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MCQEasyJEE 2026Calorimetry & Change of State

JEE Physics 2026 Question with Solution

Which of the following best represents the temperature versus heat supplied graph for water, in the range of −20∘C-20^\circ \text{C} to 120∘C120^\circ \text{C}?

Four temperature versus heat graphs labeled 1 to 4 for water heating, showing different combinations of sloping and horizontal segments near 0 degree Celsius, 100 degree Celsius, and 120 degree Celsius.
  • A

    Graph 1

  • B

    Graph 2

  • C

    Graph 3

  • D

    Graph 4

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: Water is heated from −20∘C-20^\circ \text{C} to 120∘C120^\circ \text{C}.

Find: Which temperature versus heat supplied graph is correct.

Concept: In a temperature–heat graph, horizontal portions indicate phase change. During phase change, heat is absorbed but temperature remains constant.

For water, the important temperatures are:

  • Ice melts at 0∘C0^\circ \text{C}
  • Water boils at 100∘C100^\circ \text{C}

From −20∘C-20^\circ \text{C} to 0∘C0^\circ \text{C}, water is ice, so temperature increases with heat. The graph must have a rising line.

At 0∘C0^\circ \text{C}, ice melts into water. Temperature remains constant while latent heat of fusion is supplied. Therefore, the graph must have a horizontal segment at 0∘C0^\circ \text{C}.

From 0∘C0^\circ \text{C} to 100∘C100^\circ \text{C}, water is liquid, so temperature again rises with heat. The graph must have another rising segment.

At 100∘C100^\circ \text{C}, water boils into steam. Temperature remains constant while latent heat of vaporization is supplied. Therefore, the graph must have a horizontal segment at 100∘C100^\circ \text{C}.

From 100∘C100^\circ \text{C} to 120∘C120^\circ \text{C}, steam is heated further, so temperature rises again.

Thus the correct graph must show:

  • rising line from −20∘C-20^\circ \text{C} to 0∘C0^\circ \text{C}
  • flat portion at 0∘C0^\circ \text{C}
  • rising line up to 100∘C100^\circ \text{C}
  • flat portion at 100∘C100^\circ \text{C}
  • rising line up to 120∘C120^\circ \text{C}

Among the given options, Graph 4 satisfies all these conditions. Therefore, the correct option is D.

Common mistakes

  • Choosing a graph with only one flat portion is incorrect because water undergoes two phase changes in this range: melting at 0∘C0^\circ \text{C} and boiling at 100∘C100^\circ \text{C}. Always check for both horizontal segments.

  • Assuming temperature always rises when heat is supplied is wrong. During phase change, heat goes into latent heat, so temperature remains constant. Look for flat portions at the phase-change temperatures.

  • Ignoring the starting temperature −20∘C-20^\circ \text{C} leads to an incorrect graph. The graph must begin below 0∘C0^\circ \text{C} with a rising segment for heating ice before melting starts.

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