Vapor
Definition and meaning of Vapor in chemistry.
A vapor is the gaseous form of a substance that is normally a liquid. It can also be the gaseous form of a substance that is normally a solid. These substances exist as liquids or solids at standard room temperature and normal pressure.
In more detail
In everyday life, people often use the words gas and vapor to mean the exact same thing. However, chemists make a very important distinction between these two common scientific terms. A true gas is a substance that naturally exists in the gaseous state at room temperature.
Oxygen and nitrogen are true gases because they do not pool as liquids under normal conditions. A vapor comes from a substance that usually prefers to be a liquid or a solid. Because water is a liquid at room temperature, its airborne gas phase is called a vapor.
Vapors form when molecules escape from a liquid surface through a natural process called evaporation. They can also form when solids turn directly into gas through a physical process called sublimation. Every liquid and solid constantly releases some molecules into the surrounding air as a vapor.
The overall tendency to form a vapor depends heavily on the temperature of the chemical substance. If you cool a vapor down, it will easily condense back into a liquid or a solid. You can also force a vapor to condense simply by squeezing it with high physical pressure.
Key facts
| Field | Physical Chemistry |
|---|---|
| Base State | Normally a liquid or solid at room temperature |
| Formation Process | Evaporation (from liquids) or sublimation (from solids) |
| Reversibility | Can easily condense back into a liquid or solid |
| Key Difference | A true gas is already gaseous at standard room temperature |
| Common Example | Water vapor, H2O, in the atmosphere |
When a puddle of water sits in the warm afternoon sun, it slowly disappears. The liquid water molecules are absorbing heat energy and turning into invisible water vapor. This water vapor rises high up into the much cooler parts of the earth's atmosphere. The cold temperature forces the water vapor to condense back into tiny liquid water droplets. We see these massive collections of floating liquid droplets as large clouds in the sky.
Frequently asked questions
What is the main difference between a vapor and a true gas?
A true gas is naturally gaseous at standard room temperature and normal atmospheric pressure. A vapor is just the gaseous form of a normal liquid or solid.
Can a vapor turn back into a liquid?
Yes, a vapor will quickly turn back into a liquid if you lower its temperature. You can also force it to condense by increasing the pressure on it.
Does dry ice produce a gas or a vapor?
Dry ice is solid carbon dioxide, so it produces a vapor when it sublimates. It turns directly from a frozen solid block into a cold, drifting vapor.