1. Equilibrium Relative Humidity

2. Colligative Properties
Depend on the number of solute molecules or ions added to the solvent.
Boiling Point Elevation
Freezing Point Depression
Osmotic Pressure
The above are colligative (collective) properties and are used to determine the molecular weights and to measure water activity

3. Solutions
Solutions are involved in most chemical reactions, refining and purification, industrial processing, and biological processes.

4. A solution is a homogenous mixture of 2 or more substances
The solute is(are) the substance(s) present in the smaller amount(s)
The solvent is the substance present in the larger amount
12.1

5. A saturated solution contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature.
An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature.
A supersaturated solution contains more solute than is present in a saturated solution at a specific temperature.
Sodium acetate crystals rapidly form when a seed crystal is
added to a supersaturated solution of sodium acetate.
12.1

6. Ideal Solution – Raoult’s Law
A solution can be defined as ideal if the cohesive forces inside a solution are uniform
A mixture of A and B will form an ideal solution if the forces holding the molecules together in the pure liquids are of equivalent strength to those holding the molecules together in the mixture – the forces between A and B, A and A, and B and B are all the same
In an ideal solution, the tendency to escape as vapor is the same as that in the pure liquids
The partial pressure exerted by each component in the mixture is given by Raoult’s Law, which states that the vapor pressure of any component in an ideal solution is given by the products of the vapor pressure of that pure component and its mole fraction in the solution

7. Raoult's law Raoult's law states:
the vapor pressure of an ideal solution is dependent on the vapor pressure of each chemical component and the mole fraction of the component present in the solution

8. Colligative Properties of Nonelectrolyte Solutions
Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles.
Vapor-Pressure Lowering
P1 = X1 P 1
P 1
= vapor pressure of pure solvent
X1 = mole fraction of the solvent
Raoult’s law
If the solution contains only one solute:
X1 = 1 – X2
P 1
- P1 = DP = X2
X2 = mole fraction of the solute
12.6

9. For example
The addition of salt to water causes the water to freeze below its normal freezing point (0°C) and to boil above its normal boiling point (100°C).
The fact that an appropriate solute can both lower the freezing point and raise the boiling point of a pure liquid is the basis for year-round antifreeze for automobile cooling systems. In the winter the antifreeze lowers the freezing point of the water, preventing it from freezing at its normal freezing point; in the summer it guards against boilover by raising the boiling point of the water.

10. Colligative Properties of Nonelectrolyte Solutions
Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles.
Vapor-Pressure Lowering
P1 = X1 P 1
Boiling-Point Elevation
DTb = Kb m
Freezing-Point Depression
DTf = Kf m
Osmotic Pressure (p)
p = MRT
12.6

11. Boiling-Point Elevation
DTb = Tb – T b
T b is the boiling point of
the pure solvent
T b is the boiling point of
the solution
Tb > T b
DTb > 0
DTb = Kb m
m is the molality of the solution
Kb is the molal boiling-point
elevation constant (0C/m)
12.6

12. Freezing-Point Depression
DTf = T f – Tf
T f is the freezing point of
the pure solvent
T f is the freezing point of
the solution
T f > Tf
DTf > 0
DTf = Kf m
m is the molality of the solution
Kf is the molal freezing-point
depression constant (0C/m)
12.6

13. Osmotic Pressure (p)
Osmosis is the selective passage of solvent molecules through a porous membrane from a dilute solution to a more concentrated one.
A semipermeable membrane allows the passage of solvent molecules but blocks the passage of solute molecules.
Osmotic pressure (p) is the pressure required to stop osmosis.
more
concentrated
dilute
12.6

14. Osmotic Pressure (p) p = MRT High P Low P
M is the molarity of the solution
R is the gas constant
T is the temperature (in K)
12.6

15. Colligative Properties of Electrolyte Solutions
0.1 m NaCl solution
0.1 m Na+ ions & 0.1 m Cl- ions
Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles.
0.1 m NaCl solution
0.2 m ions in solution
van’t Hoff factor (i) =
actual number of particles in soln after dissociation
number of formula units initially dissolved in soln
i should be
nonelectrolytes 1
NaCl 2
CaCl2 3
12.6

16. Colligative Properties of Electrolyte Solutions
Boiling-Point Elevation
DTb = i Kb m
Freezing-Point Depression
DTf = i Kf m
Osmotic Pressure (p)
p = iMRT
12.7

