Boiling Point Elevation: Videos & Practice Problems

Boiling point elevation occurs when a solute is added to a solvent, resulting in a higher boiling point for the solution compared to the pure solvent's normal boiling point. The change in boiling point, denoted as ΔTb, can be calculated using the formula:

ΔTb=i×Kb×m where 'i' represents the Van't Hoff factor, 'Kb' is the boiling point elevation constant of the solvent (in °C/m), and 'm' is the molality of the solution (moles of solute per kilogram of solvent). The boiling point of the solution is then found by adding ΔTb to the normal boiling point of the solvent. Common solvents like water, benzene, chloroform, and ethanol have specific Kb values provided for these calculations. Notably, covalent, non-volatile, or non-ionic compounds have a Van't Hoff factor of 1. Understanding these concepts is crucial for predicting the boiling point of a solution after solute addition.

Boiling Point Elevation

Here we're going to say the phenomenon when adding a solute to a pure solvent results in increasing of the boiling point of the solvent. Remember the more solute you add, the higher your boiling point will be. Now with this idea of boiling point, we have what's called the normal boiling point and then the boiling point of the solution that's created.

We're going to say normal boiling point, which is BP and we're going to say here of the solvent. This is the boiling point of the pure solvent before the addition of solute. So before I've added any solute, this is basically the boiling point of the pure solvent. Now boiling point of solution, we're going to say BP solution. This is the boiling point of the solvent after the addition of the solute. Remember solution is when we add solute to a solvent.

Now if we take a look at boiling point elevation, remember it goes up as we add more solute. We have 4 areas that we need to focus on. The first area deals with the boiling point elevation formula. Here we're going to say ΔT_B. So the change in our boiling point equals I times K_B times lowercase m. I is related to our van't Hoff factor, so these are our variables.

We're going to say here that K_B is the boiling point constant of our pure solvent in units of degree Celsius over molality and then lowercase m is molality of the solution in moles of solute per kilogram of solvent. Once we've used this boiling point elevation formula and know its different variables, we then configure out the boiling point of our solution. Here we're going to say the boiling point of solution equals the boiling point of our pure solvent. So this is our normal boiling point plus ΔT_B.

Now the solvents that are customarily used in questions like this are water, benzene, chloroform, and ethanol. Here are their normal boiling points. If we add solute to them, we'd expect their boiling points to be higher than these that are reported here. Their K_B values, their boiling point constants are these values respectively: 0.51, 2.53, 3.60, and 1.20. No, you don't need to memorize these numbers. These are just different types of solvent that you might see pop up in a question dealing with the colligative properties.

Now one last thing, recall if a compound is covalent, nonvolatile, or nonionic, meaning non-electrolyte, then its van't Hoff factor is going to be equal to 1. And so just keep in mind when it comes to boiling point elevation, these are the key areas you need to pay attention to in order to find the boiling point of a solution.

Boiling Point Calculation Example

Calculate the boiling point of a 3.71 mol aqueous calcium bromide solution. All right, so they want us to find our boiling point. So remember, your boiling point of solution equals the boiling point of your pure solvent plus the change in the boiling point.

So the boiling point of our pure salt of our pure solvent. What exactly is our solvent here? Well, here they're saying aqueous. When we say aqueous that means the solvent is water. Pure water boils at 100℃. All we have to do now is figure out what our new changing boiling point will be.

So we're going to say ΔT_B equals I times K_B times M. Remember, I is the number of ions your solute will dissolve into when placed in a solvent. Here, calcium bromide is ionic. It breaks up into a calcium ion +2 bromide ions, for a total of 3 ions. The solvent is water. The boiling point constant of water is oh, .51℃ over molality.

Then finally we're going to say our molality has given us given to us as 3.71 mole. So molalodies cancel out and I'll have my answer here as degrees Celsius, which comes out to 5.68℃. So we take that and we plug it here. So when we add those together, our new temperature is 105.68℃ Celsius.

