Boiling point elevation is the increase in the boiling point of a solvent by dissolving a nonvolatile solute into it. For example, dissolving salt in water raises the boiling point of water so that it is higher than 100 °C. Like freezing point depression and osmotic pressure, boiling point elevation is a colligative property of matter. In other words, the effect depends on how many solute particles dissolve into the solvent and not on the nature of the solute.
How Boiling Point Elevation Works
Dissolving a solute in a solvent lowers the vapor pressure above the solvent. Boiling happens when the vapor pressure of the liquid equals the vapor pressure of the air above it. So, it takes more heat to give the molecules enough energy to transition from the liquid to vapor phase. In other words, boiling occurs at a higher temperature.
The reason this happens is because the solute particles are not volatile, so at any given time they are most likely in the liquid phase and not the gas phase. Boiling point elevation also occurs with volatile solvents, partly because the solute dilutes the solvent. The extra molecules affect interactions between solvent molecules.
While electrolytes have the largest effect on boiling point elevation, it occurs regardless of the nature of the solute. Electrolytes, like salts, acids, and bases, break into their ions in solution. The more particles added to the solvent, the greater the effect on boiling point. For example, sugar has less of an effect than salt (NaCl), which in turn has less of an effect than calcium chloride (CaCl2). Sugar dissolves but does not dissociate into ion. Salt breaks into two particles (Na+ and Cl–), while calcium chloride breaks into three particles (one Ca+ and two Cl–).
Similarly, a solution of higher concentration has a higher boiling point than one of lower concentration. For example, a 0.02 M NaCl solution has a higher boiling point than a 0.01 M NaCl solution.
Boiling Point Elevation Formula
The boiling point formula calculates the temperature difference between the normal boiling point of the solvent and the boiling point of the solution. The temperature difference is the boiling point elevation constant (Kb) or ebullioscopic constant, multiplied by the molal solute concentration. So, boiling point elevation is directly proportional to solute concentration.
ΔT = Kb · m
Another form of the boiling point formula uses the Clausius-Clapeyron equation and Raoult’s law:
ΔTb = molality * Kb * i
Here, i is the van’t Hoff factor. The van’t Hoff factor is the number of moles of particles in solution per mole of solute. For example, the van’t Hoff factor for sucrose in water is 1 because sugar dissolves, but doesn’t dissociate. The van’t Hoff factors for salt and calcium chloride in water are 2 and 3, respectively.
Note: The boiling point elevation formula only applies to dilute solutions! You can use it for concentrated solutions, but it only gives an approximate answer.
Boiling Point Elevation Constant
The boiling point elevation constant is a proportionality factors that is the change in boiling point for a 1 molal solution. Kb is a property of the solvent. Its value depends on temperature, so a table of values includes temperature. For example, here are some boiling point elevation constant values for common solvents:
|Solvent||Normal Boiling Point, oC||Kb, oC m-1|
Boiling Point Elevation Problem – Dissolving Salt in Water
For example, find the boiling point of a solution of 31.65 g of sodium chloride in 220.0 mL of water at 34 °C. Assume all of the salt dissolves. The density of water at 35 °C is 0.994 g/mL and Kb water is 0.51 °C kg/mol.
The first step is calculating the molality of the salt solution. From the periodic table, the atomic weight of sodium (Na) is 22.99, while the atomic weight of chlorine is 35.45. The formula of salt is NaCl, so its mass is 22.99 plus 35.45 or 58.44.
Next, determine how many moles of NaCl are present.
moles of NaCl = 31.65 g x 1 mol/(22.99 + 35.45)
moles of NaCl = 31.65 g x 1 mol/58.44 g
moles of NaCl = 0.542 mol
In most problems, you assume the density of water is essentially 1 g/ml. Then, the salt concentration is the number of moles divided by the number of liters of water (0.2200). But, in this example, the water temperature is high enough that its density is different.
kg water = density x volume
kg water = 0.994 g/mL x 220 mL x 1 kg/1000 g
kg water = 0.219 kg
mNaCl = moles of NaCl/kg water
mNaCl = 0.542 mol/0.219 kg
mNaCl = 2.477 mol/kg
Find the van’t Hoff Factor
For nonelectrolytes, the van’t Hoff factor is 1. For electrolytes, it is the number of particles that form when the solute dissociates in the solvent. Salt dissociates into two ions (Na+ and Cl–), so the van’t Hoff factor is 2.
Apply the Boiling Point Elevation Formula
The boiling point elevation formula tells you the temperature difference between the new and original boiling point.
ΔT = iKbm
ΔT = 2 x 0.51 °C kg/mol x 2.477 mol/kg
ΔT = 2.53 °C
Find the New Boiling Point
From the boiling point elevation formula, you know the new boiling point is 2.53 degrees higher than the boiling point of the pure solvent. The boiling point of water is 100 °C.
Solution boiling point = 100 °C + 2.53 °C
Solution boiling point = 102.53 °C
Note that adding salt to water does not change its boiling point a whole lot. If you want to raise the boiling point of water so food cooks faster, it takes so much salt that it makes the recipe inedible!
- Atkins, P. W. (1994). Physical Chemistry (4th ed.). Oxford: Oxford University Press. ISBN 0-19-269042-6.
- Laidler, K.J.; Meiser, J.L. (1982). Physical Chemistry. Benjamin/Cummings. ISBN 978-0618123414.
- McQuarrie, Donald; et al. (2011). “Colligative Properties of Solutions”. General Chemistry. University Science Books. ISBN 978-1-89138-960-3.
- Tro, Nivaldo J. (2018). Chemistry: Structure and Properties (2nd ed.). Pearson Education. ISBN 978-0-134-52822-9.