Water Vapour Pressure vs Temperature: Data & Reference Table
Vapour pressure is a fundamental property of liquids and plays a key role in thermodynamics, chemical engineering, HVAC, and process industries. The vapour pressure of a substance is defined as the pressure exerted by its vapor when the liquid (or solid) is in thermodynamic equilibrium with the vapor phase at a given temperature in a closed system. In simpler terms, it measures a liquid’s tendency to evaporate at a particular temperature.
For pure substances like water, vapour pressure is primarily determined by temperature. As temperature rises, more molecules gain enough kinetic energy to escape into the vapor phase, increasing the vapour pressure. This property is crucial for designing boilers, condensers, evaporators, distillation units, and pressurized containers.
Key Points in the Definition:
- Applies to equilibrium conditions—not just any evaporation.
- Measured in a closed container to ensure vapor cannot escape.
- Units are usually Pa (kPa), atm, or psi.
Dependence on Temperature:
- Primary factor: Temperature.
- As temperature increases, more molecules have enough kinetic energy to escape → vapour pressure rises.
- Not significantly dependent on other factors like container volume or liquid amount, as long as equilibrium is maintained.
Exceptions:
- In mixtures, composition affects vapour pressure (Raoult’s law).
- Presence of dissolved substances (e.g., salts) slightly lowers vapour pressure (vapor pressure lowering).
How Vapour Pressure Works
Vapour pressure is a result of the dynamic balance between molecules leaving the liquid surface and those returning from the vapor phase. At the microscopic level, molecules in a liquid constantly move with varying kinetic energy. Some molecules near the surface gain enough energy to escape into the gas phase, forming vapor. Simultaneously, vapor molecules collide with the liquid surface and condense back into the liquid.
When the rate of evaporation equals the rate of condensation, the system reaches thermodynamic equilibrium. The pressure exerted by the vapor at this point is the vapour pressure. This equilibrium explains why vapour pressure depends almost entirely on temperature: higher temperatures increase molecular energy, causing more molecules to escape, and thus increasing pressure.
Practical Example: In a sealed boiler operating at 100°C, water develops a vapour pressure of 101.3 kPa (1 atm). If the temperature rises, the pressure inside the boiler increases, which must be considered for material selection and safety valves. Similarly, in storage tanks, knowing the vapour pressure helps prevent over-pressurization when liquids are stored at elevated temperatures.
Engineering Insight: Understanding vapour pressure is crucial for:
- Determining boiling points at various pressures for distillation and evaporation processes.
- Preventing cavitation in pumps and turbines.
- Designing safety systems for pressurized containers and pipelines.
Critical Point of Water & Supercritical Fluid
The critical point of a substance is the temperature and pressure at which the liquid and vapor phases become indistinguishable. For water, this occurs at a temperature of 374.14°C and a pressure of 22.064 MPa (3200.1 psi).
At the critical point:
- There is no distinct liquid or vapor phase—water becomes a supercritical fluid.
- Vapour pressure is no longer defined, since there is no phase boundary.
- The fluid exhibits unique properties: it can diffuse through solids like a gas and dissolve materials like a liquid.
Practical Applications: Supercritical water is used in:
- Advanced power generation systems.
- Supercritical extraction processes in chemical and food industries.
- Waste treatment and supercritical oxidation processes.
Understanding the critical point is important in engineering design because conventional pressure-temperature relationships, such as vapour pressure tables, are no longer valid beyond this point. Engineers must consider supercritical properties for high-pressure and high-temperature systems.
Vapour Pressure of Water Against Temperature – Data Table
| °C | °F | kPa | atm | psi |
|---|
1. 0–100°C: NIST Chemistry WebBook (Antoine Equation)
2. 101–374°C: IAPWS-95 Formulation (Wagner Equation)
3. Critical Point (374.14°C): IAPWS Guidelines
Calculations verified against ASHRAE Fundamentals (2021), Chapter 1.