During boiling at saturation, what happens to temperature as heat is added?

During boiling at saturation, what happens to temperature as heat is added?

• A. Temperature increases
• B. Temperature remains constant
• C. Temperature decreases
• D. Temperature fluctuates

Answer: (B)

When a liquid boils at its boiling point, the energy you add goes into changing the phase from liquid to vapor, not into increasing the kinetic energy of the molecules. At this moment, the liquid and its vapor are in equilibrium at a specific saturation temperature for the given pressure. Because that energy is used for latent heat of vaporization, the temperature stays the same until all the liquid has vaporized. Only after the liquid is fully converted will further heat raise the temperature of the resulting vapor (or the system if the pressure changes). So, the temperature remains constant during boiling at saturation. The other options would require either adding or removing heat without a phase change, or a fluctuating temperature, which doesn’t describe the steady boiling process.

When a liquid boils at its boiling point, the energy you add goes into changing the phase from liquid to vapor, not into increasing the kinetic energy of the molecules. At this moment, the liquid and its vapor are in equilibrium at a specific saturation temperature for the given pressure. Because that energy is used for latent heat of vaporization, the temperature stays the same until all the liquid has vaporized. Only after the liquid is fully converted will further heat raise the temperature of the resulting vapor (or the system if the pressure changes).

So, the temperature remains constant during boiling at saturation. The other options would require either adding or removing heat without a phase change, or a fluctuating temperature, which doesn’t describe the steady boiling process.