Vapor Pressure and Phase Data

Introduction

Boiling point is not a fixed temperature independent of pressure. Tables of vapor pressure and phase behavior show how equilibrium changes with temperature and external pressure. A useful entry states the pure substance, phase, pressure units and applicable temperature range. It should not be used as a universal description of mixtures or supercritical conditions.

Core explanation

For a pure liquid in equilibrium with its vapor, vapor pressure rises with temperature. Boiling begins when vapor pressure reaches the surrounding pressure, allowing bubbles to persist within the liquid. The normal boiling point uses 1 atm, while a boiling point at 1 bar is slightly different. At reduced pressure, boiling occurs at a lower temperature; under pressure it occurs higher, as long as the liquid-vapor boundary exists. The NIST Chemistry WebBook provides vapor-pressure and phase-transition data with sources and conditions.

Vapor-pressure tables may give measured points or empirical fits. An Antoine form such as log₁₀P = A − B/(T + C) is only meaningful with stated pressure and temperature units and a fitted validity interval. Coefficients for one pressure unit cannot be inserted into another without conversion. A fit for a liquid-vapor region should not be extrapolated across melting, the critical region or far outside measured temperatures. A NIST Antoine-data example shows coefficients accompanied by units and temperature ranges.

Phase diagrams add solid-liquid and solid-vapor boundaries. Below a substance's triple-point pressure, the liquid phase may not be stable, so heating can cause sublimation rather than boiling. Near a critical point, liquid and vapor become indistinguishable and a simple “boiling point” description ceases to apply. For mixtures, boiling begins when the mixture's vapor-liquid equilibrium condition is met; composition can change as evaporation proceeds. Do not substitute the pure-solvent boiling point for a multicomponent system without a justified approximation.

Vapor-pressure values can also be affected by chemical decomposition. A compound that reacts before reaching an expected boiling point may have a reported decomposition temperature rather than a reliable normal boiling point. Read footnotes. If a source gives several conflicting values, examine purity, pressure, method and evaluation date before choosing one.

Step-by-step reasoning

1. Identify the substance, purity, phase and whether the system is pure or mixed. 2. Check pressure units, temperature scale and reference pressure for any boiling point. 3. Use tabulated vapor pressures or a correlation only within its stated range. 4. Locate the relevant phase boundary at the target external pressure. 5. Note critical, triple-point, decomposition and mixture limitations.

Visual explanation

Plot vapor pressure against temperature as a rising curve. Draw horizontal lines at 0.5 atm and 1 atm: their intersections with the curve give two boiling temperatures. A second pressure-temperature phase diagram shows solid, liquid and vapor regions meeting at the triple point. Mark that at very low pressure a heating path crosses the sublimation boundary instead of the liquid-vapor boundary.

Real-world analogy

A door opens when an inside push exceeds an outside push. Vapor bubbles persist when vapor pressure can match surrounding pressure. The analogy is limited because phase equilibrium involves molecular chemical potential, but it helps explain why reducing outside pressure lowers boiling temperature.

Real-world example

Vacuum distillation lowers pressure so a heat-sensitive liquid can boil below its atmospheric boiling point. A technician checks vapor-pressure data over the actual operating range rather than using only the normal boiling point. If the mixture contains several components, vapor-liquid equilibrium and composition changes must also be considered.

Why?

Why does a pressure cooker raise water's boiling temperature? It maintains pressure above atmospheric, so water must reach a higher temperature before its vapor pressure matches the surroundings. Why does this not imply that water molecules are “more strongly bonded” under pressure? The equilibrium condition changes, while molecular interactions and chemical identity remain broadly the same.

Common misconception

“Every liquid has one boiling temperature” ignores pressure. “Vapor pressure requires visible boiling” is false; evaporation and vapor equilibrium occur below boiling. “Antoine coefficients are universal constants” ignores fitted ranges and units. “A pure-component boiling point predicts mixture behavior exactly” ignores composition-dependent equilibrium.

Worked example

Suppose a hypothetical pure liquid has vapor pressure 40 kPa at 320 K and 100 kPa at 360 K. At external pressure 100 kPa, its boiling temperature is about 360 K based on the given table point. At 40 kPa, it is about 320 K. Do not linearly extrapolate these two points to 200 kPa without more data; vapor pressure varies nonlinearly with temperature, and a phase transition or decomposition might intervene. If a source instead defines the “normal” boiling point at 101.325 kPa, the 100 kPa point is close but not definitionally identical.

Quick check

1. What happens to a liquid's boiling temperature when external pressure decreases, assuming the liquid phase remains stable? Answer: It decreases because vapor pressure reaches the lower external pressure at a lower temperature.

Exam focus

Read vapor-pressure tables with units and temperature range. Distinguish normal boiling point at 1 atm from a stated 1 bar point. Explain a pressure shift using phase equilibrium. Identify when sublimation, mixtures or decomposition make a simple boiling-point lookup inappropriate.

Advanced insight

The Clausius–Clapeyron relation can approximate the slope of ln vapor pressure versus reciprocal temperature when vaporization enthalpy is nearly constant and the vapor behaves ideally. Near a critical point or across a wide temperature range those assumptions weaken, so critically evaluated equations of state or measured data are preferable.

Summary

Vapor pressure and boiling data are pressure- and temperature-dependent equilibrium information. A boiling point must name its reference pressure, while correlations must retain units and fitted range. Phase boundaries and mixture behavior determine which table entry applies.

Practice questions

1. Is a normal boiling point defined at 1 bar or 1 atm? Answer: Conventionally 1 atm; a 1 bar boiling point is close but is a distinct pressure condition. 2. Can a liquid have measurable vapor pressure below its boiling point? Answer: Yes. Equilibrium vapor exists before the vapor pressure reaches external pressure. 3. Why should Antoine parameters not be extrapolated far beyond their fitted range? Answer: They are empirical and may fail across phase changes, critical behavior or unmeasured temperatures. 4. What phase transition may occur when heating below the triple-point pressure? Answer: Sublimation directly from solid to vapor.