is in a single phase and its temperature is rising, while flat portions correspond to phase changes at fixed temperatures.
Meaning of the flat portion
During the flat portion at the melting point, the solid and liquid phases coexist, and all added energy becomes latent heat of fusion rather than increasing temperature. Once all the solid has melted, the curve resumes an upward slope, showing that the now-liquid substance warms above its melting point.
Applying the concept to the question
In the given question, the flat region between 9 and 18 minutes indicates that the sample is melting at a constant temperature while heat is still being supplied. Therefore, the best explanation is that the substance begins melting at 9 minutes and is completely liquid by 18 minutes, which corresponds to option (b).
