Solutes Affect the Colligative Properties of Aqueous Solutions

Solutes of all kinds alter certain physical properties of the solvent, water: its vapor pressure, boiling point, melting point (freezing point), and osmotic pressure. These are called colligative ("tied together") properties, because the effect of solutes on all four properties has the same basis: the concentration of water is lower in solutions than in pure water. The effect of solute concentration on the colligative properties of water is independent of the chemical properties of the solute; it depends only on the number of solute particles (molecules, ions) in a given amount of water. A compound such as NaCl, which dissociates in solution, has twice the effect on osmotic pressure, for example, as does an equal number of moles of a nondissociating solute such as glucose.

Solutes alter the colligative properties of aqueous solutions by lowering the effective concentration of water. For example, when a significant fraction of the molecules at the surface of an aqueous solution are not water but solute, the tendency of water molecules to escape into the vapor phase—that is, the vapor pressure—is lowered (Fig. 2-11). Similarly, the tendency of water molecules to move from the aqueous phase to the surface of a forming ice crystal is reduced when some of the molecules that collide with the crystal are solute, not water. In that case, the solution will freeze more slowly than pure water and at a lower temperature. For a 1.00 molal aqueous solution (1.00 mol of solute per 1,000 g of water) of an ideal, nonvolatile, and nondis-sociating solute at 101 kPa (1 atm) of pressure, the freezing point is 1.86 °C lower and the boiling point is 0.543 °C higher than for pure water. For a 0.100 molal solution of the same solute, the changes are one-tenth as large.

Water molecules tend to move from a region of higher water concentration to one of lower water concentration. When two different aqueous solutions are separated by a semipermeable membrane (one that allows the passage of water but not solute molecules), water molecules diffusing from the region of higher water concentration to that of lower water concentration produce osmotic pressure (Fig. 2-12). This pressure, n, measured as the force necessary to resist water movement (Fig. 2-12c), is approximated by the van't Hoff equation:

in which R is the gas constant and T is the absolute temperature. The term ic is the osmolarity of the solution, the product of the solute's molar concentration c and the van't Hoff factor i, which is a measure of the extent to which the solute dissociates into two or more ionic species. In dilute NaCl solutions, the solute completely

Forming ice crystal c:

In pure water, every molecule at the surface is H2O, and all contribute to the vapor pressure. Every molecule in the bulk solution is H2O, and can contribute to formation of ice crystals.

In this solution, the effective concentration of H2O is reduced; only 3 of every 4 molecules at the surface and in the bulk phase are H2O. The vapor pressure of water and the tendency of liquid water to enter a crystal are reduced proportionately.

osmolarity than the cytosol, the cell shrinks as water flows out. In a hypotonic solution, with lower osmo-larity than the cytosol, the cell swells as water enters. In their natural environments, cells generally contain higher concentrations of biomolecules and ions than their surroundings, so osmotic pressure tends to drive water into cells. If not somehow counterbalanced, this inward movement of water would distend the plasma membrane and eventually cause bursting of the cell (osmotic lysis).

Several mechanisms have evolved to prevent this catastrophe. In bacteria and plants, the plasma membrane is surrounded by a nonexpandable cell wall of sufficient rigidity and strength to resist osmotic pressure and prevent osmotic lysis. Certain freshwater protists that live in a highly hypotonic medium have an organelle (contractile vacuole) that pumps water out of the cell. In multicellular animals, blood plasma and interstitial fluid (the extracellular fluid of tissues) are maintained at an osmolarity close to that of the cytosol. The high concentration of albumin and other proteins in blood plasma contributes to its osmolarity. Cells also actively pump out ions such as Na+ into the interstitial fluid to stay in osmotic balance with their surroundings.

FIGURE 2-11 Solutes alter the colligative properties of aqueous solutions. (a) At 101 kPa (1 atm) pressure, pure water boils at 100 °C and freezes at 0°C. (b) The presence of solute molecules reduces the probability of a water molecule leaving the solution and entering the gas phase, thereby reducing the vapor pressure of the solution and increasing the boiling point. Similarly, the probability of a water molecule colliding with and joining a forming ice crystal is reduced when some of the molecules colliding with the crystal are solute, not water, molecules. The effect is depression of the freezing point.

dissociates into Na+ and Cl-, doubling the number of solute particles, and thus i = 2. For nonionizing solutes, i is always 1. For solutions of several (n) solutes, n is the sum of the contributions of each species:

Osmosis, water movement across a semipermeable membrane driven by differences in osmotic pressure, is an important factor in the life of most cells. Plasma membranes are more permeable to water than to most other small molecules, ions, and macromolecules. This permeability is due partly to simple diffusion of water through the lipid bilayer and partly to protein channels (aquaporins; see Fig. 11-XX) in the membrane that selectively permit the passage of water. Solutions of equal osmolarity are said to be isotonic. Surrounded by an isotonic solution, a cell neither gains nor loses water (Fig. 2-13). In a hypertonic solution, one with higher

Pure water

Pure water

Nonpermeant solute dissolved in water

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Responses

  • Feorie
    How solut afect the collegative proportie of water?
    12 months ago
  • Jane Shiffer
    How solutes can affect the colligative properties of water?
    12 months ago
  • Eden Anderson
    How would 1 mole of gluclose affect the colligtive properties of water?
    11 months ago
  • janice
    Why solute concentration of collective properties is independent of chemicals and give sr example?
    9 months ago

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