ChemBench

Boiling Point Elevation Calculator

Salt in the pot, a higher boiling point.

Van 't Hoff factor (i)

Ebullioscopic constant Kb (°C·kg/mol)

Molality (mol/kg)

Boiling point elevation

0.512 °C

What you entered

Van 't Hoff factor (i): 1Ebullioscopic constant Kb (°C·kg/mol): 0.512Molality (mol/kg): 1

1
ΔTb = i × Kb × m
1 × 0.512 × 1= 0.512 °C

Result

Boiling point elevation: 0.512 °C

This solution's boiling point rises by 0.512°C above pure solvent.

Boiling point elevation for solutions in water

Delta Tb equals i times Kb times molality. Water's Kb is 0.512 C per molal, and i is the number of particles each formula unit releases.

Solute	van't Hoff i	Molality	Elevation	Boils at
Glucose	1	1 m	0.512 C	100.512 C
Glucose	1	2 m	1.024 C	101.024 C
NaCl	2	0.5 m	0.512 C	100.512 C
NaCl	2	1 m	1.024 C	101.024 C
NaCl	2	2 m	2.048 C	102.048 C
CaCl2	3	1 m	1.536 C	101.536 C

Salting pasta water barely raises its boiling point: a heavily salted pot is under 0.1 molal, worth about 0.05 C. Salt is there for flavour, not for cooking speed.

Dissolved particles get in the way of boiling

Adding a solute lowers the solvent's vapor pressure at any given temperature, so the solution needs a higher temperature to reach the boiling point — more dissolved particles (higher molality × van 't Hoff factor) means a bigger elevation.

Why the van 't Hoff factor matters

A solute that splits into multiple ions in solution (like NaCl splitting into Na⁺ and Cl⁻) raises the boiling point roughly twice as much as a solute that stays as one particle, for the same molar amount — this is captured by the factor i.

Frequently asked questions

I dissolve 29.2 g of NaCl in 500 g of water — by how much does the boiling point rise?

Moles = 29.2/58.44 = 0.5 mol. Molality = 0.5/0.500 = 1.0 m. ΔTb = i × Kb × m = 2 × 0.512 × 1.0 = 1.024°C. The boiling point rises from 100°C to about 101°C.

How is boiling point elevation different from freezing point depression?

Both are colligative properties driven by the same solute-disruption principle. Boiling point goes up; freezing point goes down. The constants differ (Kb vs Kf), but the formula structure ΔT = i × K × m is identical. Use the freezing point depression calculator for the other direction.

Does adding salt to pasta water really make it boil faster?

No — it raises the boiling point slightly (about 0.5°C per tablespoon of salt per liter), meaning it actually takes marginally longer to boil. The amount used in cooking raises the temperature by less than 1°C.