Boiling Point Elevation Calculator
Solve ΔTb = i·Kb·m for the boiling point elevation caused by dissolving a solute in a solvent — a colligative property that depends only on how many particles are dissolved, not what they are. Two 3D diagrams compare pure solvent to a solution at the molecular level, and charts show how elevation scales with molality and with the van't Hoff factor.
Reviewed by the ToolNestr Editorial Team — July 2026
Pure solvent vs solution
1. Pure solvent
Only solvent molecules (blue) — nothing obstructs their escape into vapor, so boiling starts at the normal boiling point.
2. Solution with dissolved solute
Solute particles (amber) mixed among the solvent — extra heat is needed to reach the same escaping tendency, raising the boiling point.
Boiling point elevation graphs
How it works
The core idea in one line: dissolved solute particles get in the way of solvent molecules escaping into vapor, so a solution always needs a bit more heat than the pure solvent to reach its boiling point — and the effect scales directly with how many particles are actually dissolved.
ΔTb = i·Kb·m
boiling point elevation — i = van't Hoff factor, Kb = ebullioscopic constant, m = molality (mol/kg)
BP_new = BP_pure + ΔTb
the solution's new boiling point
Boiling occurs when a liquid's vapor pressure equals atmospheric pressure. Dissolved solute particles lower the solvent's vapor pressure at any given temperature (by simple dilution of the solvent at the surface), so a higher temperature is needed to reach the same vapor pressure — this vapor-pressure-lowering effect is what ΔTb=i·Kb·m quantifies directly. Because it depends only on particle count, a solute that dissociates into more ions (higher i) produces proportionally more elevation at the same molal concentration.
Worked example 1 — table salt (NaCl) in water
Given: 2 mol of NaCl dissolved in 1 kg of water (m = 2 mol/kg). NaCl dissociates into 2 ions (i=2). Water's Kb = 0.512 °C·kg/mol.
Because NaCl splits into two ions per formula unit, it raises the boiling point twice as much as a nonelectrolyte at the same molality would.
Worked example 2 — glucose (a nonelectrolyte) in water
Given: 1 mol of glucose dissolved in 0.5 kg of water (m = 2 mol/kg). Glucose does not dissociate (i=1). Water's Kb = 0.512 °C·kg/mol.
Same molality as the NaCl example, but exactly half the boiling point elevation — because glucose contributes only one particle per formula unit instead of two.
How the van't Hoff factor changes boiling point elevation
Same molality (2 mol/kg) and solvent (water, Kb=0.512) — only the number of dissolved particles per formula unit changes.
| Solute | i | ΔTb |
|---|---|---|
| Glucose (nonelectrolyte) ★ | 1 | 1.024°C |
| NaCl | 2 | 2.048°C |
| CaCl₂ | 3 | 3.072°C |
| AlCl₃ | 4 | 4.096°C |
★ Reference row (worked example 2). Elevation scales directly with i — a solute that splits into more ions raises the boiling point proportionally more, at the same molality.
Where boiling point elevation actually matters
🍝 Cooking — salting pasta water
Adding salt to boiling water does technically raise its boiling point, though the effect at typical cooking concentrations is small — usually less than 1°C, more relevant to seasoning than to cooking speed.
🚗 Engine coolant formulation
Antifreeze/coolant mixtures use boiling point elevation deliberately, raising the boiling point of the coolant so engines can run hotter without the coolant boiling over.
🧪 Determining unknown molar masses
Boiling point elevation measurements can be used in reverse — measuring ΔTb for a solution of known mass lets chemists calculate the molar mass of an unknown solute.
🏭 Industrial solution processing
Chemical and food processing industries account for boiling point elevation when concentrating solutions by evaporation, since a more concentrated solution boils at a progressively higher temperature.
Common misconceptions
"Boiling point elevation depends on what the solute actually is, chemically."
It's a colligative property — it depends only on how many particles are dissolved (accounting for dissociation via i), not on the solute's chemical identity. Two different solutes at the same effective particle concentration raise the boiling point by the same amount.
"Any salt added to water doubles the boiling point elevation compared to sugar."
It depends on exactly how many ions the salt dissociates into — NaCl (i=2) does double the effect of a nonelectrolyte like sugar, but CaCl₂ (i=3) triples it, and not all "salts" behave the same way.
"Boiling point elevation and freezing point depression use the same constant."
They use different solvent-specific constants — Kb (ebullioscopic constant) for boiling point elevation and Kf (cryoscopic constant) for freezing point depression — and for water these have quite different values (0.512 vs 1.86 °C·kg/mol).
"Molarity and molality give the same result in this formula."
The formula specifically requires molality (moles per kg of solvent), not molarity (moles per litre of solution) — molality is used because it doesn't change with temperature, while molarity does as the solution's volume expands when heated.
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, Chemistry 2e — Chapter 11, "Solutions and Colloids" (free, peer-reviewed). openstax.org
- • Brown, LeMay & Bursten, Chemistry: The Central Science — Chapter 13, Properties of Solutions.
- • Zumdahl & Zumdahl, Chemistry — Colligative properties of solutions.
ΔTb = i·Kb·m. Water Kb = 0.512 °C·kg/mol (standard reference value). Results are rounded for display.
How to use this calculator
Enter the van't Hoff factor
Use i=1 for molecular solutes, i=2/3/4 for salts based on how many ions they dissociate into.
Enter Kb and molality
Default Kb=0.512 for water — change it for other solvents. Provide the molality in mol/kg.
Read the result
Boiling point elevation and the new boiling point solve instantly.
Related tools
Frequently asked questions
What is boiling point elevation?
Boiling point elevation is the increase in a solvent's boiling point caused by dissolving a non-volatile solute in it. The formula is ΔTb = i·Kb·m, where i is the van't Hoff factor, Kb is the ebullioscopic constant, and m is molality.
What is the van't Hoff factor (i)?
The van't Hoff factor is the number of particles a solute dissociates into in solution. Nonelectrolytes like glucose have i=1 (they don't dissociate), NaCl has i=2 (splits into Na⁺ and Cl⁻), and CaCl₂ has i=3 (one Ca²⁺ plus two Cl⁻).
What is the ebullioscopic constant (Kb)?
Kb is a property of the solvent itself, describing how much the boiling point rises per unit of molality. For water, Kb = 0.512 °C·kg/mol — different solvents have very different Kb values.
Why does boiling point elevation depend on particle count, not identity?
Boiling point elevation is a colligative property — it depends only on the number of dissolved particles, not their chemical identity. Two solutions with the same total particle concentration raise the boiling point by the same amount, regardless of what the solute actually is.
Does concentration or molality matter more for this calculation?
Molality (moles of solute per kg of solvent) is used specifically because it doesn't change with temperature, unlike molarity (which is volume-based and shifts as the solution expands or contracts with heat).