Remember, it's called boiling point elevation. We go up as we add solute to our pure solvent. Here pure water was at 100℃. After the addition of calcium bromide, it's now at 105.68℃.

An ethylene glycol solution contains 25.2 g of ethylene glycol (C₂H₆O₂) in 99.5 mL of water. Determine the change in boiling point. Assume a density of 1.00 g/mL for water.

• A. 18.4°C
• B. 9.22°C
• C. 2.08°C
• D. 0.572°C

Answer: (C)

Pure water boils at 100°C. What is the new boiling point of water after the addition of 13.12 g aluminum chloride, AlCl₃, to 615 g water?  

• A. 100.25°C
• B. 100.08°C
• C. 100.33°C
• D. 100.16°C

Answer: (C)

What is the molality of glucose in an aqueous solution if the boiling point of the solution is 103.15°C?

• A. 0.15 m
• B. 0.29 m
• C. 6.18 m
• D. 1.6 m

Answer: (C)

Carbon dioxide is dissolved in 722 mL of benzene with a density of 1.59 g/mL. What mass of carbon dioxide would you add to make the boiling point of the solution 104.7°C?

• A. 11 g
• B. 500 g
• C. 22.9 g
• D. 491 g

Answer: (D)

Here’s what students ask on this topic:

What are some practical applications of boiling point elevation?

Boiling point elevation, a colligative property, has several practical applications across different industries and everyday life. In cooking, adding salt to water increases its boiling point, allowing food to cook at a higher temperature, which can alter cooking times and results. This principle is also used in candy making, where sugar syrups are boiled to specific temperatures to achieve different textures.

In colder climates, boiling point elevation is utilized for road safety. Salts or chemicals are spread on icy roads to lower the freezing point of water, effectively melting ice and snow, which prevents re-freezing and makes roads safer for travel.

In the automotive industry, antifreeze, a solution with a higher boiling point than pure water, is used in car radiators. This prevents the coolant from boiling over in hot weather and from freezing in cold temperatures, ensuring the engine operates within an optimal temperature range.

In industrial processes, boiling point elevation is important in the design of heat exchangers and other equipment where control over phase changes of liquids is crucial for efficiency and safety.

How do you calculate boiling point elevation?

To calculate boiling point elevation, you'll use the formula:

ΔT b = i ⋅ K b ⋅ m where ΔT_b is the boiling point elevation, i is the van't Hoff factor (which represents the number of particles the solute splits into or generates in solution), K_b is the ebullioscopic constant (a property specific to the solvent), and m is the molal concentration of the solute in the solution.

Here's a step-by-step process:

1. Determine the molal concentration (m) of the solute in the solution, which is the number of moles of solute per kilogram of solvent.
2. Find the ebullioscopic constant (K_b) for the solvent, which is usually provided in textbooks or reference materials.
3. Calculate the van't Hoff factor (i) for the solute, which is the number of ions into which a compound dissociates in solution. For non-electrolytes, i is typically 1.
4. Multiply these three values together to find the boiling point elevation (ΔT_b).

Finally, add the boiling point elevation (ΔT_b) to the pure solvent's normal boiling point to get the new boiling point of the solution.

What is boiling point elevation?

Boiling point elevation is a phenomenon where the boiling point of a liquid (a solvent) increases when a solute is dissolved in it. This occurs because the presence of the solute particles makes it more difficult for the solvent molecules to escape into the gas phase, which is necessary for boiling to occur. The added particles disrupt the solvent's natural tendency to vaporize by creating additional intermolecular forces that the molecules must overcome to transition into the gas phase.

To boil the solvent, you now need to apply more heat to give the molecules enough energy to break these interactions. The amount of boiling point elevation is directly proportional to the molal concentration of the solute particles in the solution. This relationship is described by the boiling point elevation equation:

ΔT_b = iK_bm

where ΔT_b is the change in boiling point, K_b is the ebullioscopic constant (a property of the solvent), m is the molal concentration of the solution, and i is the van 't Hoff factor, which accounts for the number of particles the solute breaks into or forms in the solution.