Boiling occurs when the pressure exerted by the liquid towards the atmosphere ________ the pressure exerted by the atmosphere down on the liquid molecules.

Boiling occurs when the pressure exerted by the liquid towards the atmosphere ________ the pressure exerted by the atmosphere down on the liquid molecules.

• A. exceeds
• B. has no relation
• C. is less than
• D. equals

Answer: (D)

Boiling happens when the vapor pressure of the liquid—the pressure the liquid molecules exert into the gas phase—reaches the external pressure pushing down on the liquid. When these two pressures are equal, bubbles of vapor can form inside the liquid and rise to the surface, marking the onset of boiling. So the pressure exerted by the liquid toward the atmosphere must equal the atmospheric pressure. This is why boiling points change with altitude: lower external pressure lowers the temperature at which the liquid’s vapor pressure matches it. At sea level (about 1 atm), water boils at 100°C; at higher altitudes, boiling occurs at lower temperatures.

Boiling happens when the vapor pressure of the liquid—the pressure the liquid molecules exert into the gas phase—reaches the external pressure pushing down on the liquid. When these two pressures are equal, bubbles of vapor can form inside the liquid and rise to the surface, marking the onset of boiling. So the pressure exerted by the liquid toward the atmosphere must equal the atmospheric pressure.

This is why boiling points change with altitude: lower external pressure lowers the temperature at which the liquid’s vapor pressure matches it. At sea level (about 1 atm), water boils at 100°C; at higher altitudes, boiling occurs at lower temperatures.