Boiling Point Elevation — Overview & Examples

We've seen that adding a solute lowers a solvent's freezing temperature. We called it freezing point depression. What about the boiling point? It turns out that the boiling point increases. That's boiling point elevation.

Both freezing point depression and boiling point elevation are examples of colligative properties. Colligative properties are physical properties that change because of solute addition. But, the solute's identity does not matter! We only care about the amount. So, they are extensive properties.

Only the amount? Really!? For colligative properties, we make an assumption. We assume the solute is not volatile. Typically we are talking about adding ionic salts to water. The salt's vaporization point is well above (about $1300^{o}C$) the water's. So, only the water vaporizes. Our assumption is valid.

What Causes Boiling Point Elevation?

What causes boiling point elevation? Entropy! Remember, entropy is a measure of energy dispersal. Solute addition increases the variety of Van Der Waal's forces. We have more electrostatic interactions. So, our energy is more spread out. Higher entropy means our system is more stable.

We can also consider the vapor pressure. Remember, boiling happens when vapor pressure equals atmospheric pressure. By adding a solute, we lowered the vapor pressure of the solvent. On the surface, some solute molecules replace the solvent. But, the solutes only vaporize at very high temperatures. So, there are fewer surface molecules that can vaporize.

What about the surface tension? Let's say our solvent is water. It's the most common! Let's say our solute is an ionic salt. It strengthens the surface tension! That's because ion-dipole interactions have a larger bond energy than hydrogen bonds. A greater surface tension means lower vapor pressure.

Boiling Point Elevation Formula

The boiling point elevation formula is similar to freezing point depression. The equation is $\Delta T_{B}=K_{B}\times m$.

• $\Delta T_{B}$ is the change in the boiling point.

• $K_{B}$ is the boiling point elevation constant.

• Its units are $\frac{^{o}C}{m}$.

• The constant is unique for each solvent. The most common solvent is water. Its $K_{B}=0.51\frac{^{o}C}{m}$.

• $m$ is the solution's molality.

• Remember molality's units are $\frac{mol_{solute}}{kg_{solvent}}$.

Boiling Point Elevation Practice Problem

Let's return to our antifreeze problem. Our cars also use boiling point elevation. Remember, the antifreeze circulates through the coolant system. The engine transfers heat to the antifreeze. So, your car's engine stays at safe operating temperatures. If your coolant started to boil, your engine could overheat. Let's look at an example problem.

Again, we'll say we mixed one gallon of antifreeze solution (about 3.8 liters). The ratio of ethylene glycol to water is 1:1. The density of ethylene glycol is $0.7857\frac{g}{ml}$. Its molar mass is $62.1\frac{g}{mol}$. Calculate the boiling point elevation of water.

Step 1: Find the kilograms of water and moles of ethylene glycol. Use dimensional analysis.

• $3.8 L_{solution} \times \frac{1 L_{ethylene\,glycol}}{2L_{solution}} \times \frac{1000.0 ml}{L} \times \frac{0.7857g}{ml} \times \frac{1mol}{62.1g}=24.0 mols_{ethlyene\,glycol}$

• $3.8 L_{solution}\times \frac{1.0 L_{water}}{2.0 L_{solution}} \times\frac{1.0kg}{L}=1.9\,kg_{water}$

Step 2: Calculate the molality.

• $m=\frac{mol_{solute}}{kg_{solvent}}=\frac{24.0mols_{ethylene\,glycol}}{1.9kg_{water}}= 12.6 molal$

Step 3: Calculate the freezing point depression.

• $\Delta T_{B}=K_{B}\times m \rightarrow 0.51\frac{^{o}C}{m}\times 12.6m=6.4^{o}C$

• We raised the boiling point by $6.4^{o}C$. So the new boiling point would be $106.4^{o}C$ or $223.5^{o}F$.

• Most engines run between $90^{o}C$ and $104^{o}C$. That's $195^{o}F$ and $220^{o}F$. If you added less ethylene glycol, your car could overheat!

(Video) Boiling Point Elevation With Example Problem

by Denovo Tutor

In this video, Denovo Tutor explains boiling point elevation. He starts with a quick description of vapor pressure, atmospheric pressure, and boiling. Next, he explains why adding a solvent affects the vapor pressure. Finally, he demonstrates the boiling point elevation equation and shows an example problem. Do you need help with boiling point elevation? This video will explain the concept and the math!