Dans une pièce fermée de 40 m^3 on place un récipient ouvert contenant 0,5 litre d'eau; la temp

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Dans une pièce fermée de 40 m^3 on place un récipient...

Dans une pièce fermée de $40 \mathrm{~m}^3$ on place un récipient ouvert contenant 0,5 litre d'eau; la température y est $20^{\circ} \mathrm{C}$ et le taux d'humidité $65 \%(*)$. Théoriquement, cette eau s'évaporera-t-elle totalement ou non? Si non, quel volume en restera-t-il dans le récipient? Pratiquement, on sait bien que, dans ces conditions, de l'eau finira toujours par s'évaporer totalement, quelle que soit sa quantité. Pourquoi y a-t-il un désaccord entre le réel et le théorique? (*) La pression partielle de la vapeur d'eau $=65 \%$ de la pression de vapeur saturante à cette température.

Dans une pièce fermée de $40 \mathrm{~m}^3$ on place un récipient ouvert contenant 0,5 litre d'eau; la température y est $20^{\circ} \mathrm{C}$ et le taux d'humidité $65 \%(*)$.
Théoriquement, cette eau s'évaporera-t-elle totalement ou non? Si non, quel volume en restera-t-il dans le récipient?
Pratiquement, on sait bien que, dans ces conditions, de l'eau finira toujours par s'évaporer totalement, quelle que soit sa quantité. Pourquoi y a-t-il un désaccord entre le réel et le théorique?
(*) La pression partielle de la vapeur d'eau $=65 \%$ de la pression de vapeur saturante à cette température.

Key Concepts

Saturation Vapor Pressure

Saturation vapor pressure is the maximum pressure that a vapor can exert at a given temperature when in equilibrium with its liquid form. It sets the threshold beyond which additional evaporation cannot occur because the air can no longer absorb more vapor without condensation, and it plays a key role in determining the amount of liquid that can coexist with its vapor in a closed system.

Relative Humidity

Relative humidity is the ratio of the current partial pressure of water vapor to the saturation vapor pressure at a specific temperature, expressed as a percentage. It is a crucial measure because it determines the degree of saturation of the air and influences the dynamic balance between evaporation and condensation in a system.

Phase Equilibrium

Phase equilibrium refers to the state at which a liquid and its vapor coexist without a net change in their amounts, meaning the rate of evaporation equals the rate of condensation. In a closed system, this concept is used to theoretically predict the remaining volume of liquid when equilibrium is reached at a given relative humidity.

Closed System Dynamics

In a closed system, such as a sealed room, the overall amount of water (in liquid and vapor forms) is fixed. Evaporation continues only until the partial pressure of the water vapor reaches the saturation threshold corresponding to the system’s temperature and relative humidity. This concept explains why, under theoretical equilibrium conditions, not all the water evaporates if the saturation point is reached before the complete evaporation of the liquid.

Theoretical vs Practical Discrepancies

The discrepancy between theoretical predictions and practical observations arises from dynamic factors not accounted for in a simple equilibrium model. In practice, factors like slight temperature variations, air currents, imperfect sealing, and continuous removal or addition of moisture cause deviations from the ideal conditions assumed in theory, leading to scenarios where water may eventually evaporate completely despite the predictions from a static equilibrium analysis.

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