The primary reason for water's low vapor pressure is hydrogen bonding.
Understanding Vapor Pressure
Vapor pressure refers to the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase (liquid or solid) at a given temperature. A substance with high vapor pressure evaporates more readily. Water, however, has a relatively low vapor pressure compared to many other liquids.
The Role of Hydrogen Bonding
What are Hydrogen Bonds?
Hydrogen bonds are relatively strong intermolecular forces that form between a hydrogen atom that is covalently bonded to a highly electronegative atom (such as oxygen in the case of water) and another highly electronegative atom.
How Hydrogen Bonds Affect Vapor Pressure
- Intermolecular Attraction: In water, the strong hydrogen bonds between water molecules create a cohesive network. This means the molecules are strongly attracted to each other.
- Reduced Escape: Because of the strong attraction, water molecules require more energy to break free from the liquid surface and enter the vapor phase. This means that fewer molecules will have sufficient kinetic energy to evaporate at a given temperature.
- Lower Vapor Pressure: This reduced escape of molecules translates to a lower vapor pressure. Only a small amount of water molecules can escape from the surface to turn into vapors.
Comparing with Other Liquids
Many liquids do not form hydrogen bonds, or they form weaker intermolecular attractions, which means they have a higher vapor pressure than water. These liquids have molecules that can more easily escape into the vapor phase.
Table of Comparative Properties
| Property | Water | Other Liquids (e.g., Ether, Acetone) |
|---|---|---|
| Intermolecular Forces | Strong Hydrogen bonds | Weak Van der Waals/ Dipole Forces |
| Escape of Molecules | Less | More |
| Vapor Pressure | Lower | Higher |
In Summary
The presence of strong hydrogen bonding between water molecules is the main reason why water exhibits a low vapor pressure. These bonds make it difficult for water molecules to transition into the vapor phase, meaning fewer of them become vapor at any given temperature. This is why water evaporates slower than other similar liquids.
