Each point on this line therefore describes the vapor pressure of the pure solvent at that temperature. The dotted line in this figure describes the properties of a solution obtained by dissolving a solute in the solvent. At any given temperature, the vapor pressure of the solvent escaping from the solution is smaller than the vapor pressure of the pure solvent. The dotted line therefore lies below the solid line. According to this figure, the solution can't boil at the same temperature as the pure solvent.
If the vapor pressure of the solvent escaping from the solution is smaller than the vapor pressure of the pure solvent at any given temperature, the solution must be heated to a higher temperature before it boils. The lowering of the vapor pressure of the solvent that occurs when it is used to form a solution therefore increases the boiling point of the liquid.
When phase diagrams were introduced, the triple point was defined as the only combination of temperature and pressure at which the gas, liquid, and solid can exist at the same time. The figure above shows that the triple point of the solution occurs at a lower temperature than the triple point of the pure solvent.
By itself, the change in the triple point is not important. But it results in a change in the temperature at which the solution freezes or melts. To understand why, we have to look carefully at the line that separates the solid and liquid regions in the phase diagram. This line is almost vertical because the melting point of a substance is not very sensitive to pressure.
Adding a solute to a solvent doesn't change the way the melting point depends on pressure. The line that separates the solid and liquid regions of the solution is therefore parallel to the line that serves the same function for the pure solvent.
This line must pass through the triple point for the solution, however. The decrease in the triple point that occurs when a solute is dissolved in a solvent therefore decreases the melting point of the solution. The figure above shows how the change in vapor pressure that occurs when a solute dissolves in a solvent leads to changes in the melting point and the boiling point of the solvent as well.
Because the change in vapor pressure is a colligative property, which depends only on the relative number of solute and solvent particles, the changes in the boiling point and the melting point of the solvent are also colligative properties. Colligative Properties Calculations. The best way to demonstrate the importance of colligative properties is to examine the consequences of Raoult's law.
Raoult found that the vapor pressure of the solvent escaping from a solution is proportional to the mole fraction of the solvent. But the vapor pressure of a solvent is not a colligative property.
Only the change in the vapor pressure that occurs when a solute is added to the solvent can be included among the colligative properties of a solution. Because pressure is a state function, the change in the vapor pressure of the solvent that occurs when a solute is added to the solvent can be defined as the difference between the vapor pressure of the pure solvent and the vapor pressure of the solvent escaping from the solution.
Explanation : We can use the freezing point depression equation in order to determine the molar mass of the unknown solute. Example Question 3 : Colligative Properties. Possible Answers: Volume. Correct answer: Volume.
Explanation : Intensive properties are not dependent on the amount of substance. Copyright Notice. View Physical Chemistry Tutors. Omar Certified Tutor. Andrew Certified Tutor. Britany Certified Tutor. Physical Chemistry Tutoring in Top Cities:. Report an issue with this question If you've found an issue with this question, please let us know. Do not fill in this field. Louis, MO Or fill out the form below:. Company name. Before discussing these important properties, let us first review some definitions.
Solutions can exist in solid alloys of metals are an example of solid-phase solutions , liquid, or gaseous aerosols are examples of gas-phase solutions forms.
For the most part, this discussion will focus on liquid-phase solutions. In general and as will be discussed in Chapter 8 in more detail a liquid will freeze when. As such, the freezing point of the solvent in a solution will be affected by anything that changes the chemical potential of the solvent. As it turns out, the chemical potential of the solvent is reduced by the presence of a solute.
To evaluate the temperature dependence of the chemical potential, it is useful to consider the temperature derivative at constant pressure. After a small bit of rearrangement, this results in an expression for freezing point depression of. It is important to keep in mind that for a real solution, freezing of the solvent changes the composition of the solution by decreasing the mole fraction of the solvent and increasing that of the solute.
The derivation of an expression describing boiling point elevation is similar to that for freezing point depression. In short, the introduction of a solute into a liquid solvent lowers the chemical potential of the solvent, cause it to favor the liquid phase over the vapor phase.
As sch, the temperature must be increased to increase the chemical potential of the solvent in the liquid solution until it is equal to that of the vapor-phase solvent. The increase in the boiling point can be expressed as.
A very elegant derivation of the form of the models for freezing point depression and boiling point elevation has been shared by F. Schubert Schubert, Cryoscopic and ebullioscopic constants are generally tabulated using molality as the unit of solute concentration rather than mole fraction. In this form, the equation for calculating the magnitude of the freezing point decrease or the boiling point increase is.
A we have discussed, solutions have different properties than either the solutes or the solvent used to make the solution. Those properties can be divided into two main groups--colligative and non-colligative properties. Colligative properties depend only on the number of dissolved particles in solution and not on their identity.
Non-colligative properties depend on the identity of the dissolved species and the solvent. To explain the difference between the two sets of solution properties, we will compare the properties of a 1.
Therefore, any difference in the properties of those two solutions is due to a non-colligative property.
Both solutions have the same freezing point, boiling point, vapor pressure, and osmotic pressure because those colligative properties of a solution only depend on the number of dissolved particles. The taste of the two solutions, however, is markedly different.
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