Beginner Topic 254 of 1102 States of Matter & Solutions
Vapor Pressure
The pressure exerted by a vapor in equilibrium with its liquid at a given temperature.
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In-depth explanation of Vapor Pressure
Vapour pressure is the pressure exerted by a vapour in equilibrium with its liquid or solid at a given temperature in a closed container. Some molecules escape from the liquid while others return to it, and when these rates are equal the system is at dynamic equilibrium. Vapour pressure rises steeply with temperature, often by about a factor of two for every 10°C near room temperature. Liquids with weak intermolecular forces have higher vapour pressures and are more volatile. The vapour pressure of a liquid at its boiling point equals the external pressure.
Going deeper: Vapor Pressure
Key ideas: Vapour pressure depends only on temperature and the liquid, not on the amount of liquid or the container volume, so long as some liquid remains. Boiling occurs when it reaches the external pressure. Water: 0.031 atm at 25 C, 0.47 at 80 C, 1.00 at 100 C. Higher intermolecular forces mean lower vapour pressure at a given temperature.
Common mistakes and connections: A frequent error is thinking vapour pressure increases when the container volume is made smaller; it stays the same while liquid remains and more vapour condenses. Another is mixing it up with partial pressure of the gas in air. Vapour pressure of a solution is lowered by nonvolatile solutes. Links to the Clausius-Clapeyron equation, boiling point and Raoult's law.
Visualization of Vapor Pressure
Vapour pressure rises steeply with temperature; a liquid boils when its vapour pressure equals the external pressure. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Diethyl ether has a high vapour pressure and evaporates quickly, while glycerol has a very low vapour pressure and barely evaporates at room temperature.
Frequently asked questions about Vapor Pressure
What is Vapor Pressure?
The pressure exerted by a vapor in equilibrium with its liquid at a given temperature.
Is Vapor Pressure a beginner, intermediate or advanced topic?
Vapor Pressure is a beginner topic in States of Matter & Solutions. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Vapor Pressure?
Study these earlier topics in States of Matter & Solutions first: Joule-Thomson Effect, Liquefaction of Gases, Liquid State.
What comes after Vapor Pressure?
The next topics in States of Matter & Solutions are Evaporation and Boiling Point.
Can I practise Vapor Pressure with quizzes?
Yes. The interactive lesson includes a self-check question on Vapor Pressure, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Vapor Pressure fits in States of Matter & Solutions
Vapor Pressure is topic 35 of 109 in States of Matter & Solutions.