Which of the following best represents the temperature versus heat supplied graph for water, in the range of -20°C to 120°C?

Solution

Physics sir · 3 steps

Water changes phase twice in this range, so look for exactly two flat steps: at 0 °C and at 100 °C.

1. Sloped parts. While the water stays in one phase, the heat supplied raises its temperature by Q = mc Δ T. On the graph that is a rising line. This happens three times: ice from -20°C to 0°C, water from 0°C to 100°C, and steam above 100°C.
2. Flat parts. During a phase change, the heat Q = mL goes into breaking bonds, not into raising temperature. So the graph runs flat. Ice melts at 0°C and water boils at 100°C, so there are exactly two flat steps.

   Option	Flat at 0 °C	Flat at 100 °C	Other flat parts
   (a)	yes	only near 100 °C	an extra one between 0 and 100 °C
   (b)	no	yes	none
   (c)	yes	yes	none
   (d)	no	no	none

3. So. Only graph (c) has a sloped segment, a flat step at 0°C, another sloped segment, a flat step at 100°C, and a final sloped segment. The answer is (c).

The idea to keep. On a temperature versus heat graph, every phase change is a flat step at a fixed temperature, and every single-phase stretch is a rising line. Count the phase changes first, then match the steps.

Answer from the NTA final answer key. Solution by PrepWiser.