Boiling Point Elevation Calculator

The Boiling Point Elevation Calculator works out how far a dissolved solute raises a solvent's boiling point, using the colligative formula ΔT_b = i × K_b × m. Pick a solvent to pull in its real ebullioscopic constant and boiling point, enter the molality (directly or from mass), set the van't Hoff factor for ionic solutes, and the tool returns the boiling point elevation, the new boiling point of the solution, an animated thermometer, a salt-to-boil reality check, and a full step-by-step walkthrough.

What is Boiling Point Elevation?

Boiling point elevation is the rise in a solvent's boiling point that occurs when a solute is dissolved in it. It is one of the four colligative properties — properties that depend on the number of dissolved particles rather than what those particles are. Dissolved particles lower the solvent's vapor pressure, so the solution must be heated to a higher temperature before its vapor pressure matches the surrounding atmosphere and it boils. This is why a salty or sugary solution boils a little above 100 °C and why antifreeze raises an engine coolant's boiling point in summer.

Boiling Point Elevation Formula

The calculation takes two short steps: find the elevation, then add it to the pure solvent's boiling point.

Where:

• ΔT_b — the boiling point elevation (°C)
• i — the van't Hoff factor, the number of particles each formula unit produces
• K_b — the ebullioscopic (molal boiling point elevation) constant of the solvent (°C·kg/mol)
• m — the molality of the solution (mol solute per kg solvent)
• T_b^° — the boiling point of the pure solvent (°C)

Does Adding Salt Make Water Boil Hotter? (The Pasta Myth)

A favorite kitchen myth says salting pasta water makes it boil hotter and cook food faster. The math says otherwise. Water's ebullioscopic constant K_b is only 0.512 °C·kg/mol, and salt (NaCl) gives i = 2, so to raise water's boiling point by a single degree you need roughly 57 grams of salt per litre — about three tablespoons. The pinch most cooks add raises the boiling point by only a few hundredths of a degree, which has no measurable effect on cooking time. Salt is added for flavor, not for heat. This calculator's built-in salt-to-boil reality check shows the exact salt equivalent for any solution you enter.

The van't Hoff Factor (i)

Because boiling point elevation depends on the number of dissolved particles, an ionic compound that splits into several ions raises the boiling point much more than the same molality of a non-electrolyte. The van't Hoff factor (i) captures this multiplier.

Ebullioscopic Constants (Kb) of Common Solvents

Each solvent has its own K_b. The calculator's solvent menu fills these in automatically, but you can always enter your own.

Notice that for water K_b (0.512) is much smaller than K_f (1.86) — a given molality lowers water's freezing point about 3.6× more than it raises its boiling point.

Worked Example

Dissolve 1 mol of NaCl in 1 kg of water (molality = 1 mol/kg). NaCl dissociates into Na⁺ and Cl⁻, so i = 2, and water's K_b = 0.512 °C·kg/mol:

The salt water now boils at about 101.02 °C instead of 100 °C — and that already needs a full tablespoon of salt per litre.

Molality vs Molarity

Colligative formulas use molality (moles of solute per kilogram of solvent), not molarity (moles per liter of solution). Mass does not change with temperature, but volume does — so molality keeps the result temperature-independent, which matters when you are deliberately heating a solution to its boiling point.

How to Use This Calculator

1. Choose your solvent: Selecting one auto-fills its ebullioscopic constant K_b and normal boiling point. Choose "Custom solvent" to enter your own.
2. Enter the molality: Type it directly, or switch to "Calculate molality from mass" and enter the solute mass, molar mass, and solvent mass.
3. Set the van't Hoff factor: Pick the solute type to set i automatically, or enter a measured value for real (non-ideal) solutions.
4. Click Calculate: Review the boiling point elevation, the new boiling point, the animated thermometer, the salt-to-boil reality check, the dissociation diagram, and the step-by-step math.

Frequently Asked Questions

What is boiling point elevation?

Boiling point elevation is the raising of a solvent's boiling point when a solute is dissolved in it. It is a colligative property, meaning it depends on the number of dissolved particles, not their chemical identity. Adding salt or sugar to water, for example, raises water's boiling point above 100 °C.

What is the formula for boiling point elevation?

The formula is ΔT_b = i × K_b × m, where i is the van't Hoff factor, K_b is the ebullioscopic constant of the solvent, and m is the molality. The new boiling point is the pure solvent's boiling point plus ΔT_b.

Does adding salt make water boil hotter?

Yes, but only by a tiny amount. To raise water's boiling point by just 1 °C you would need about 57 g of table salt per litre — roughly three tablespoons. The pinch cooks usually add raises the boiling point by a few hundredths of a degree, so it has no meaningful effect on cooking time. Salt is added for flavor, not heat.

What is the van't Hoff factor?

The van't Hoff factor (i) is the number of particles a solute produces when it dissolves. Non-electrolytes such as sugar give i = 1; NaCl gives i = 2; CaCl₂ gives i = 3; AlCl₃ gives i = 4. Real values are slightly lower than these ideals because of ion pairing.

What is the ebullioscopic constant Kb?

K_b, the molal boiling point elevation constant, is a property of the solvent that gives the boiling point rise per unit of molality. For water it is 0.512 °C·kg/mol; for benzene 2.53; for acetic acid 3.07. It is generally smaller than the same solvent's freezing point constant K_f.

How is molality different from molarity?

Molality is moles of solute per kilogram of solvent, while molarity is moles per liter of solution. Colligative formulas use molality because mass — unlike volume — does not change with temperature.

Additional Resources

• Boiling-point elevation - Wikipedia
• Colligative properties - Wikipedia
• van't Hoff factor - Wikipedia