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    the boiling point of a solvent containing non volatile solute

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    Physical Properties of Solutions

    The physical properties of a solution are different from those of the pure solvent. Many differences in physical properties are predictable if the solute in the pure state is nonvolatile - that is, if it has a very low vapor pressure. Sugar, sodium chloride, and potassium nitrate are examples of nonvolatile solutes. Colligative properties are those physical properties of solutions of nonvolatile solutes that depend only on the number of particles present in a given amount of solution, not on the nature of those particles. We will consider four colligative properties: vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure.

    A. Vapor Pressure Lowering

    At any given temperature, the vapor pressure of a solution containing a nonvolatile solute is less than that of the pure solvent (see Section 10.3A for a discussion of vapor pressure). This effect is called vapor pressure lowering. The solid line in Figure 11.6 is a plot of the vapor pressure of pure water versus temperature. The break in the curve at 0°C is the intersection of the curve of the vapor pressure of the solid with the curve of the vapor pressure of the liquid. The dashed line in Figure 11.6 is a plot of the vapor pressure of an aqueous solution of sugar versis temperature. Notice that the vapor pressure of the solution is always less than that of the pure solvent. What causes this difference?

    FIGURE 11.6 The vapor pressure of pure water is shown as a solid line; the vapor pressure of an aqueous solution is shown as a dashed line. Note the differences between the solution and the pure substance in melting point and boiling point.

    The surface of a pure solvent (Figure 11.7a) is populated only by solvent molecules. Some of these molecules are escaping from the surface, and others are returning to the liquid state (see Section 10.3A). The surface of a solution is populated by two kinds of molecules; some are solvent molecules,

    FIGURE 11.7 Vapor pressure lowering: (a) the vapor pressure of a pure liquid; (b) the vapor pressure of a solution. In (b), the number of solvent molecules on the surface of the liquid has been decreased by the presence of the solute molecules. Fewer solvent molecules can vaporize, and the vapor pressure is lower.

    others are solute molecules. Only the solvent molecules are volatile. They alone can escape to build up the vapor pressure of the solution. There are fewer solvent molecules on the surface of the solution than on the surface of the pure liquid. Fewer will vaporize and, as a consequence, the vapor pressure of the solution will be less than that of the pure liquid at the same temperature (see Figure 11.7b).

    B. Boiling Point Elevation

    The boiling point of a substance is the temperature at which the vapor pressure of the substance equals atmospheric pressure. A solution containing a nonvolatile solute, having a lower vapor pressure than the pure solvent, must be at a higher temperature before its vapor pressure equals atmospheric pressure and it boils. Thus, the boiling point of a solution containing a nonvolatile solute is higher than that of the pure solvent (see Figure 11.6) This effect is called boiling point elevation.

    C. Freezing Point Depression

    Recall that freezing and melting point are two terms that describe the same temperature, the temperature at which the vapor pressure of the solid equals the vapor pressure of the liquid and at which the solid and the liquid are in equilibrium. Remember, too, that vapor pressure decreases as the temperature decreases. The vapor pressure of a solution is lower than that of the solvent, so the vapor pressure of a solution will equal that of the solid at a lower temperature than in the case of the pure solvent. Thus, the freezing point will be lower for a solution than for the pure solvent (see Figure 11.6). This effect is called freezing point depression. Remember that, just as it is the solvent that vaporizes when a solution boils, it is the solvent, not the solution, that becomes solid when a solution freezes. When a salt solution freezes, the ice is pure water (solid); the remaining solution contains all the salt.

    Application of this principle leads us to add antifreeze (a nonvolatile solute) to the water in the radiators in our cars. We thus lower the freezing point of the solvent (water), and the solution remains a liquid even at subfreezing temperatures.

    D. Osmosis and Osmotic Pressure

    Osmosis and osmotic pressure depend on the ability of small molecules to pass through semipermeable membranes like a thin piece of rubber, a cell membrane, or a thin piece of plastic wrap. Think of the membrane as a sieve with very tiny holes. Solvent particles are small and can very easily pass through these holes; solute particles are larger and cannot pass through (Figure 11.8).

    FIGURE 11.8 A semipermeable membrane allows small solvent molecules to pass through but prevents the passage of larger particles like those of a nonvolatile solute.

    स्रोत : www.chem.uiuc.edu

    Which of the following statements is correct for the boiling point of solvent containing a dissolved solid substance

    Click here👆to get an answer to your question ✍️ Which of the following statements is correct for the boiling point of solvent containing a dissolved solid substance

    Question

    Which of the following statements is correct for the boiling point of solvent containing a dissolved solid substance

    A

    Boiling point of the liquid is depressed

    B

    Boiling point of the liquid is elevated

    C

    There is no effect on the boiling point

    D

    The change depends upon the polarity of liquid

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    Updated on : 2022-09-05

    Solution Verified by Toppr

    Correct option is B)

    Dissolution of a non - volatile solute raises the boiling point of a liquid. It is due to decrease in free surface area for vapourisation.

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    Which statement is CORRECT for the boiling point of solvent containing a dissolved non

    Which statement is CORRECT for the boiling point of solvent containing a dissolved non-volatile solute?

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    Which statement is CORRECT for the boiling point of solvent containing a dissolved non-volatile solute?

    OPTIONS

    Boiling point of the liquid gets lowered.

    Boiling point of the liquid gets elevated.

    There is no effect of the added solute on the boiling point.

    The change depends upon the nature of the solute.

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    SOLUTION

    Boiling point of the liquid gets elevated.Explanation:

    When a non-volatile solute is dissolved in a liquid solvent, the boiling point of the resulting solution is raised due to a reduction in the solution's vapour pressure. Boiling point elevation is a colligative feature that is reliant only on the amount of solute particles and is unaffected by the solute's type.

    Concept: Boiling Point Elevation

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