Question

substance remaining constant? a - b, c - d, e - f b - c, d - e b - c only. a - b only.

Explanation:

When a substance undergoes a phase change (such as melting or boiling), its temperature remains constant while the heat energy is used to break the intermolecular forces. In the given graph, the segments where the temperature is constant (horizontal lines) represent phase - change processes. During phase - change, the substance exists in two states simultaneously (e.g., solid - liquid during melting, liquid - gas during boiling), and the amount of the substance in each state is changing. However, the total amount of the substance (mass) remains constant throughout the entire process (both during phase - change and when the temperature is rising). But if we consider the state of the substance (e.g., solid, liquid, gas), during the horizontal line segments (B - C and D - E which are phase - change segments), although the temperature is constant, the state of the substance is changing. But if we consider the amount of the substance (in terms of mass), it is constant throughout all segments. But if the question is about the state - related'substance' (e.g., a pure solid, then a mixture of solid - liquid, then pure liquid etc.), during the phase - change (B - C: melting, D - E: boiling), the substance is in a transition state. But if we assume the question is about the mass of the substance, it is constant in all segments. However, if we consider the common interpretation in such heating - curve questions (related to state - change and the idea of a particular state of the substance), during the phase - change (where temperature is constant, B - C and D - E), the amount of the substance in a particular state (e.g., pure solid before B - C, pure liquid after B - C but before D - E, pure gas after D - E) is changing. But if we consider the total amount of the substance (mass), it is constant in all segments. Since the options are given, and in the context of heating - curve problems (where the horizontal lines are phase - change, and during phase - change, although there is a state change, the mass of the substance is constant. And in non - phase - change segments (A - B, C - D, E - F), the substance is in a single state (solid, liquid, gas respectively) and its mass is also constant. But if we consider the intended answer (matching with the phase - change concept where the question might be misphrased as'substance' meaning state - related), in the phase - change (B - C and D - E) the'substance' (in terms of state composition) is in transition but mass is constant. Since the options are given, and B - C and D - E are the phase - change (temperature - constant) segments.

Answer:

B. B - C, D - E