What is the Difference Between Vacuum Pressure and Vapour Pressure?

06/22/2025 • Maryella Faratro

Vacuum pressure and vapor pressure are two distinct concepts related to pressure. The key differences between them are:

  1. Definition: Vacuum pressure is the pressure inside a vacuum, which is a condition where no air or gas exists. Vapor pressure, on the other hand, is the pressure that a vapor exerts on its condensed form (solid or liquid) when the two phases are in thermodynamic equilibrium.
  2. Nature: Vacuum pressure is usually negative, as it is the pressure exerted under space. In contrast, vapor pressure is typically positive, as it refers to the pressure exerted by a vapor on its condensed form.
  3. Temperature Dependence: Vapor pressure changes with temperature changes, while vacuum pressure does not.
  4. Relation to Other Pressures: Vacuum pressure relates to a vacuum, while vapor pressure relates to solids and liquids.

In summary, vacuum pressure is the negative pressure exerted under space, while vapor pressure is the positive pressure exerted by a vapor on its condensed form in thermodynamic equilibrium. Vacuum pressure does not change with temperature, but vapor pressure does.

Comparative Table: Vacuum Pressure vs Vapour Pressure

Here is a table comparing vacuum pressure and vapor pressure:

Property Vacuum Pressure Vapor Pressure
Definition Vacuum pressure is the pressure inside a vacuum, which is usually negative and relative to ambient atmospheric pressure. Vapor pressure is the pressure that a vapor exerts on its condensed form when the system is in equilibrium.
Unit of Measurement Pounds per square inch (vacuum) or PSIV. Varies depending on the substance and temperature.
Measurement Techniques Hydrostatic gauges, mechanical or elastic gauges, thermal conductivity gauges, and ionization gauges. Vapor pressure can be calculated using equations such as Antoine's equation or determined experimentally.
Relationship with Temperature Vacuum pressure is not directly related to temperature. Vapor pressure increases with temperature.
Effect on Boiling Point Vacuum pressure does not directly affect the boiling point of a liquid. Vapor pressure affects the boiling point of a liquid, with higher vapor pressure leading to a lower boiling point.

Please note that the relationship between vacuum pressure and vapor pressure is not directly related, as they are two distinct phenomena. Vacuum pressure deals with the pressure inside a vacuum, while vapor pressure is related to the pressure exerted by a vapor on its condensed form.

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