Boiling Point Elevation Calculator

Boiling Point Elevation Calculator

A boiling point elevation calculator finds how much higher a solution boils than the pure solvent it is made from. It uses the molality of the dissolved substance, the solvent's ebullioscopic constant, and the number of particles each solute unit breaks into in solution.

What is boiling point elevation?

Boiling point elevation is the rise in boiling temperature that happens when a solute is dissolved in a solvent. Pure water boils at 100 °C at normal atmospheric pressure. Water with salt or sugar dissolved in it boils a little higher.

The effect happens because dissolved particles get in the way of solvent molecules trying to escape into vapor. More thermal energy is needed to boil the liquid, so the boiling point goes up.

Boiling point elevation is a colligative property. That means it depends on how many solute particles are in the solution, not on what the particles are. A molecule of sugar and a molecule of salt raise the boiling point differently only because salt splits into more particles when it dissolves, not because the two substances are chemically different in some other relevant way.

How to calculate boiling point elevation

The formula is:

$\Delta T_b = i \times K_b \times m$

• ΔTb is the boiling point elevation, in °C.
• i is the van 't Hoff factor, the number of particles one unit of solute produces when it dissolves.
• Kb is the ebullioscopic constant of the solvent, in °C·kg/mol.
• m is the molality of the solution: moles of solute per kilogram of solvent, in mol/kg.

The new boiling point of the solution is the solvent's normal boiling point plus this elevation:

$T_b(\text{solution}) = T_b(\text{pure solvent}) + \Delta T_b$

The van 't Hoff factor

A solute that stays as whole molecules in solution, such as glucose or sucrose, has i = 1. An ionic compound that splits apart, such as table salt, has a higher i because each formula unit becomes more than one particle:

• i = 1: non-electrolytes, such as glucose and sucrose, which do not split apart.
• i = 2: NaCl and KCl, each splitting into two ions (for example Na⁺ and Cl⁻).
• i = 3: CaCl₂ and Na₂SO₄, each splitting into three ions.

These are ideal values that assume complete dissociation. Real solutions, especially concentrated ones, dissociate slightly less than this, so the measured elevation can be a little lower than the ideal formula predicts.

Ebullioscopic constants of common solvents

Each solvent has its own ebullioscopic constant and its own normal boiling point:

Worked examples

Example 1: sugar dissolved in water

Sugar (sucrose) does not split into ions in water, so i = 1.

• Solvent: water, Kb = 0.512 °C·kg/mol, normal boiling point 100 °C
• Molality: 1.5 mol/kg
• van 't Hoff factor: i = 1

$\Delta T_b = 1 \times 0.512 \times 1.5 = 0.768\ °C$

The solution boils at 100 + 0.768 = 100.768 °C.

Example 2: table salt dissolved in water

Table salt (NaCl) splits into a sodium ion and a chloride ion, so i = 2.

• Solvent: water, Kb = 0.512 °C·kg/mol, normal boiling point 100 °C
• Molality: 1.5 mol/kg
• van 't Hoff factor: i = 2

$\Delta T_b = 2 \times 0.512 \times 1.5 = 1.536\ °C$

The solution boils at 100 + 1.536 = 101.536 °C. At this same molality, salt raises the boiling point exactly twice as much as a non-electrolyte would, because each dissolved unit produces two particles instead of one.

How to use the calculator

1. Enter the molality of the solution, in mol/kg.
2. Enter the ebullioscopic constant of the solvent, or pick a common solvent from the dropdown to fill it in automatically.
3. Enter the van 't Hoff factor: 1 for a molecular solute, 2 for a salt like NaCl, 3 for a salt like CaCl₂.
4. Read the boiling point elevation and the resulting boiling point of the solution.

If molality is zero, the elevation is zero and the solution boils at the solvent's normal boiling point. Negative values are not accepted, since molality, the ebullioscopic constant, and the particle count cannot be negative.

Why boiling point elevation matters

• Cooking. Salted water boils a little higher than plain water, though the change is too small to noticeably affect cooking speed on its own.
• Distillation and chemical engineering. Knowing how solutes shift boiling points helps in designing separation processes.
• Molecular weight determination. Measuring the boiling point elevation of a solution with a known mass of solute can be used to estimate the solute's molar mass, a method called ebullioscopy.
• Antifreeze and coolant systems. The same colligative principle that raises boiling points also lowers freezing points, which is why coolant mixtures use dissolved solutes to widen the liquid range of the fluid.

Related colligative properties

Boiling point elevation is one of several colligative properties:

• Freezing point depression: ΔTf = i × Kf × m, where Kf is the cryoscopic constant. This is why salt is spread on icy roads.
• Vapor pressure lowering, described by Raoult's law.
• Osmotic pressure, which drives water movement across membranes in biological cells.

Frequently asked questions

What is boiling point elevation? It is the rise in a liquid's boiling temperature that happens when a solute is dissolved in it. It is a colligative property, so it depends on the number of dissolved particles rather than what they are made of.

What is the formula for boiling point elevation? ΔTb = i × Kb × m, where i is the van 't Hoff factor, Kb is the solvent's ebullioscopic constant, and m is the molality of the solution.

What is the van 't Hoff factor? It is the number of particles one formula unit of solute produces when it dissolves. Molecular solutes like sugar have i = 1. Salts that split into ions have higher values: i = 2 for NaCl, i = 3 for CaCl₂.

Why does salt raise the boiling point of water more than sugar does? At the same molality, NaCl produces twice as many dissolved particles as sugar because it splits into two ions. Since boiling point elevation depends on particle count, salt's effect is about twice as large.

Does boiling point elevation depend on which solute is used? Only through the number of particles it produces. Two different non-electrolytes at the same molality raise the boiling point by the same amount.

Can boiling point elevation be negative? No, for a non-volatile solute the boiling point always rises, never falls. A solute that is itself volatile behaves differently and is not covered by this formula.