Boiling Point Elevation Calculator
Boiling Point Elevation (°C)
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New Boiling Point (°C)
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How Boiling Point Elevation Works
Boiling point elevation is a colligative property of solutions, meaning it depends only on the number of dissolved solute particles, not on their chemical identity. When a non-volatile solute is dissolved in a solvent, it raises the solvent's boiling point by interfering with the escape of solvent molecules from the liquid phase. According to the American Chemical Society (ACS), this phenomenon is fundamental to physical chemistry and has practical applications ranging from cooking to automotive coolant systems and industrial chemical processes.
The magnitude of boiling point elevation is governed by the formula ΔTb = i × Kb × m, where i is the van't Hoff factor, Kb is the ebullioscopic constant of the solvent, and m is the molality of the solution. For water, Kb = 0.512 °C·kg/mol, meaning each mole of non-electrolyte solute per kilogram of water raises the boiling point by 0.512°C. This calculator computes the elevation and new boiling point for any solvent-solute combination. Related chemistry tools include our molarity calculator and dilution calculator.
The Boiling Point Elevation Formula
The boiling point elevation equation, as defined in standard chemistry textbooks and referenced by LibreTexts Chemistry:
ΔTb = i × Kb × m
- ΔTb — boiling point elevation in °C
- i — van't Hoff factor (number of particles the solute dissociates into)
- Kb — ebullioscopic constant of the solvent (°C·kg/mol)
- m — molality of the solution (moles of solute per kg of solvent)
Worked example: Dissolving 1 mole of NaCl (i = 2) in 1 kg of water (Kb = 0.512): ΔTb = 2 × 0.512 × 1 = 1.024°C. New boiling point = 100 + 1.024 = 101.024°C.
Key Terms You Should Know
- Colligative Property — a property of a solution that depends on the number (not identity) of solute particles. Boiling point elevation, freezing point depression, osmotic pressure, and vapor pressure lowering are all colligative properties.
- Van't Hoff Factor (i) — the number of particles a solute produces in solution. NaCl: i = 2 (Na⁺ + Cl⁻). Glucose: i = 1 (does not dissociate). CaCl₂: i = 3 (Ca²⁺ + 2Cl⁻).
- Ebullioscopic Constant (Kb) — a solvent-specific constant representing the boiling point elevation per unit molality. Water: 0.512. Benzene: 2.53. Chloroform: 3.63.
- Molality (m) — concentration expressed as moles of solute per kilogram of solvent. Unlike molarity, molality is independent of temperature because it is based on mass, not volume.
- Non-volatile Solute — a solute with negligible vapor pressure at the boiling point of the solution. The boiling point elevation formula applies specifically to non-volatile solutes.
Ebullioscopic Constants for Common Solvents
Different solvents have different ebullioscopic constants, which determine how much their boiling point rises per unit of dissolved solute. Water has a relatively low Kb, which is why adding salt to cooking water has minimal effect on boiling point. Solvents like camphor (Kb = 5.95) are used in molecular weight determination because their large Kb produces a measurable temperature change even with small solute amounts.
| Solvent | Normal BP (°C) | Kb (°C·kg/mol) |
|---|---|---|
| Water | 100.0 | 0.512 |
| Benzene | 80.1 | 2.53 |
| Chloroform | 61.2 | 3.63 |
| Acetic Acid | 118.1 | 3.07 |
| Ethanol | 78.4 | 1.22 |
| Camphor | 204.0 | 5.95 |
Practical Examples
Example 1 — Cooking with salt: Adding 1 tablespoon of salt (~17 g NaCl, 0.29 mol) to 4 liters (4 kg) of water creates a 0.073 molal solution. ΔTb = 2 × 0.512 × 0.073 = 0.075°C. The boiling point rises from 100.000°C to just 100.075°C — essentially imperceptible. Salt is added to pasta water for flavor, not to meaningfully change the boiling temperature.
