Phase Diagrams Chapter 13 section 4

Phase Diagrams Show the relationship between temperature, pressure and state of matter Points on the line: exist as both phases Triple point: all three phases at once Critical point: can’t exist as a liquid above this point At critical temperature: increasing pressure won’t change a gas to a liquid

Normal Boiling/Freezing Points: occur at 1 atm of pressure

Dynamic Equilibrium Dynamic equilibrium: movement of an equal # of particles back and forth from one state of matter to another. rate of one phase change must equal the rate of another phase change. (a)– Open Container NOT at equilibrium (b) Closed Container - at equilibrium between liquid and vapor phases - rate of evaporation equals the rate of condensation.

How Temperature Affects the Vapor Pressure Vapor Pressure is simply the push of a gas above its liquid. As the temperature of a liquid increases, so does the number of vapor particles. More vapor particles cause more collisions, therefore more vapor pressure. [Direct Relationship: (T ↑ Vapor Pressure ↑)] Low Temperature High Temperature

Why Liquids Boil Any liquid will boil when the atmospheric (air) pressure equals the vapor pressure of the liquid. Inside of the bubble there is water vapor pressure pushing outward which keeps the shape of the bubble. vapor This image shows the inner- right-side of a bubble. liquid

Why Liquids Boil (Continued) Before the water gets hot enough, the bubble gets squished by the external atmospheric pressure pushing in as the bubble moves up through the water The bubble won’t quite make it to the surface—no boiling When the internal water vapor pressure equals the external air pressure, the bubble keeps its shape to the top water boils. Key: = interior water vapor pressure = exterior atmospheric pressure

3 Ways to Make Water Boil: How to Get the Pressures Equal (1) Heat it up: Raising the temperature of the water will increase the vapor pressure until it equals the atmospheric pressure. (2) Use a vacuum pump: Lower the external pressure above the liquid by pulling out the air until the atmospheric pressure is equal to the vapor pressure of the water. (3) Do both: Lower the air pressure and heat it.

“Normal Boiling Point” of Water At standard pressure (760 mm Hg), water’s normal B.P = 100˚C. Once a liquid is boiling, adding heat DOES NOT increase its temperature. The liquid simply bubbles more. On a mountain, the atm. pressure is lower, therefore the temperature at which the water boils is less than 100˚C. Foods in boiling water on a mountain will take longer to cook since the boiling water is cooler . Some foods, like breads and cakes, have special cooking instructions for high altitudes.

Boiling Water Above 100 ºC In a pressure cooker, the atm. pressure is higher, therefore the temperature at which the water boils is more than 100˚C. Foods in a pressure cooker will cook faster since the water is hotter. Water that’s next to volcanic vents on the ocean’s floor will reach temperatures well above 100˚ C, but it won’t “boil” since the surrounding pressure is so great!

Boiling Points vs. Pressure Practice Problems: What is the normal boiling point for ethanoic acid? _______ At what temperature will ethanoic acid boil on Mt. Everest when the pressure is only 30 kPa? _______ 120 ºC 80 ºC