Key Takeaways 11

Question 1

Group IV Hydrides

Answer 1

On the periodic table include carbon (C), silicon (Si), germanium (Ge) and tin (Sn), with each having four
valence electrons. Each valence electron can form a covalent
bond with hydrogen; the resultant molecule is symmetric and
includes methane (CH4, SiH4, GeH4, SnH4), with bond angles of
109.5o, which only interact by weak van der Waals interactions which control adhesion. Arising from transient dipole moments and are nonpolar molecules.
- significantly lower melting and
boiling points compared to group VI hydrides

Question 2

Group VI hydrides (Chalcogen atoms)

Answer 2

include oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po), these molecules lack symmetry when covalently bound to two hydrogen atoms due to lone electron pairs in their valence shells (i.e., H2O, H2S, H2Se, and H2Te) forming polar molecules
-significantly higher melting and
boiling points compared to group IV hydrides

Question 3

Hydrogen chalcogenides (H2X)

Answer 3

are triatomic molecules with a chalcogen, bent molecular configuration, and the presence of lone electron pairs that cause a deviation in the tetrahedral bond angle from 109.5 in methane (CH4) to 104.5 in the bent configuration for water and to 92.1o for H2S, 91.0o for H2Se and 90.0o for H2Te. Due to the uneven charge distribution, water
can align to and shield both negative and positive charges on dissolved molecules, making them
ideal solvents able to solubilize a wide range of molecules.

Question 4

Unlike methane, water molecules:

Answer 4

attract each other due to the permanent dipole moments arising
from the valance electron pairs on oxygen, causing an uneven charge distribution between the
electronegative oxygen and positively charged hydrogen atoms.

Question 5

When two water molecules
hydrogen bond:

Answer 5

one partially donates a hydrogen atom to the other molecule’s lone pair of electrons on its oxygen. Interactions between the positively charged hydrogen atom on one molecule of water and a negatively charged oxygen atom on another neutralize the dipole moment and result in a network of water molecules that form cooperative and anti-cooperative hydrogen bonds

Question 6

cooperative hydrogen bonding

Answer 6

A water molecule that donates an electron pair to form a hydrogen bond with another water
molecule causes an electron-withdrawing effect, thereby increasing the negative charge of the
oxygen atom. The subsequent increase in electronegativity amplifies the strength of the dipole of the adjacent water molecule, which is donating its hydrogen

Question 7

anti-cooperative hydrogen bonding

Answer 7

The other water molecule that accepts a hydrogen atom
attains a greater positive charge, causing its lone pair of electrons to be less electronegative,
causing the next water molecule to
interact less strongly.

Question 8

liquid-vapor transition temperature of the water

Answer 8

is pressure dependent,
and water boils only at 100oC under atmospheric pressure (101kPa), while it would boil at ~71oC
atop Mt. Everest. Elevated pressure increases the liquid-vapor transition temperature, while
decreased pressure, or under vacuum, allows water to boil below 100oC. The pressure affects the
vaporization temperature with important consequences on food processing when sterilizing
canned liquid foods

Question 9

examples of liquid-vapour transition being pressure dependent

Answer 9

• A can is placed into a pressurized retort and uses steam to
  increase temperature and pressure, allowing the contents to heat above 100oC while remaining
  liquid.
• Cans sterilized in an unpressurized vessel cannot exceed 100oC because water vaporization
  causes expansion of the can, which may rupture.
• Conversely, a vacuum is applied to lower the boiling point when making concentrate for heat-sensitive foods prone to browning reactions, such
  as orange juice.
• Rising film evaporators reduce the pressure to ~2.9 kPa, allowing water to be removed at or near ambient temperature and preserving the color and taste

Question 10

ice-liquid boundary

Answer 10

is less sensitive to pressure than the vapor-liquid or solid-vapor boundaries because
the density of liquid and ice are more similar than vapor density; thus, no processing techniques
use pressure to modify the ice-liquid boundary.

