the boiling point of a substance which is solid at room temperature is 150° . what can be the melting point.
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Contents Home Campus Bookshelves College of Marin CHEM 114: Introductory Chemistry 13: Solutions Expand/collapse global location
13.9: Freezing Point Depression and Boiling Point Elevation- Making Water Freeze Colder and Boil Hotter
Last updatedMay 21, 2018
13.8: Solution Stoichiometry
13.10: Osmosis- Why Drinking Salt Water Causes Dehydration
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Chapter 1 • Chapter 2 • Chapter 3 • Chapter 4 • Chapter 5 • Chapter 6 • Chapter 7 • Chapter 8
Chapter 9 • Chapter 10 • Chapter 11 • Chapter 12 • Chapter 13 • Chapter 14
Learning Objectives
Explain what the term "colligative" means, and list the colligative properties.
Indicate what happens to the boiling point and the freezing point of a solvent when a solute is added to it.
Calculate boiling point elevations and freezing point depressions for a solution.
People who live in colder climates have seen trucks put salt on the roads when snow or ice is forecast. Why is this done? As a result of the information you explore in this section, you will understand why these events occur. You will also learn to calculate exactly how much of an effect a specific solute can have on the boiling point or freezing point of a solution.
The example given in the introduction is an example of a colligative property. Colligative properties are properties that differ based on the concentration of solute in a solvent, but not on the type of solute. What this means for the example above is that people in colder climates do not necessarily need salt to get the same effect on the roads—any solute will work. However, the higher the concentration of solute, the more these properties will change.
Boiling Point Elevation
Water boils at 100oC at 1atm of pressure, but a solution of saltwater does not . When table salt is added to water, the resulting solution has a higher boiling point than the water did by itself. The ions form an attraction with the solvent particles that prevents the water molecules from going into the gas phase. Therefore, the saltwater solution will not boil at 100oC . In order for the saltwater solution to boil, the temperature must be raised about 100oC . This is true for any solute added to a solvent; the boiling point will be higher than the boiling point of the pure solvent (without the solute). In other words, when anything is dissolved in water, the solution will boil at a higher temperature than pure water would.
The boiling point elevation due to the presence of a solute is also a colligative property. That is, the amount of change in the boiling point is related to the number of particles of solute in a solution and is not related to the chemical composition of the solute. A 0.20m solution of table salt and a 0.20m solution of hydrochloric acid would have the same effect on the boiling point.
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. A solution will have a lower freezing point than a pure solvent. 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. The solute-solvent interactions require the temperature to decrease further in order to solidify the solution. A common example is found when salt is used on icy roadways. Salt is put on 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 monopropylene glycol, is used to de-ice an aircraft. The aircrafts are sprayed with the solution when the temperature is predicted to drop below the freezing point. The freezing point depression is the difference in the freezing points of the solution from the pure solvent. This is true for any solute added to a solvent; the freezing point of the solution will be lower than the freezing point of the pure solvent (without the solute). Thus, when anything is dissolved in water, the solution will freeze at a lower temperature than pure water would.
The freezing point depression due to the presence of a solute is also a colligative property. That is, the amount of change in the freezing point is related to the number of particles of solute in a solution and is not related to the chemical composition of the solute. A 0.20m solution of table salt and a 0.20m solution of hydrochloric acid would have the same effect on the freezing point.
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Figure 13.9.1 : Comparison of boiling and freezing points of a pure liquid (right side) with a solution (left side).
Comparing the Freezing and