Chapter 3: Pure Substances

Question 1

what is a pure substance?

Answer 1

• 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

Question 2

what is a compressed or sub cooled liquid?

Answer 2

liquid not about to vaporize

Question 3

what is a saturated liquid

Answer 3

liquid about to vaporize

Question 4

what is a saturated liquid-vapour mixture?

Answer 4

liquid and vapour coexist

Question 5

what is saturated vapour?

Answer 5

last drop of liquid just vaporized or vapour about to condense

Question 6

what is superheated vapour?

Answer 6

vapour not about to condense, temperature of the vapour rises
- above 100°C for water

Question 7

what is saturation pressure?

Answer 7

pressure at which pure substance changes phase

Question 8

what is saturation temperature?

Answer 8

temperature at which a pure substance changes phase

Question 9

are saturation temperature and pressure dependant or independent properties?

Answer 9

they are dependant properties, i.e. at different saturation pressures, water will boil at different temperatures

Question 10

Adiabatic compression of air masses

Answer 10

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)

Question 11

what is the critical point in a T-v diagram?

Answer 11

When saturated liquid and saturated vapour states are identical
Above the critical point, liquid and vapour cannot be distinguished from one another

Question 12

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?

Answer 12

left: compressed liquid
right: superheated liquid
connecting both lines: critical point
middle: wet/mixed region

Question 13

In which diagram does the area under the curve represent work transfer?

Answer 13

Pressure-volume diagram (P-V)

Question 14

What is the triple line on property diagrams?

Answer 14

where solid, vapour and liquid co-exist under some specific conditions

Question 15

what is the triple point on phase diagrams (P-T diagram)?

Answer 15

where line of sublimation (solid to vapor), line of vaporization (liquid to vapor) and line of melting (solid to liquid) all meet

Question 16

what is latent heat?

Answer 16

energy absorbed/released during a phase-change process

Question 17

what is latent heat of fusion?

Answer 17

melting and freezing (liquid - solid)

Question 18

what is latent heat of vaporization?

Answer 18

vaporization and condensation (liquid - vapor)

Question 19

what is enthalpy?

Answer 19

combination of properties, hence internal energy + pressure times volume
often used in control volume analysis

Question 20

what does change in enthalpy during phase change process mean?

Answer 20

represents latent heat of change

Question 21

what is entropy?

Answer 21

measure of molecular disorder (higher the disorder, the higher the entropy)
at absolute zero, no entropy

Question 22

what is quality (x)?

Answer 22

• 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

Question 23

for ideal gases, what does the internal energy and enthalpy of a system depend on?

Answer 23

temperature
b/c low pressure and low density

Question 24

how does vacuum cooling work?

Answer 24

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)