17. aw measurement Colligative properties Isopiestic transfer
Measure vapor pressure of water in food directly
Freezing point depression
Isopiestic transfer
Hygroscopicity of salts
Hygrometers
Wet and dry bulb temperature
Electrical resistance/capacitance of salt

18. Vapor Pressure Measurement

19. Moisture sorption isotherms
Simplest method
Sorption data of weighed sample stored in enclosed container maintained at a certain RH, at constant temperature, and reweighing after equilibrium reached
Moisture content of sample determined from time to time
RH environments using saturated salt soln, sulfuric acid, glycerol
Long equilibration time, risk of mold or bacterial growth at high relative humidity

21. Moisture sorption isotherms
A moisture sorption isotherm describes the relationship between water activity and the equilibrium moisture content of a food product at constant temperature
Equilibrium moisture content curve
Equilibrium moisture content is the moisture content of a substance at equilibrium with a given partial pressure of the vapor. Used to describe the final moisture content reached during drying
Moisture content data
Dry basis
Wet basis

22. Moisture Content Dry basis
Expressed as ratio of amount of moisture in food to amount of dry solid
(kg of moisture / kg of dry solid)
Wet basis
Ratio of amount of moisture in food to total amount of wet solid
(kg of moisture / kg of wet solid)

23. Sorption isotherm Useful to determine shelf life
Assess background of operations such as drying, conditioning, mixing, packaging and storage
Gives information of specific interaction between water and product
Adjustment of aw vs sorption data
Integral technique
Differential technique

24. Sorption isotherms – sigmoid/types

25. Generation of Sorption isotherms
Adsorption process
Starting from a dry system having a zero water activity
Desorption process
Starting with a wet system having a water activity value of 1)
Hysteresis – difference between these 2 curves
Observed in most hygroscopic foods
Different models – Langmuir, BET, Oswin etc to describe moisture sorption isotherms

26. Sorption isotherms

27. Pause

28. Raoult's law
Raoult's law (räūlz') [for F. M. Raoult, a French physicist and chemist] states that the addition of solute to a liquid lessens the tendency for the liquid to become a solid or a gas, i.e., reduces the freezing point and the vapor pressure (see solution).
Qualitatively, depression of the freezing point and reduction of the vapor pressure are due to a lowering of the concentration of water molecules, since the more solute is added, the less the percentage of water molecules in the solution as a whole and therefore the less their tendency to form into a crystal solid or to escape as a gas.
Quantitatively, Raoult's law states that the solvent's vapor pressure in solution is equal to its mole fraction times its vapor pressure as a pure liquid, from which it follows that the freezing point depression and boiling point elevation are directly proportional to the molality of the solute, although the constants of proportion are different in each case. This mathematical relation, however, is accurate only for dilute solutions.

29. Solution
In chemistry, a homogeneous mixture of two or more substances in relative amounts that can vary continuously up to the limit of solubility (saturation), if any, of one in the other.
Most solutions are liquids, but solutions also can be of gases or solids — for example, air (composed primarily of oxygen and nitrogen)
In solutions comprising a solid dissolved in a liquid, the liquid is the solvent, and the solid is the solute; if both components are liquids, the one present in a smaller amount is usually considered the solute. If the saturation point is passed, excess solute separates out.
Substances with ionic bonds (e.g., salts) and many with covalent bonds (e.g., acids, bases, alcohols) undergo dissociation into ions on dissolving and are called electrolytes. Their solutions can conduct electricity and have other properties that differ from those of nonelectrolytes.
Solutions are involved in most chemical reactions, refining and purification, industrial processing, and biological processes.

30. The number of moles of a solute in a litre of solution is its molarity (M); the number of moles of solute in 1,000 g of solvent is its molality (m). The two measures differ slightly and have different uses.
For any chemical, a mass of 1 kilogram of the sample contains a large number of molecules, of the order 1023–1024. The mole is defined so that 1 mole of any substance always contains the same number of molecules. This number approximately 6.02 × 1023, and is known as the Avogadro number. The mole is a more convenient unit in which to measure the amount of a chemical than counting the number of molecules, and it has the same advantages.

31. and the individual vapor pressure for each component is
Once the components in the solution have reached equilibrium, the total vapor pressure p of the solution is:
and the individual vapor pressure for each component is
where
p*i is the vapor pressure of the pure component
xi is the mole fraction of the component in solution