Question 11

triple point

Answer 11

• three-phase boundaries converge at this point
• at this pressure/temperature
  combination, all three states, solid, liquid, and vapor, coexist in equilibrium
• a water molecule in the vapor phase can condense into a liquid or a solid; a water molecule from ice either melts into a liquid or sublimates into a vapor

Question 12

Allowing foods to sublimate below the triple point:

Answer 12

allows frozen food to be freeze dried

Question 13

Transitions between these three phases are essential for:

Answer 13

• numerous aspects of food science, such as freezing crystallizing melting chocolate, evaporating water from foods to make concentrates,
  isolates, and powders, and sublimating ice while retaining the structure of freeze-dried berries found in breakfast cereals and ‘astronaut’ freeze-dried ice cream.

Question 14

critical point

Answer 14

The critical point in food science refers to the specific temperature and pressure at which a substance (typically a fluid) undergoes a phase transition from liquid to gas without a distinct boundary between the two phases. At this point, the properties of the liquid and gas phases become indistinguishable.

Question 15

above the critical point

Answer 15

a substance is termed a supercritical fluid

Question 16

below the critical point

Answer 16

classified as a superheated vapor
- commonly used in processing plants to transfer thermal energy.

Question 17

supercritical fluid

Answer 17

behave as gas that can diffuse through porous materials and as liquid because they can dissolve small molecules.
For example, supercritical CO2, removes caffeine from coffee beans and tea.

Question 18

physical properties of water are influenced by temperature

Answer 18

• particularly the dipole moment and the strength and length of hydrogen bonds
• Maximum hydrogen bonding strength occurs at approximately 4°C, resulting in the highest density of water
• The transition from liquid to solid ice alters molecular arrangement, leading to ice’s lower density and causing it to float

Question 19

Heat capacity (cp), thermal conductivity (k), and thermal diffusivity (α) of water:

Answer 19

vary with temperature and physical state (solid vs. liquid). When a product freezes, ice forms at the surface and spreads inward, with ice exhibiting higher thermal diffusivity and conductivity than water, which accelerates freezing. However, thawing is slower because the water at the surface acts as an insulator.

Question 20

heat capacity (water vs carb vs fat vs protein)

Answer 20

• Water has a high specific heat capacity (4.2 kJ/kg/K), much greater than fat (1.7 kJ/kg/K), protein (1.5 kJ/kg/K), carbohydrates (1.4 kJ/kg/K), and ash (0.8 kJ/kg/K).
• Considering the adult human body contains 60%
  water, the high specific heat capacity regulates body temperature at 37o, requiring significant
  energy input or loss to cause a deviation in this temperature.

Question 21

the body further regulates temperature through:

Answer 21

evaporative cooling during
perspiration, taking advantage of the high latent heat of vaporization.

Question 22

The high specific heat of
water is also essential in maintaining climate:

Answer 22

as large bodies of water contain significant thermal
energy, which resists temperature changes when air temperature drops or increases; as solar
radiation increases the temperature near the equator, natural convection establishes ocean
currents regulating global climate.

Question 23

latent heat of fusion

Answer 23

The amount of heat required to convert a unit mass of a solid into a liquid at its melting point without changing its temperature.

Question 24

latent heat of vaporization

Answer 24

The amount of heat required to convert a unit mass of a liquid into a gas at its boiling point without changing its temperature.

Question 25

water cohesion

Answer 25

the attraction between water molecules

Question 26

water adhesion

Answer 26

the attraction of
water to other molecules

Question 27

what happens when other molecules surround water

Answer 27

they arrange their dipole moments to create the “lowest energy state,”
balancing the net dipole moments of water molecules in the lattice, which minimizes the surface
area forming spherical drops

Question 28

when adhesive force exceeds cohesive force

Answer 28

capillary action generates
an upward force drawing liquid up at the liquid edges, forming a meniscus.

Question 29

why is capillary action essential

Answer 29

• to move water from the roots to the leaves of plants
• and to move through the capillaries of the human vascular system; adhesion is also central
  to solubilizing and dispersing nutrients in the blood.

Question 30

heat capacity

Answer 30

is the amount of heat required to produce a unit change in its temperature

Question 31

thermal conductivity

Answer 31

is the ability to conduct heat

Question 32

thermal diffusivity

Answer 32

is the rate at which heat is transferred through a material from the hotter region to the cooler region.