Chapter 3: Pure Substances Flashcards
what is a pure substance?
- a substance having a fix chemical composition throughout (single chemical element/compound)
- homogenous mixture of various chemical elements or compounds is also a pure substance
- a mixture of 2 or more phases of a pure substance is also a pure substance
what is a compressed or sub cooled liquid?
liquid not about to vaporize
what is a saturated liquid
liquid about to vaporize
what is a saturated liquid-vapour mixture?
liquid and vapour coexist
what is saturated vapour?
last drop of liquid just vaporized or vapour about to condense
what is superheated vapour?
vapour not about to condense, temperature of the vapour rises
- above 100°C for water
what is saturation pressure?
pressure at which pure substance changes phase
what is saturation temperature?
temperature at which a pure substance changes phase
are saturation temperature and pressure dependant or independent properties?
they are dependant properties, i.e. at different saturation pressures, water will boil at different temperatures
Adiabatic compression of air masses
Generates dry, warm winds
- Adiabatic expansion: air becomes cooler as it rises (due to pressure decreasing exponentially) and can hold less humidity, leading to cloud formation and rainfall
- Adiabatic compression: dry air is compressed adiabatically during descent (due to increase pressure), the air takes up humidity from the ground
Air masses are blown towards a mountain range and rise on the windward side (side of adiabatic expansion)
what is the critical point in a T-v diagram?
When saturated liquid and saturated vapour states are identical
Above the critical point, liquid and vapour cannot be distinguished from one another
what is to the left, the right, connecting both and in between the saturated liquid line and saturated vapour line in a T-v diagram?
left: compressed liquid
right: superheated liquid
connecting both lines: critical point
middle: wet/mixed region
In which diagram does the area under the curve represent work transfer?
Pressure-volume diagram (P-V)
What is the triple line on property diagrams?
where solid, vapour and liquid co-exist under some specific conditions
what is the triple point on phase diagrams (P-T diagram)?
where line of sublimation (solid to vapor), line of vaporization (liquid to vapor) and line of melting (solid to liquid) all meet
what is latent heat?
energy absorbed/released during a phase-change process
what is latent heat of fusion?
melting and freezing (liquid - solid)
what is latent heat of vaporization?
vaporization and condensation (liquid - vapor)
what is enthalpy?
combination of properties, hence internal energy + pressure times volume
often used in control volume analysis
what does change in enthalpy during phase change process mean?
represents latent heat of change
what is entropy?
measure of molecular disorder (higher the disorder, the higher the entropy)
at absolute zero, no entropy
what is quality (x)?
- ratio of mass of vapour to total mass of mixture
- only has meaning in wet region and only has meaning between 0 & 1
saturated liquid: x=0
saturated vapour: x=1
for ideal gases, what does the internal energy and enthalpy of a system depend on?
temperature
b/c low pressure and low density
how does vacuum cooling work?
Cooling method where the produce (vegetables) are cooled by the evaporation of moisture from inside itself. The heat of vaporization during evaporation is absorbed from the products, which lowers the product temperature:
1. products are maintained at ambient temperature, the temperature of the chamber remains constant until the saturation pressure is reached
2. saturation conditions are maintained inside at progressively lower pressures and the corresponding lower temperatures until the desired temperature is reached
evaporative cooling (adiabatic cooling)
