READ: Colligative Properties

READ: Colligative Properties

Colligative Properties

People who live in colder climates have seen the trucks put salt on the roads when snow or ice is forecast. Why do they do that? When planes fly in cold weather, the planes need to be de-iced before liftoff. How is that done? It turns out that pure solvents differ from solutions in their boiling points and freezing points when a solute is added. In this lesson, you will understand why these events occur. Boiling and freezing point changes are both examples of colligative properties (properties of solutions that are due only to the number of particles in solution and not to the chemical properties of the solute).

Boiling Point Elevation

At 1 atm of pressure, pure water boils at 100ºC, but salt water does not. When table salt is added to water, the resulting solution has a higher boiling point than water alone.

Essentially, the solute particles take up space in the solvent, physically blocking some of the more energetic water molecules from escaping into the gas phase. This is true for any solute added to a solvent. Boiling point elevation (increase in a solvent’s boiling temperature when a solute is added) is an example of a colligative property, meaning that the change in boiling point is related only to the number of solute particles in solution, regardless of what those particles are. A 0.20 m solution of table salt would have the same change in boiling point as a 0.20 m solution of KNO₃.

Freezing Point Depression

The effect of adding a solute to a solvent has the opposite effect on the freezing point of a solution as it does on the boiling point. Recall that the freezing point is the temperature at which the liquid changes to a solid. At a given temperature, if a substance is added to a solvent (such as water), the solute-solvent interactions prevent the solvent from going into the solid phase, requiring the temperature to decrease further before the solution will solidify. This is called freezing point depression (decrease in a solvent’s freezing temperature when a solute is added).

A common example is found when salt is used on icy roadways. Here the salt is put on the roads so that the water on the roads will not freeze at the normal 0ºC but at a lower temperature, as low as -9ºC. The de-icing of planes is another common example of freezing point depression in action. A number of solutions are used, but commonly a solution such as ethylene glycol or a less toxic propylene glycol is used to de-ice an aircraft. The aircraft are sprayed with the solution when the temperature is predicted to drop below the freezing point. The freezing point depression, then, is the difference between the freezing points of the solution and the pure solvent.

Remember that colligative properties are due to the number of solute particles in the solution. Adding 10 molecules of sugar to a solvent will produce 10 solute particles in the solution. However, when the solute is an ionic compound, such as NaCl, adding 10 molecules of solute to the solution will produce 20 ions (solute particles) in the solution. Therefore, adding enough NaCl solute to a solvent to produce a 0.20 m solution will have twice the effect of adding enough sugar to a solvent to produce a 0.20 m solution.

Colligative properties have practical applications, such as the salting of roads in cold-weather climates. By applying salt to an icy road, the melting point of the ice is decreased, and the ice will melt more quickly, making driving safer. Sodium chloride (NaCl) and either calcium chloride (CaCl₂) or magnesium chloride (MgCl₂) are used most frequently, either alone or in a mixture. Sodium chloride is the least expensive option, but is less effective because it only dissociates into two ions instead of three.

The van’t Hoff factor (usually abbreviated as ‘i’) is the number of particles that the solute will dissociate -(split apart)- into upon mixing with the solvent. For example, sodium chloride (NaCl) will dissociate into two ions, so the van’t Hoff factor for NaCl is i = 2. For lithium nitrate (LiNO₃), i = 2, and for calcium chloride (CaCl₂), i = 3. More ions will have a greater effect on colligative properties. In other words, the boiling point will increase more when there are more ions and the freezing point will decrease more when there are more ions.

Summary

• Colligative properties are properties that are due only to the number of particles in solution and not to the chemical properties of the solute.
• Boiling point elevation and freezing point depression are colligative properties.
• For electrolyte solutions, the van’t Hoff factor is added to account for the number of ions that the solute will dissociate into in solution.

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