Chapter 3 Flashcards
A substance that has a fixed chemical composition throughout
Pure substance
Air
Pure substance even though it is a mixture of several gases
Nitrogen and gaseous air
Pure substances
Mixture of liquid and gaseous water
Pure substance
Mixture of liquid and gaseous air
Not a pure substance
Molecules are at relatively fixed positions in a ___
Solid
Groups of molecules move about each other in the ____
Liquid phase
Molecules move about at random in the ___
Gas phase
The molecules in a solid are kept at their positions by…
The large springlike inter-molecular forces
A substance that is not about to vaporize
Compressed liquid
Subcooled liquid
At 1 atm and 20 degrees C water exists in the ___
Liquid phase (compressed liquid)
A liquid that is about to vaporize
Saturated liquid
At 1 atm pressure and 100 degrees C, water exists as…
A liquid that is ready to vaporize (saturated liquid)
A vapor that is about to condense
Saturated vapor
The state at which the liquid and vapor phases coexist in equilibrium
Saturated liquid-vapor mixture
A vapor that is not about to condense (i.e., not a saturated vapor)
Superheated vapor
As more heat is transferred, part of the saturated liquid vaporizes
Saturated liquid-vapor mixture
At 1 atm pressure, the temperature remains constant at 100 degrees C until the last drop of liquid is vaporized
Saturated vapor
As more heat is transferred, the temperature of the vapor starts to rise
Superheated vapor
The temperature at which water starts boiling depends on the ____
Pressure
If the pressure is fixed, so is the boiling temperature
Water boils at
100 degrees C at 1 atm pressure
The temperature at which a pure substance changes phase at a given pressure
Saturation temperature (Tsat)
The pressure at which a pure substance changes phase at a given temperature
Saturation pressure (Psat)
The amount of energy absorbed or released during a phase-change process
Latent heat
The amount of energy absorbed during melting. It is equivalent to the amount of energy released during freezing
Latent heat of fusion
The amount of energy absorbed during vaporization and it is equivalent to the energy released during condensation
Latent heat of vaporization
The ____ of latent heats depend on the temperature or pressure at which the phase change occurs
Magnitudes
Latent heat of fusion of water
At 1 atm, 333.7kJ/kg
Latent heat of vaporization of water
At 1 atm, 2256.5kJ/kg
The atmospheric pressure, and thus the boiling temperature of water, ____ with elevation
Decreases
At supercritical pressures (P > Pcr)
There is no distinct phase-change (boiling) process
The point at which the saturated liquid and saturated vapor states are identical
Critical point
The pressure in a piston-cylinder device can be reduced by…
Reducing the weight of the piston
Triple point of water
Ttp = 0.01 degrees C
Ptp = 0.6117kPa
At ____ pressure and temperature, a substance exists in three phases in equilibrium
Triple-point
Passing from the solid phase directly into the vapor phase
Sublimation
At low pressures (below the triple-point value), solids…
Evaporate without melting first (sublimation)
Deposition
Vapor to solid
Enthalpy
A combination property
Energy units
Pressure*volume
The amount of energy needed to vaporize a unit mass of saturated liquid at a given temperature or pressure
Enthalpy of vaporization (latent heat of vaporization)
Hfg
The ratio of the mass of vapor to the total mass of the mixture
Quality (x)
Between 0 and 1
Quality x = 0
Sat. Liquid
Quality x = 1
Sat. Vapor
The properties of the ____ are the same whether it exists alone or in a mixture with saturated vapor
Saturated liquid
Temperature and pressure are ___ properties for a mixture
Dependent
The relative amounts of liquid and vapor phases in a saturated mixture are specified by the ____
Quality (x)
In the region to the right of the saturated vapor line and at temperatures above the critical point temperature, a substance exists as _____
Superheated vapor
In the superheated vapor region, temperature and pressure are ____ properties
Independent
Superheated vapor is characterized by:
Lower pressures (P < Psat at a given T)
Higher temperatures (T > Tsat at a given P)
High specific volumes (v > vg at a given P or T)
Higher internal energies (u > ug at a given P or T)
Higher enthalpies (h > hg at a given P or T)
Compressed liquid properties depend more on what?
Temperature than pressure
Compressed liquid is characterized by:
Higher pressures (P > Psat at a given T)
Lower temperatures (T < Tsat at a given P)
Lower specific volumes (v < vf at a given P or T)
Lower internal energies (u < uf at a given P or T)
Lower enthalpies (h < hf at a given P or T)
Any equation that relates the pressure, temperature, and specific volume of a substance
Equation of state
The simplest and best-known equation of state for substances in the gas phase is the ____ of state
Ideal gas equation
Ideal gas equation
Predicts the P-v-T behavior of a gas quite accurately within some properly selected region
Real gases behave as an ideal gas at ____
Low densities (low pressure, high temperature)
When can water vapor be treated as an ideal gas?
At pressures below 10kPa
Ideal gas applications
Air-conditioning applications
When can ideal gas not be used?
At higher pressures (unacceptable errors in the vicinity of the critical point and the saturated vapor line)
Steam power plant applications
A factor that accounts for the deviation of real gases from ideal-gas behavior at a given temperature and pressure
Compressibility factor Z
The farther away Z is from unity,
The more the gas deviates from ideal-gas behavior
What is the criteria for low pressure and high temperature?
The pressure or temperature of a gas is high or low relative to its critical temperature or pressure
Ideal gas Z
Z=1
Real gases Z
Z>1
Z=1
Z<1
As P goes to 0,
Real gas to ideal gas
Van Der Waals Equation
Intermolecular attraction forces, volume occupied by the molecules themselves
2 constants, accurate over a limited range
Beattie-Bridgeman
5 constants
Accurate for density<=0.8Pcr
Benedict-Webb-Rubin
8 constants
Accurate for density<=2.5Pcr
Strobridge
16 constants
More suitable for computer calculations
Virial
May vary.
Accuracy depends on the number of terms used
The same of the individual pressures each component exerts on the system
Pressure of a gas mixture
Atmospheric pressure can be seen as…
The sum of “dry” air pressure (Pa) and vapor pressure (Pv) from water in the air
Air can only hold so much ____
Water vapor
The ratio of the actual amount of water vapor in the air to the maximum amount is called ___
Relative humidity (phi)
The relative humidity range
0 (dry air) to 1 (saturated air)
The desirable range for “thermal comfort” is ____
40-60%
Amount of water air can hold
Directly related to saturation pressure which increases with temperature
Air can hold more water at ____ temperatures
Higher
At low temperatures, water in the air ____
Condenses
What happens in the early morning hours when it is still cool?
Fog and dew
Example of phase equilibrium
At 100%, the air cannot hold any more water, so drying will not take place
Large differences between _____ causes air to evaporate
Vapor pressure and saturated pressure
Boiling vs. Evaporation
Liquid to vapor
Boiling - vapor bubbles
Evaporation occurs…
At a liquid-vapor interface when the vapor pressure is less than the saturation pressure
Evaporation examples
Evaporation of water from a pool or lake
Sweating
Boiling occurs…
At a solid-liquid interface when the solid surface temperature is higher than Tsat for the liquid
Boiling examples
Boiling water on a stove top
