We see and even use the changes of physical states, or phase transitions, in many different ways. The most significant example of this is the evaporation, condensation, freezing, and melting of water. These are most commonly referenced when talking about the water cycle. --- # Vaporization and Condensation [Vapor Pressure Video](https://www.youtube.com/watch?v=re9r0kzQp_M) 1. The change from a gas phase to a liquid is called **condensation**. 2. The change from a liquid phase to a gas is called **vaporization**. 3. When a liquid **vaporizes** in a closed container, the gas molecules will be trapped inside. 4. If these gas molecules are unable to escape, they will continue to **build pressure** in the space above the liquid. 5. As these vapor molecules in the gas phase continue to move around, they can **collide back** into the molecules in the **condensed phase** and convert back to their liquid form. ![[Pasted image 20220610122147.png|400]] > When the rate of **condensation** becomes equal to the rate of **vaporization**, the vapor is said to be ***in equilibrium*** with the liquid. This concept is often referred to as **dynamic equilibrium**, as the there is *no net change*, but there is still some fluctuation between the two rates. --- ## Variations in Vapor Pressure As we know, the identities of the molecules within a liquid determine the types and strengths of [[Intermolecular Forces]] that occur. Because of this, **different substances will exhibit different equilibrium vapor pressures**. --- # Boiling Points The **boiling point** of a liquid is the temperature at which its equilibrium vapor pressure is equal to the pressure exerted on the liquid by its gaseous surroundings. The **normal boiling point** of a liquid is defined as its boiling point when the surrounding pressure is equal to 1 atm (atmosphere). --- ## Clausius - Clapeyron Equation Defines the quantitative relationship between a substance's vapor pressure and its temperature. $ln(\frac{P_2}{P_1})=\frac{\Delta H_{\text{vap}}}{R} \cdot (\frac{1}{T_1}-\frac{1}{T_2})$ $P$ = pressure (atm, mmHg, psi, etc.) > Any unit of pressure is acceptable within this equation, so long as the units of P1 and P2 are identical. $T$ = temperature (kelvin) $\Delta H$ = enthalpy of vaporization $R$ = gas constant = $8.314 \text{ J/mol}\cdot K$ > [!example] Finding Vapor Pressure > At $34.0 \degree C$, the vapor pressure of isooctane is $10.0$ kPa, and at $98.8 \degree C$, its vapor pressure is $100.0$ kPa. Use this information to calculate the enthalpy of vaporization for isooctane in kJ/mol. > > Step