A block of ice at -10° C is slowly heated and converted to steam at 100° C. Which of the following curves represent the phenomenon qualitatively:

Solution

Physics sir · 3 steps

Heat either raises the temperature or changes the phase, never both: slopes and plateaus alternate.

1. Two kinds of heating. While a single phase is being heated, its temperature rises steadily, which gives a sloping line. While a phase change is happening, all the heat goes into latent heat and the temperature stays fixed, which gives a flat line.

   Stage	What happens	Shape on the graph
   1	Ice warms from -10°C to 0°C	rising slope
   2	Ice melts at 0°C	flat
   3	Water warms from 0°C to 100°C	rising slope
   4	Water boils into steam at 100°C	flat, and the process ends here

2. Check the curves. A vertical jump would mean the temperature changes with no heat supplied, which is impossible, so (c) and (d) fail. A curve that begins with a flat part skips the warming of ice from -10°C, so (b) and (d) fail. A curve that ends rising heats steam above 100°C, which is not part of this process.
3. So. Only (a) shows rise, flat, rise, flat in that order, ending on the boiling plateau. The answer is (a).

The idea to keep. On a temperature versus heat graph, every phase change is a horizontal plateau and every single-phase stage is a slope. There are no vertical jumps, and you start and stop exactly where the question says.

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