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Unit 8: When Chemicals Meet Water—The Properties of Solutions

Section 9: Colligative Properties—Freezing and Boiling

Boiling Point Elevation

As mentioned earlier, vapor pressure increases as we raise the temperature. This means that we can compensate for the vapor pressure depression by increasing the temperature. For a particular decrease in vapor pressure due to the presence of dissolved solutes, there will be some increase in temperature that will raise the vapor pressure back up to what it was for the pure solvent. This is important for understanding the next colligative property, boiling point elevation.

Boiling Point Elevation

Figure 8-14. Boiling Point Elevation

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Boiling Point Elevation

Figure 8-14. Boiling Point Elevation

A solution has a higher boiling point than the corresponding pure solvent. Dissolving a solute in the solvent decreases the mole fraction of the solvent relative to what it would be if it were pure. This decreases the vapor pressure at any given temperature. Since the boiling point is the temperature at which the vapor pressure equals the atmospheric pressure, the solution must therefore be heated to a higher temperature to reach this equivalence. This is referred to as "boiling point elevation."

When we increase the temperature of a liquid, the vapor pressure above it increases. At some point, the vapor pressure of the liquid equals the pressure of the atmosphere above it. When this happens, the liquid stops merely evaporating from the surface of the liquid and begins to bubble vigorously and rapidly turn into the vapor phase. This is called "boiling," and the temperature at which it happens is called the "boiling point." As we saw in Unit 2, for pure water the boiling point occurs at 100°C.

Since decreasing the mole fraction of a solvent lowers the vapor pressure, salt water will not boil at 100°C. Because the mole fraction of the water is less than one, the vapor pressure of salt water at 100°C is lower than what it would be for pure water, and so it is less than atmospheric pressure.

We can compensate for this, though, by heating the water to a higher temperature until the vapor pressure increases to equal atmospheric pressure again. Salt water will boil at some temperature above 100°C; exactly how much above depends on how much we have lowered the mole fraction of the water (in other words, how much salt we have added). This phenomenon is called "boiling point elevation." (Figure 8-14)

Freezing Point Depression

While dissolving a solute in solvent increases the boiling point, dissolving a solute in solvent decreases the freezing point. As the mole fraction of solvent goes down, the freezing point of the solution goes down as well.

Freezing Point Depression

Figure 8-15. Freezing Point Depression

© Wikimedia Commons, Creative Commons License 2.0. Author: Michael Pereckas, 3 December 2006.

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Freezing Point Depression

Figure 8-15. Freezing Point Depression

Just as adding solute to a solvent raises the boiling point, it also lowers the freezing point. This means that solutions will melt at a colder temperature than the pure solvent. This is why putting a sand/salt mixture on roads prevents them from icing up.

This phenomenon is called "freezing point depression," and it is what makes salt water freeze at a lower temperature than pure water. This is why we salt roads in the winter: By lowering the freezing point of the water, any ice will melt at a higher temperature. (Figure 8-15)

For salt in water, freezing point depressions are more dramatic than are boiling point elevations. A 36 percent solution of sodium chloride (which is saturated) boils at about 109°C, an increase of only nine degrees from the boiling point of pure water (100°C). In contrast, this same saturated salt solution freezes at about -22°C, a decrease of more than 20 degrees from the freezing point of pure water (0°C).

Glossary

Boiling point

The temperature at which a liquid begins to boil, i.e., its phase changes from liquid to gas. At this temperature, the vapor pressure of the liquid is equal to the pressure of the atmosphere.

Boiling point elevation

The increase in boiling point of a solution due to an increased concentration of dissolved solute.

Freezing point depression

The decrease in freezing point of a solution due to an increased concentration of dissolved solute.

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