While testing an open, unpressurized engine cooling system, a mechanic observes that the pure water inside has reached its boiling point and is actively boiling. If a constant source of thermal energy continues to be applied to this boiling water, which of the following best describes what will happen to its temperature?
- It will remain constant until all the water has changed into steam.Answer
- BIt will continuously increase as long as the thermal energy is being added.
- CIt will increase, but at a significantly slower rate than before boiling began.
- DIt will decrease because the escaping steam carries away the extra energy.
Answer
The temperature will remain constant until all the water has changed into steam.
During a phase change, such as boiling (transitioning from liquid to gas), the temperature of a pure substance remains constant. The continuous thermal energy being added is used entirely as latent heat to break the intermolecular bonds holding the liquid together, rather than increasing the kinetic energy (which would raise the temperature).
Step-by-Step Solution
Key Concept
During a phase change, such as boiling or melting, the temperature of a pure substance remains constant despite the continuous addition of heat. The energy (latent heat) is used to alter the molecular structure rather than increase kinetic energy.