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Notebook Review and Final Preparation

Figures (10)

Preparing casein glue
Figure 11.18 The presence of nonvolatile solutes lowers the vapor pressure of a solution by impeding the evaporation of solvent molecules.
Figure 11.19 A typical laboratory distillation unit is shown in (a) a photograph and (b) a schematic diagram of the components. (credit a: modification of work by “Rifleman82”/Wikimedia commons; credit b: modification of work by “Slashme”/Wikimedia Commons)
Figure 11.20 Crude oil is a complex mixture that is separated by large-scale fractional distillation to isolate various simpler mixtures.
This is a diagram with three boxes connected with two arrows pointing to the right. The first box is labeled, “Molality of solution,” followed by an arrow labeled, “1,” pointing to a second box labeled, “Change in boiling point,” followed by an arrow labeled, “2,” pointing to a third box labeled, “New boiling point.”
This is a diagram with five boxes oriented horizontally and linked together with arrows numbered 1 to 4 pointing from each box in succession to the next one to the right. The first box is labeled, “Mass of iodine.” Arrow 1 points from this box to a second box labeled, “Moles of iodine.” Arrow 2 points from this box to to a third box labeled, “Molality of solution.” Arrow labeled 3 points from this box to a fourth box labeled, “Change in boiling point.” Arrow 4 points to a fifth box labeled, “New boiling point.”
Figure 11.21 Rock salt (NaCl), calcium chloride (CaCl2), or a mixture of the two are used to melt ice. (credit: modification of work by Eddie Welker)
This is a diagram with three boxes connected with two arrows pointing to the right. The first box is labeled, “Molality of solution,” followed by an arrow labeled, “1,” pointing to a second box labeled, “Change in freezing point,” followed by an arrow labeled, “2” pointing to a third box labeled, “New freezing point.”
Figure 11.22 Freezing point depression is exploited to remove ice from (a) roadways and (b) the control surfaces of aircraft.
Figure 11.23 Phase diagrams for a pure solvent (solid curves) and a solution formed by dissolving nonvolatile solute in the solvent (dashed curves).

Colligative properties depend on the total number of solute particles in a solution, not their identities. These properties include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. When a nonvolatile solute is added to a solvent, it reduces the solvent's vapor pressure.

This happens because solute particles occupy surface area, reducing the number of solvent molecules that can escape into the vapor phase. The vapor pressure lowering is directly proportional to the mole fraction of the solute. Boiling point elevation occurs because the lowered vapor pressure requires a higher temperature to reach atmospheric pressure.

The increase in boiling point, ΔTb, is directly proportional to the molal concentration of solute particles and the boiling point elevation constant, Kb. Similarly, freezing point depression happens because solute particles disrupt the formation of a solid, lowering the freezing point. The decrease in freezing point, ΔTf, is directly proportional to the molal concentration of solute particles and the freezing point depression constant, Kf.

Osmotic pressure is another colligative property. It is the pressure needed to stop the flow of solvent through a semipermeable membrane into a solution. Osmotic pressure, Π, is directly proportional to the molarity of the solute, the absolute temperature, and the gas constant.

These relationships are described by equations that allow for the calculation of colligative effects based on solute concentration. The van’t Hoff factor, i, accounts for the actual number of particles a solute dissociates into in solution. For electrolytes, this factor is crucial because it affects the magnitude of colligative properties.

For example, NaCl dissociates into two ions, so its van’t Hoff factor is 2. However, due to ion pairing and other interactions, the measured van’t Hoff factor is often less than the ideal value. This factor is used in calculations to adjust for the actual number of particles in solution, ensuring accurate predictions of colligative effects.

Key Points

  • Casein is a family of related phosphoproteins that are commonly found in mammalian milk, comprising about 80% of the proteins in cow's milk and between 20% and 60% of the proteins in human milk.
  • Milk fat is a component of milk that contributes to its creamy texture and is present in varying concentrations, with human milk having a particularly low fat content compared to cow's milk.
  • Freezing point depression is the decrease in freezing point of a dilute solution compared to that of the pure solvent, directly proportional to the molal concentration of the solute.
  • Colligative properties are solution properties that depend only upon the total concentration of solute species, regardless of their identities, and include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure.
  • An emulsion is a mixture of two or more liquids that are normally immiscible, with one liquid dispersed in the other, such as oil-in-water or water-in-oil emulsions.
  • Curd is the solid mass formed when milk is coagulated, typically through the action of rennet or acid, and is a key component in the production of cheese.

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