# Colligative solutions

Use Back Button to Return Colligative Properties Problems - Answers Remember that Colligative properties depend only on the number or concentration of particles in a solution. The properties are, for ideal solutions, independent of the kind or size of the particles, whether ionic, large or small etc.  Owing to the strong hydrogen-bonding properties of this double alcohol, this substance is miscible with water in all proportions, and contributes only a very small vapor pressure of its own.

Besides lowering the freezing point, antifreeze also raises the boiling point, increasing the operating range of the cooling system. The pure glycol freezes at — Assume that we use 1 L of glycol and 2 L of water the actual volumes do not matter as long as their ratios are as given.

The mass of the glycol will be 1. Any ionic species formed by dissociation will also contribute to the freezing point depression. This can serve as a useful means of determining the fraction of a solute that is dissociated.

If the solution was prepared by adding 0. The nominal molality of the solution is. The fraction of HNO2 that is dissociated is. You may already be familiar with the phase map for water, shown at the right.

For more on these, see here. The image shown below expands on this by plotting lines for both pure water and for its "diluted" state produced by the introduction of a non-volatile solute. The normal boiling point of the pure solvent is indicated by point where the vapor pressure curve intersects the 1-atm line — that is, where the escaping tendency of solvent molecules from the liquid is equivalent to 1 atmosphere pressure.

Addition of a non-volatile solute reduces the vapor pressures to the values given by the blue line. To understand freezing point depression, notice that the vapor pressure line intersects the curved black vapor pressure line of the solid ice atwhich corresponds to a new triple point at which all three phases ice, water vapor, and liquid water are in equilibrium and thus exhibit equal escaping tendencies.

Note that the above analysis assumes that the solute is soluble only in the liquid solvent, but does not remain in the frozen solvent. This is generally more or less true. For example, when arctic ice forms from seawater, the salts get mostly "squeezed" out.

This has the interesting effect of making the water that remains more saline, and hence more dense, causing it to sink to the bottom part of the ocean where it gets taken up by the south-flowing deep current.

Those readers who have some knowledge of thermodynamics will recognize that what we have been referring to as "escaping" tendency is really a manifestation of the Gibbs free energy.

The rule is that the phase with the most negative free energy rules! The phase that is most stable and which therefore is the only one that exists is always the one having the most negative free energy indicated here by the thicker portions of the plotted lines. The melting and boiling points correspond to the respective temperatures where the free energies of the solid and liquidand of the liquid and vapor are identical: The relationships shown in these plots depend on the differing slopes of the lines representing the free energies of the phases as the temperature changes.

These slopes are proportional to the entropy of each phase. Because gases have the highest entropies, the slope of the "gaseous solvent" line is much greater than that of the others. This principle is explained here. Note that this plot is not to scale. As we saw above, adding a solute to the liquid dilutes it, making its free energy more negative, with the result that the freezing and boiling points are shifted to the left and right, respectively.

The key role of the solvent concentration is obscured by the greatly-simplified expressions used to calculate the magnitude of these effects, in which only the solute concentration appears.

The details of how to carry out these calculations and the many important applications of colligative properties are covered elsewhere. Our purpose here is to offer a more complete explanation of why these phenomena occur.

## Gen Chem Discuss_Colligative Properties

Basically, these all result from the effect of dilution of the solvent on its entropy, and thus in the increase in the density of energy states of the system in the solution compared to that in the pure liquid.

Equilibrium between two phases liquid-gas for boiling and solid-liquid for freezing occurs when the energy states in each phase can be populated at equal densities. The temperatures at which this occurs are depicted by the shading. Dilution of the solvent adds new energy states to the liquid, but does not affect the vapor phase.

This raises the temperature required to make equal numbers of microstates accessible in the two phases. Dilution of the solvent adds new energy states to the liquid, but does not affect the solid phase. This reduces the temperature required to make equal numbers of states accessible in the two phases.

The pressure acts to compress the liquid very slightly, effectively narrowing the potential energy well in which the individual molecules reside and thus increasing their tendency to escape from the liquid phase.

In terms of the entropy, we can say that the applied pressure reduces the dimensions of the "box" within which the principal translational motions of the molecules are confined within the liquid, thus reducing the density of energy states in the liquid phase.Colligative Properties Problems - Answers Remember that Colligative properties depend only on the number or concentration of particles in a solution.

The properties are, for ideal solutions, independent of the kind or size of the particles, whether ionic, large or small etc. Colligative Properties- Page 1 Lecture 4: Colligative Properties • By definition a colligative property is a solution property (a property of mixtures) for which it is the amount of solute.

Jul 14,  · This chemistry review video tutorial focuses on the equations and formulas that you know regarding colligative properties of solutions such as boiling point. Since all of the colligative properties of solutions depend on the concentration of the solvent, their measurement can serve as a convenient experimental tool for determining the concentration, and thus the molecular weight, of a solute. Osmotic pressure is especially useful in this regard. The colligative properties that we will consider in this and the next unit apply to to solutions in which the solute is non-volatile; that is, it does not make a significant contribution to the overall vapor pressure of the solution.

• For colligative properties, the ratio of the number of solute particles to the number of solvent molecules in a solution can be related to various units for concentration of solutions such as molarity and molality.

SparkNotes: Colligative Properties of Solutions: Colligative Properties