Boiling Point Elevation

Learning Objectives

  • Define boiling point elevation.
  • Calculate the boiling point of a solution when given the molal boiling-point elevation constant.

Does adding salt help water boil?

Salt is often added to boiling water when preparing spaghetti or other pasta. One reason is to add flavor to the food. Some people believe that the addition of salt increases the boiling point of the water. Technically, they are correct, but the increase is rather small. You would need to add over 100 grams of NaCl to a liter of water to increase the boiling point a couple of degrees, which is just not healthy.

Boiling Point Elevation

The figure below shows the phase diagram of a solution and the effect that the lowered vapor pressure has on the boiling point of the solution compared to the solvent. In this case, the solution has a higher boiling point than the pure solvent. Since the vapor pressure of the solution is lower, more heat must be supplied to the solution to bring its vapor pressure up to the pressure of the external atmosphere. The boiling point elevation is the difference in temperature between the boiling point of the pure solvent and that of the solution. On the graph, the boiling point elevation is represented by Delta T_b .

The presence of solute causes an increase in boiling point by decreasing vapor pressure

Figure 1. The lowering of the vapor pressure in a solution causes the boiling point of the solution to be higher than that of the pure solvent. Figure from the CK-12 Foundation – Christopher Auyeung.

The magnitude of the boiling point elevation is also directly proportional to the molality of the solution. The equation is:

Delta T_b=K_b times m

The proportionality constant, K_b , is called the molal boiling-point elevation constant. It is a constant that is equal to the change in the boiling point for a 1-molal solution of a nonvolatile molecular solute. For water, the value of  K_b is 0.512°C/ m . So the boiling temperature of a 1-molal aqueous solution of any nonvolatile molecular solute is 100.512°C.

Summary

  • Boiling point elevation is defined.
  • Calculations involving the molal boiling point elevation constant are outlined.

Practice

Solve the problems at the link below:

http://home.comcast.net/~cochranjim/PDFS3/COLIGWS2A.pdf

Review

  1. Why does addition of a solute to water increase the boiling point?
  2. What does the molal boiling point elevation constant tell us?
  3. Addition of KCl raises the boiling point of 1.0 kg water by 1.014°C. How many moles of KCl were added?

 Glossary

  • boiling point elevation: The difference in temperature between the boiling point of the pure solvent and that of the solution.
  • molal boiling-point elevation constant: A constant that is equal to the change in the boiling point for a 1-molal solution of a nonvolatile molecular solute.