DAT General Chemistry: Boiling Point and External Pressure

DAT High-Yield Concept

Boiling Condition: Boiling occurs when a liquid's internal vapor pressure equals the external atmospheric pressure pushing down on it ( $P_{\text{vap}} = P_{\text{ext}}$ ).
Normal Boiling Point: Defined specifically as the boiling temperature measured under standard atmospheric pressure of exactly $\pu{1.00 atm}$ ( $\pu{760 mmHg}$ ).
Temperature Dependence: Vapor pressure increases exponentially with increasing temperature according to the Clausius-Clapeyron equation.
External Pressure Shift: Lower external pressure decreases the boiling point (e.g., high altitude), whereas higher external pressure increases the boiling point (e.g., pressure cooker/autoclave).

1. Temperature Dependence of Vapor Pressure

When a liquid is heated, its molecules acquire higher kinetic energy, enabling a greater fraction of molecules to overcome intermolecular forces and transition into the gas phase across the phase interface: $\ce{H2O(l) <=> H2O(g)}$ . (Dynamic liquid-vapor equilibrium is covered in depth in its own lesson.) As more vapor forms above the liquid surface, the vapor exerts an increasing partial pressure known as vapor pressure.

The quantitative relationship between vapor pressure and absolute temperature is exponential, described by the two-point Clausius-Clapeyron equation:

\ln\left(\frac{P_2}{P_1}\right) = \frac{\Delta H_{\text{vap}}}{R}\left(\frac{1}{T_1} - \frac{1}{T_2}\right)

where $\Delta H_{\text{vap}}$ is the molar enthalpy of vaporization in $\pu{J/mol}$ , $R = \pu{8.314 J/(mol K)}$ , and temperatures $T_1$ and $T_2$ are in Kelvin ( $\pu{K}$ ).

Liquefied Gas Vapor Pressure Curves (Source: Tosaka · CC BY-SA 3.0 · Wikimedia Commons)

2. Normal Boiling Point

Key Definition: The normal boiling point is the temperature at which the vapor pressure of a liquid equals exactly

\pu{1.00 atm}

(

\pu{760 mmHg}

\pu{101.3 kPa}

Evaporation occurs only at the liquid surface, but boiling is characterized by vapor bubble formation throughout the bulk liquid. For bubbles to expand and rise without collapsing, the internal vapor pressure inside the bubble must equal or exceed the external pressure exerted on the liquid. Liquids with weaker intermolecular forces (e.g., diethyl ether compared to water) exert higher vapor pressures at any given temperature and thus exhibit lower normal boiling points.

3. How External Pressure Shifts the Boiling Point

Because boiling requires $P_{\text{vap}} = P_{\text{ext}}$ , altering the external pressure directly shifts the temperature needed for boiling to occur:

Decreased External Pressure: Less thermal energy is required for vapor pressure to match external pressure. Consequently, the boiling point decreases.
Increased External Pressure: More thermal energy must be added to elevate vapor pressure to match external pressure. Consequently, the boiling point increases.

Boiling happens where the vapor pressure curve meets the pressure pushing down — Every liquid has a vapor pressure that climbs steeply with temperature, and it boils at the temperature where that vapor pressure becomes equal to the pressure pushing down on its surface. Reading each curve against the 1 atm line gives the normal boiling point: 34.6 degrees Celsius for diethyl ether, 78.4 for ethanol and 100 for water, in the order of how weakly each liquid holds itself together. Because the boiling point is set by that crossing and not by the liquid alone, changing the external pressure moves it. Water boils at about 90 degrees Celsius where the air pressure is 526 torr, roughly 3 km up, and not until about 104 degrees Celsius when a sealed pot holds the pressure at 875 torr.

Environment	External Pressure ( $P_{\text{ext}}$ )	Boiling Point of $\ce{H2O}$	Clinical / Practical Application
High Altitude (e.g., Denver, $\pu{5280 ft}$ )	$\pu{0.83 atm}$ ( $\pu{630 mmHg}$ )	$\pu{95 degC}$	Water boils at a lower temperature; cooking takes longer.
Sea Level (Standard)	$\pu{1.00 atm}$ ( $\pu{760 mmHg}$ )	$\pu{100 degC}$	Standard baseline (normal boiling point).
Autoclave / Pressure Cooker	$\pu{2.00 atm}$ ( $\pu{1520 mmHg}$ )	$\pu{120 degC}$	High temperature sterilizes dental instruments by killing endospores.

Worked Quantitative Example

Problem: Ethanol has a normal boiling point of $\pu{78.0 degC}$ ( $\pu{351.15 K}$ ) at $\pu{760 mmHg}$ and a molar enthalpy of vaporization $\Delta H_{\text{vap}} = \pu{38.56 kJ/mol}$ . Calculate the equilibrium vapor pressure of ethanol at $\pu{25.0 degC}$ ( $\pu{298.15 K}$ ).

Step-by-Step Calculation:

Identify parameters: $P_1 = \pu{760 mmHg}$ , $T_1 = \pu{351.15 K}$ , $T_2 = \pu{298.15 K}$ , $\Delta H_{\text{vap}} = \pu{38560 J/mol}$ .
Evaluate inverse temperature difference: $\left(\frac{1}{T_1} - \frac{1}{T_2}\right) = \left(\frac{1}{351.15} - \frac{1}{298.15}\right) = -0.0005062 \pu{ K^{-1}}$
Apply Clausius-Clapeyron: $\ln\left(\frac{P_2}{760}\right) = \frac{38560 \pu{ J/mol}}{\pu{8.314 J/(mol K)}} \times (-0.0005062 \pu{ K^{-1}}) = -2.348$
Solve for $P_2$ : $\frac{P_2}{760} = e^{-2.348} = 0.09556 \implies P_2 = 760 \times 0.09556 = \pu{72.6 mmHg}$

DAT Exam Trap: Always convert temperatures to Kelvin (

\pu{K}

) and enthalpy to Joules per mole (

\pu{J/mol}

) before applying Clausius-Clapeyron! Also remember that the phrase "normal boiling point" gives you an implicit reference pressure of

P_1 = \pu{1 atm} = \pu{760 mmHg}

Quick Mnemonic: Pressure UP = Boiling point UP. (Push down harder, must Burn hotter).

Critical Thinking Questions

Why does water boil at approximately $\pu{70 degC}$ on top of Mount Everest, and why is an open pot of boiling water ineffective for rapidly cooking food at high elevations?
Dental autoclaves operate at elevated pressures (e.g., $\pu{2 atm}$ ) to achieve liquid water temperatures of $\pu{121 degC}$ . Why is liquid water at $\pu{121 degC}$ significantly more effective at sterilizing surgical instruments than steam or air at $\pu{100 degC}$ ?
If compound X has a higher vapor pressure than compound Y at $\pu{25 degC}$ , which compound has stronger intermolecular forces, and which has a higher normal boiling point?

References

Sources returned by Google Search grounding during lesson verification.

5.5: Vapor Pressure - Chemistry LibreTexts — chem.libretexts.org
Clausius-Clapeyron Equation - Chemistry LibreTexts — chem.libretexts.org

Boiling Point and External Pressure

Summary

Practice Questions

AI Tutor