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Step-by-step Explanation
Step 1: Identify the Initial State
We start with liquid oxygen at 50 K kept at a constant atmospheric pressure (1 atm). Since the substance is in the liquid phase below its boiling point, as we supply heat, its temperature rises from 50 K until it reaches the boiling point.
Step 2: Heating in the Liquid Phase
When liquid oxygen is heated below its boiling point (approximately 90 K at 1 atm), the temperature increases uniformly with time if the rate of heat supply is constant. This part of the temperature–time graph is an upward-sloping line.
Step 3: Phase Change at the Boiling Point
Upon reaching the boiling point, the liquid begins to convert into the gaseous phase. During this phase change, the supplied heat is used to overcome intermolecular forces (latent heat of vaporization), so the temperature remains constant until all the liquid has converted into vapor. Hence, we get a plateau (horizontal line) in the temperature–time graph.
Step 4: Heating in the Gaseous Phase
After all the liquid oxygen has been converted into gas, any further heating causes the temperature of the oxygen gas to rise again. Thus, the graph resumes an upward slope from the boiling point temperature up to 300 K.
Step 5: Conclusion and Correct Graph
The correct temperature–time variation shows two segments with upward slopes (in liquid and then in gas phase) separated by a flat segment (during phase change). Therefore, the correct graph is the one that first shows an increase, then a plateau, and finally another increase in temperature — matching Option (1).
