Vapor Pressure and Boiling
A liquid boils when its vapor pressure equals external pressure.
About this topic
Vapor pressure is the pressure exerted by a vapour in equilibrium with its liquid in a closed container. It rises with temperature and is lower for liquids with stronger intermolecular forces. A liquid boils when its vapour pressure equals the external pressure; the normal boiling point is where it equals 1 atm.
In a closed container, molecules continuously escape from the liquid and return from the vapour; at equilibrium these rates are equal, and the vapour pressure is steady. Volatile liquids such as ether and acetone have high vapour pressures at room temperature and evaporate quickly, while water and oil have low ones. Lowering the external pressure lowers the boiling point: at the top of Mount Everest, water boils at about 70 degrees Celsius, so cooking takes longer. A pressure cooker does the opposite, raising the boiling point to about 120 degrees.
Worked example: at 100 degrees Celsius the vapour pressure of water is 101.3 kPa, matching atmospheric pressure, so it boils. At 25 degrees it is only 3.2 kPa. Applications include distillation, perfumes that evaporate gradually, and vacuum distillation to protect heat-sensitive substances.
A misconception is that boiling and evaporation are the same; evaporation occurs at the surface at any temperature, while boiling forms bubbles throughout the liquid. Also, vapour pressure does not depend on the amount of liquid or container size.
Key ideas
- Weaker forces mean higher vapor pressure
- Boiling point falls at altitude
- Evaporation vs boiling
Quick check
Use cases: where Vapor Pressure and Boiling helps
- Homework: review the key idea behind Vapor Pressure and Boiling before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Vapor Pressure and Boiling without notes.
- Visual learning: play with the interactive lab to see how Vapor Pressure and Boiling changes when you adjust its inputs.
- Teaching a friend: explain Vapor Pressure and Boiling in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Vapor Pressure and Boiling?
A liquid boils when its vapor pressure equals external pressure.
What will I learn in Vapor Pressure and Boiling?
Weaker forces mean higher vapor pressure. Boiling point falls at altitude. Evaporation vs boiling.
Is Vapor Pressure and Boiling part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Vapor Pressure and Boiling lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Vapor Pressure and Boiling?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
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