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Difficulty: MediumThermodynamics and Heat

An open pot of pure water on a stove reaches a vigorous boil at standard atmospheric pressure. While heat continues to be supplied by the burner, which of the following describes what happens to the temperature of the boiling liquid water as it turns to steam?

  1. A
    The temperature steadily rises above 100∘C100^\circ\text{C} as the liquid absorbs additional thermal energy.
  2. The temperature remains constant at 100∘C100^\circ\text{C} because the added thermal energy is used to break intermolecular bonds during the phase change.Answer
  3. C
    The temperature rapidly drops toward 0∘C0^\circ\text{C} because escaping steam carries away all thermal energy from the remaining liquid.
  4. D
    The temperature fluctuates unpredictably between the freezing point and the boiling point due to turbulent convection currents.

Answer

The temperature remains constant at 100∘C100^\circ\text{C} because the added thermal energy is used to break intermolecular bonds during the phase change.
When a pure substance reaches its boiling point at constant pressure, its temperature remains unchanged until the entire liquid has transitioned into vapor. The continuous input of heat supplies the latent heat of vaporization, which breaks the intermolecular bonds holding liquid molecules together rather than increasing their average kinetic energy.

Step-by-Step Solution

1
Identify the physical state and process occurring in the system.
Water is actively undergoing a phase change from liquid to gas (boiling/vaporization) at standard atmospheric pressure (1 atm1\text{ atm}).
Phase transitions have specific thermodynamic properties distinct from single-phase heating.
2
Analyze how thermal energy affects substances during a phase transition.
The added energy functions as the latent heat of vaporization, which overcomes intermolecular hydrogen bonds rather than increasing molecular kinetic energy.
Temperature is a measure of average translational kinetic energy; because the kinetic energy does not increase during the phase change, temperature remains constant until all liquid has vaporized.

Key Concept

Latent heat and constant temperature during phase transitions
Estimated Time:1m 0s
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Topics

Thermodynamics and Heat

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