Physical and Chemical properties 2

Question 1

what can freezing curves be used for in regard to ice cream

Answer 1

hardness at a given temperature

Question 2

what is hardness of ice at a given temperature determined by

Answer 2

amount of water/ % of solutes

Question 3

Describe the region of A-S on a freezing curve (refer to slides if needed)

Answer 3

cooling of liquid water
removal of sensible heat
nucleation (initiation of phase change)

Question 4

What happens at point S

Answer 4

super cooling
-Cooling below the initial freezing point without ice formation

Question 5

Describe the region of S-B on a freezing curve (refer to slides if needed)

Answer 5

Ice crystals form at nucleation points
-latent heat of crystallisation is released increasing temperature.

Question 6

Describe the region of B-C on a freezing curve (refer to slides if needed)

Answer 6

-Ice crystals grow
-removal of latent heat, temp remains relatively constant (FP plateau)
-FP depression as solute become concentrated

Question 7

Describe the region of C-D on a freezing curve (refer to slides if needed)

Answer 7

-Supersaturated solutes may precipitate (crystalise) releasing heat of fusion

Question 8

Describe the region of D-E on a freezing curve (refer to slides if needed)

Answer 8

-crystlisation of ice and solutes continues
-a portion remains unfrozen water

Question 9

Describe the region of E-F on a freezing curve (refer to slides if needed)

Answer 9

-ice crystal form until theres no more water (E)
-temp falls to that of the freezer through removal of sensible heat
-glass transition can be reached

Question 10

at what stage of the feezing process do commercial freezer stop at

Answer 10

D-E

Question 11

draw a temp vs heat removal graph of pure water vs food system

Answer 11

look at notes
-water: linear decline (senisble heat removal)-> super cooling-> slight increase-> plateu (latent heat removal)-> linear decline (removal of sensible heat)
-Food: linear decline-> slight increase-> curved decline

Question 12

when does glass transition occur

Answer 12

when critial temp has been reached

Question 13

what is the glassy state?

Answer 13

when a viscoelastic liquid changes to a brittle amorphous solid

Question 14

describe the viscosity during the glass phase

Answer 14

viscosity is so high it resists shear stress (for long time)

Question 15

what is the critical temperature called ?

Answer 15

glass transition (Tg)

Question 16

what is Tg dependent on

Answer 16

solute concentration

Question 17

meaning of amorphous

Answer 17

disordered

Question 18

a glassy state has the physcial, mechanic and thermal properties of a …

Answer 18

solid

Question 19

why is a glassy state so stable

Answer 19

compounds are immobilised = no chemical reactions

Question 20

draw a simplifed state diagram
-INCLUDE Tg, Tm, T’G

Answer 20

look at notes

Question 21

what are Tm, Tg adn T’g

Answer 21

melting poing
glass transition (wholeline)
glass tranisition at freezing point (specific point)

Question 22

state of compounds below Tm

Answer 22

ice

Question 23

state of compounds above Tm

Answer 23

liquid

Question 24

name of least stable state

Answer 24

rubbery

Question 25

example of rubbery state

Answer 25

fruit leather -when it is high concentration below Tg

Question 26

describe behaviour in rubbery state

Answer 26

molecules are mobile oxygen can diffuse conc solute can pariticpate in degradation reactions