Vapor Pressure and Dynamic Equilibrium

Dynamic equilibrium Rate of evaporation = rate of condensation 

The pressure of a gas in dynamic equilibrium with its liquid is called its vapor pressure. The vapor pressure of a particular liquid depends on the intermolecular forces present in the liquid and the temperature. Weak intermolecular forces result in volatile substances with high vapor pressures because the intermolecular forces are easily overcome by thermal energy. Strong intermolecular forces result in nonvolatile substances with low vapor pressures. 

We can describe the tendency of a system in dynamic equilibrium to return to equilibrium with the following general statement  

When a system in dynamic equilibrium is disturbed, the system responds so 

If the pressure above a liquid-vapor system in equilibrium decreases, some of the liquid evaporates, restoring the equilibrium pressure. If the pressure increases, some of the vapor condenses, bringing the pressure back down to the equihbrium pressure. This basic principle— Le ChateUer s principle—is applicable to any chanical system in equilibrium, as we shall see in Chapter 14. 

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