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Difficulty: MediumStates of Matter and Phase Changes

A stationary engineer is monitoring a heavy-duty industrial boiler operating at a constant standard atmospheric pressure. The pure water inside the boiler has just reached its boiling point, and steam is actively forming. The system's burner continues to supply a massive, steady flow of thermal energy to the tank. Which of the following best describes what happens to the internal temperature of the water while it is in the process of converting from a liquid to a gas?

  1. The temperature remains perfectly constant until all of the liquid water has vaporized.Answer
  2. B
    The temperature steadily increases because the burner continues to add thermal energy.
  3. C
    The temperature continues to rise, but at a slower rate because convection currents distribute the heat.
  4. D
    The temperature rapidly spikes because the specific heat capacity of the liquid has been overcome.

Answer

The temperature remains perfectly constant until all of the liquid water has vaporized.
The correct answer accurately describes the phenomenon of latent heat. When a substance reaches its boiling point at a constant pressure, any additional thermal energy supplied to the system is used to break the intermolecular bonds holding the liquid together. Because this energy is not increasing the average kinetic energy of the molecules, the temperature remains completely stable until the entire mass of liquid has transformed into a gas.

Step-by-Step Solution

1
Identify the current thermodynamic state of the substance inside the boiler.
The water is at its boiling point and is actively converting from a liquid phase to a gas phase (vaporization).
Understanding the specific phase transition is required to apply the correct thermodynamic principles.
2
Determine how the continuous thermal energy from the burner affects the substance during this transition.
The continuous thermal energy acts as latent heat of vaporization, breaking the physical bonds between water molecules rather than increasing their average molecular kinetic energy.
Latent heat alters the physical state of a substance without altering its sensible heat (temperature).
3
Conclude the effect on the measurable temperature of the liquid water.
Because the kinetic energy of the molecules is not increasing, the temperature remains constant throughout the entire phase change plateau.
Temperature is a direct measure of average kinetic energy, which stabilizes when energy is diverted strictly to changing the phase.

Key Concept

Latent Heat and Phase Change Plateaus
Estimated Time:1m 0s
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Topics

States of Matter and Phase Changes

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