This calculation determines how much of a particular gas is available in a mixture.
Example: At sea level, the mole fraction of oxygen in the air is 21% (0.21), and the atmospheric pressure is 105 Pascals. Therefore, the partial pressure of oxygen is 0.21∗105=21 kilopascals.
Atmospheric Pressure
Atmospheric pressure is the pressure exerted by the weight of the atmosphere above us.
Measured in Pascals (Pa), which is Newtons per square meter (N/m2).
One Pascal is equivalent to the force exerted by a $50 note on the palm of your hand.
One atmosphere is equivalent to 1.01∗105 Pascals.
Atmospheric pressure is measured using a barometer.
A barometer measures the height of a column of mercury supported by atmospheric pressure.
At sea level, atmospheric pressure is approximately 105 Pascals or 100 kilopascals.
Examples of Pressure Variations:
Mount Everest: The mole fraction of oxygen is still 21%, but the total pressure is only 0.33∗105 Pascals, resulting in a partial pressure of oxygen of 7 kilopascals. Lower oxygen levels can lead to hypoxia and impaired cognitive function.
Mariana Trench: The deepest part of the ocean, with immense pressure due to the water above.
Underwater Pressure: Every 10 meters of depth increases the pressure by approximately 1 atmosphere (100 kilopascals).
At 30 meters underwater, the total pressure is 400 kilopascals (100 kPa atmosphere + 300 kPa water). The partial pressure of oxygen is 84 kilopascals (0.21 * 400 kPa), which can lead to oxygen toxicity if the gas mixture is not properly managed.
Changes in atmospheric pressure affect human physiology.
In space, where there is no atmospheric pressure, people grow taller.
Deep-sea diving requires careful management of gas mixtures to avoid oxygen toxicity and decompression sickness (“the bends”).
Phase Diagrams
Phase diagrams show the different phases of a substance (solid, liquid, gas) under various conditions of temperature and pressure.
Heating a substance adds energy to its molecules, causing them to move more.
Solids melt into liquids, and liquids vaporize into gases as temperature increases.
The state changes with temperature and pressure. For example, a solid turns into liquid at 273K, the liquid starts to get hotter, and then once the liquid reaches 373K it turns into a gas.
Increased pressure can hinder the transition from liquid to gas.
Decreased pressure can cause solids to sublimate directly into a gas
Triple point: The specific temperature and pressure at which all three phases (solid, liquid, gas) coexist in equilibrium.
For water, the triple point is at 0.006 atmospheres and approximately 273 Kelvin.
Useful for calibrating thermometers and identifying unknown substances.
Critical point: The temperature and pressure above which a distinct liquid phase does not exist.
Above the critical point, a substance exists as a supercritical fluid, which has properties of both liquids and gases.
Supercritical fluids are used in extraction, purification, sterilization, and chemical reactions.
Phase Changes
Freezing: Liquid to solid.
Melting: Solid to liquid.
Vaporization: Liquid to gas.
Condensation: Gas to liquid.
Sublimation: Solid directly to gas.
Deposition: Gas directly to solid.
The solid-liquid line represents the melting/freezing point at different pressures.
The liquid-gas line represents the boiling/condensation point at different pressures.
The solid-gas line represents the sublimation/deposition point at different pressures.
Unique Properties of Water
The solid-liquid line of water has a negative slope, indicating that increasing pressure on ice will cause it to melt.
This is because ice has a lower density than liquid water due to hydrogen bonding, which creates intermolecular spaces.
Putting ice under pressure favors the formation of the denser liquid phase.
Phase diagrams can be complex for substances with multiple solid phases or crystalline structures, such as diamond and iron, due to different arrangements of molecules and intermolecular spaces.