Test when adding substance changes vapor pressure

A pressure plateau needs enough substance

Imagine a pure substance sealed into a container with 1 L of vapor space, held at 320 K by a thermal reservoir. First predict what happens when you add more substance. This is an equilibrium investigation: it does not predict how quickly evaporation occurs.

Use water at 320 K. Compare 1 mmol with 10 mmol, then double 10 to 20 mmol. Record the pressure, vapor amount and liquid remainder. A gas-only calculation keeps increasing; the phase-equilibrium model instead reaches a plateau once liquid can coexist with saturated vapor.

The saturation pressure comes from the NIST WebBook correlation, not a measurement made by this browser. For pressure in bar and temperature in kelvin,

log₁₀(Pₛₐₜ) = A − B/(T + C).

The displayed table gives each substance’s coefficients and fitted interval. NIST calculated these coefficients from the cited authors’ data. This activity selects one correlation per substance; it does not stitch together every available fit.

The ideal-gas equation then gives the vapor capacity:

n_capacity = PₛₐₜV/(RT); n_v = min(n_total, n_capacity); n_l = n_total − n_v.

Here amounts are in mol, volume is 1 L and R = 0.08314462618 L bar mol⁻¹ K⁻¹. Multiply mol by 1000 for mmol. Actual pressure is n_vRT/V. Below capacity, all material is vapor and pressure depends on amount. Above capacity, additional material becomes liquid. At exactly the threshold there is saturated vapor but zero liquid remainder. An empty container has zero pressure, not saturation pressure.

Explain the representations

The pressure bars share a fixed scale. The amount bar shows vapor plus liquid equal to the supplied total. The particle sketch distinguishes separated gas molecules from nearby liquid molecules; its molecule count and occupied volume are qualitative. Vaporization separates intact molecules rather than breaking their covalent bonds.

Liquid molecules remain mobile. At coexistence, evaporation and condensation balance statistically; molecules do not stop moving. Both water and ethanol support hydrogen bonding and dispersion. Comparing their curves changes molecular structure as well as interactions, so it does not isolate a single force or establish a universal molar-mass ranking.

Transfer to ethanol at 340 K. Find an inventory below saturation and another above it, then explain why equal total amounts can produce different phase partitions for two substances. Capture both recipes with your reasoning.

Model boundary and sources

The container has no air, vapor is ideal, liquid volume is neglected, and temperature is externally maintained. A changing liquid volume, nonideal vapor, thermal cooling, mixtures or finite equilibration time would require additional modeling. No uncertainty interval for the selected coefficients is supplied here; that is missing evidence, not zero uncertainty.

• NIST water phase-change data, Gubkov, Fermor et al. (1964), 293–343 K.
• NIST ethanol phase-change data, Ambrose and Sprake (1970), 292.77–366.63 K.
• Continue with supported boiling conditions.

Distinguish vaporization from gas uptake into a solvent

A gas can also be absorbed into the bulk of a liquid and later released (desorbed or outgassed) without requiring a chemical reaction. This is different from adsorption at a surface and from vaporizing the solvent itself. At an established gas–solution equilibrium, entry and exit continue at equal rates while the dissolved concentration and gas partial pressure remain steady under the held conditions.

Qualitative comparison: A supplied fictional closed gas/solvent record first shows net gas uptake, then constant dissolved concentration. A second record, after lowering that gas’s partial pressure at the same temperature, shows net release. Explain how both records can fit a reversible transfer. Would a constant liquid concentration alone prove that molecular exchanges stopped?

Compare the reversible-transfer account

The first stage can have entry faster than exit, followed by balanced exchanges. Lowering the gas partial pressure can favor net transfer back from liquid to gas. A constant concentration alone cannot distinguish balanced exchange from unresolved slow change; it does not prove that exchanges stopped. These are supplied conceptual observations, not a gas-handling experiment or numerical Henry-law problem.

OpenStax solubility, checked 31 October 2026, supports the gas/solution distinction. AP topic 7.1 includes reversible gas absorption/desorption; topic 7.3 excludes equilibrium calculations coupling a dissolved species to that species in the gas phase. This qualitative action respects that boundary and does not reinterpret the pure-liquid vapor widget as a dissolved-gas model.