Which statement best differentiates evaporation from boiling?

Which statement best differentiates evaporation from boiling?

• A. Evaporation occurs at the surface at any temperature; boiling occurs throughout the liquid at a specific temperature when vapor pressure equals external pressure.
• B. Evaporation occurs only when a liquid is in contact with a solid, while boiling occurs without contact with a solid.
• C. Evaporation happens when a liquid is heated to its boiling point; boiling can occur at any temperature.
• D. Evaporation requires pressure above atmospheric; boiling requires pressure below atmospheric.

Answer: (A)

The key idea is how and where phase change happens and what sets the temperature at which it occurs. Evaporation is a surface process that can happen at any temperature. Some molecules at the liquid’s surface gain enough energy to break free into the gas phase, so the liquid slowly loses mass from the top while the bulk stays put. It doesn’t require the liquid to reach a single temperature. Boiling, on the other hand, involves the whole body of liquid. When the vapor pressure inside the liquid equals the pressure acting on the liquid from outside, bubbles form inside the liquid and rise to the surface. This happens at a specific temperature for a given external pressure, known as the boiling point. If the surrounding pressure drops, that boiling point falls; if the pressure rises, it goes up. So evaporation happens at the surface at any temperature, while boiling happens throughout the liquid at a distinct temperature determined by the external pressure.

The key idea is how and where phase change happens and what sets the temperature at which it occurs. Evaporation is a surface process that can happen at any temperature. Some molecules at the liquid’s surface gain enough energy to break free into the gas phase, so the liquid slowly loses mass from the top while the bulk stays put. It doesn’t require the liquid to reach a single temperature.

Boiling, on the other hand, involves the whole body of liquid. When the vapor pressure inside the liquid equals the pressure acting on the liquid from outside, bubbles form inside the liquid and rise to the surface. This happens at a specific temperature for a given external pressure, known as the boiling point. If the surrounding pressure drops, that boiling point falls; if the pressure rises, it goes up.

So evaporation happens at the surface at any temperature, while boiling happens throughout the liquid at a distinct temperature determined by the external pressure.