Key Difference – Gas vs Vapor
Gaseous phase is one of the four fundamental phases of all matter along with solid phase, liquid phase and plasma. Gases can be clearly distinguished from solid and liquid phases because, unlike in solids or liquids, the atoms are in free motion and are spread all around a container. Gas and vapor both seems similar due to their transparency, but are two distinct phases that matter can exist. The key difference between gas and vapor is that gas exists only in one physical state whereas vapor can coexist with another physical state.
CONTENTS
1. Overview and Key Difference
2. What is Gas
3. What is Vapor
4. Side by Side Comparison – Gas vs Vapor
5. Summary
What is Gas?
Gas in nature can be made out of a single element or a combination of atoms. However, it is a very small molecule. For example, if the group of halogen in the periodic table is considered, fluorine and chlorine exist as gases whereas bromine exists as a liquid and Iodine as a solid. This is because the size of the atom is increased down the group of halogen and large molecules cannot obtain free motion state due to the intermolecular interactions.
Gas is a substance that exists in only one state, which is the gaseous phase. This is called a thermodynamic state. A thermodynamic state is the condition of a system explained according to the thermodynamic parameters such as temperature, pressure, etc. Gas has not faced a phase change, which means it only exists as a gas and will not undergo phase changes unless special conditions are given. Therefore, it is called a monophasic substance.
The following diagram shows the relative positions of the gaseous phase and vapor phase. Here, the vapor phase is located in a lower temperature than the critical point temperature. The gaseous phase lies above the critical point.
Figure 01: Relative positions of gaseous phase and vapor phase
What is Vapor?
Vapor can be defined as a substance which is in the gaseous phase and can coexist with a liquid phase. This definition looks a bit confusing, but what happens here is that vapor is in equilibrium with a liquid. This liquid contains the same molecules as the vapor. Vapor is formed from a phase change, and it can undergo a phase change again. Therefore, it is named as a multiphasic substance. Vapor is not a state of matter as gas is. The transition of a gas into a liquid occurs by condensation followed by formation of a liquid drop and its growth. The coexistence of vapor with its liquid phase is possible because its mean temperature is below the critical point. The critical point is the temperature and pressure at which a gas and a liquid cannot be distinguished. Only gases can exist above the critical point; thus, a gas cannot coexist with a liquid. For example, steam is water vapor in high temperature whereas at room temperature it is a liquid.
A good example for vapor-liquid equilibrium in normal conditions is the equilibrium between ethanol and its vapor. The following diagram shows how these two phases are related.
Figure 02: Vapor-Liquid Equilibrium Mixture of Ethanol and Water
What is the difference between Gas and Vapor?
Gas vs Vapor |
|
| A gas can exist only in one thermodynamic phase. | Vapor can coexist with its liquid phase. |
| Physical State | |
| Gas is a fundamental state of matter. | Vapor is a temporary state of change of a liquid or a solid. |
| Nature | |
| All gases are not vapors. | All vapors are gases. |
| Properties | |
| Gases are invisible. | Vapor may be visible. (Ex: water vapor can be seen as a cloud.) |
| Phase Change | |
| Gas does not face a phase change | Vapor experiences phase change. |
| Origin | |
| Gas is always a gas in nature. | Vapor is a type of gas formed from either a liquid or a solid. |
| Formation | |
| Gas is not formed. | Vapor is formed either by boiling or evaporation |
| Critical Point | |
| The temperature of a gas is above the critical point. | The temperature of a vapor is below the critical point but above the boiling point of the particular liquid or solid. |
| Settling Down | |
| Gasses do no settle down on ground. | Vapors settle down on ground. |
Summary – Gas vs Vapor
Gas is located above the critical point whereas vapor is located below the critical point. No liquid phase can exist above the critical point. Vapor also exists below the critical point. Therefore, the main difference between gas and vapor is that gas exist only in one physical state whereas vapor can coexist with another physical state.
Image Courtesy:
1. “Phase-diag2″By Matthieumarechal (CC BY-SA 3.0) via Commons Wikimedia
2. “Vapor-Liquid Equilibrium Mixture of Ethanol and Water” By Wilfried Cordes – de:Dortmunder Datenbank; en:Dortmund Data Bank (CC BY-SA 3.0) via Commons Wikimedia