Example 2 — Automotive coolant: A 50/50 mix of ethylene glycol (MW 62.07, i = 1) and water creates a roughly 8 molal solution. ΔTb = 1 × 0.512 × 8 = 4.1°C elevation, raising the boiling point to about 104.1°C at atmospheric pressure. Under a 15 psi radiator cap (which raises the boiling point further by about 25°C), the coolant boils at approximately 129°C — well above normal engine operating temperatures of 90-105°C.
Example 3 — Sugar syrup: A candy-making simple syrup at 2 molal sucrose (i = 1): ΔTb = 1 × 0.512 × 2 = 1.024°C. At higher concentrations used in candy making (10+ molal), the boiling point can rise by 5°C or more, which is why candy thermometers are essential. Calculate solution concentrations with our mole calculator.
Tips for Solving Boiling Point Elevation Problems
- Always identify the van't Hoff factor first. Molecular compounds (glucose, sucrose, urea) have i = 1. Strong electrolytes dissociate fully: NaCl (i = 2), CaCl₂ (i = 3), AlCl₃ (i = 4). Weak electrolytes have i values between 1 and their theoretical maximum.
- Use molality, not molarity. Molality (moles per kg solvent) is temperature-independent, unlike molarity (moles per liter solution). This distinction matters for accurate calculations at elevated temperatures.
- Account for ion pairing in concentrated solutions. At high concentrations, the effective van't Hoff factor is lower than the theoretical value due to ion pairing. A 1 molal NaCl solution has an effective i of about 1.87, not exactly 2.
- Remember the formula applies to non-volatile solutes only. If the solute has significant vapor pressure, the boiling point behavior is more complex and requires Raoult's law for the full analysis.
- Use the stoichiometry calculator for mole conversions. Converting grams to moles is a common step before applying the boiling point formula.
Frequently Asked Questions
What is boiling point elevation?
Boiling point elevation is the increase in a solvent's boiling point that occurs when a non-volatile solute is dissolved in it. It is a colligative property, meaning it depends only on the number of dissolved particles, not their chemical identity. The formula is ΔTb = i × Kb × m. For water (Kb = 0.512°C·kg/mol), dissolving 1 mole of glucose in 1 kg of water raises the boiling point by 0.512°C. Electrolytes like NaCl cause a greater elevation because they dissociate into multiple ions.
What is the van't Hoff factor and how do I determine it?
The van't Hoff factor (i) represents the number of particles a solute produces when dissolved. For molecular compounds that do not dissociate (glucose, sucrose, urea), i = 1. For strong electrolytes, i equals the total number of ions produced: NaCl dissociates into Na⁺ and Cl⁻, so i = 2. CaCl₂ produces Ca²⁺ and 2 Cl⁻, so i = 3. In practice, concentrated solutions have slightly lower effective i values due to ion pairing effects.
Does adding salt to cooking water significantly raise the boiling point?
No. A typical amount of cooking salt (1 tablespoon in 4 liters of water) raises the boiling point by only about 0.075°C — completely unnoticeable. You would need to dissolve about 230 grams of salt per liter (roughly 1 cup per quart) to raise the boiling point by just 2°C, which would make the water unpalatable. Salt is added to cooking water primarily for seasoning, not for any meaningful temperature effect.
What is Kb for water and other common solvents?
The ebullioscopic constant (Kb) for water is 0.512°C·kg/mol — meaning each mole of non-electrolyte solute per kilogram of water raises the boiling point by 0.512°C. Other solvents have different Kb values: benzene (2.53), chloroform (3.63), acetic acid (3.07), ethanol (1.22), and camphor (5.95). Camphor's high Kb makes it useful in laboratory molecular weight determination because it produces large, easily measurable temperature changes.
How is boiling point elevation used in real-world applications?
Automotive coolant systems rely on boiling point elevation — a 50/50 ethylene glycol-water mix raises the boiling point by about 4°C at atmospheric pressure, and much more under radiator pressure. In candy making, sugar concentration directly determines boiling point, which is why specific temperatures correspond to candy stages (soft ball at 112-116°C, hard crack at 146-154°C). In analytical chemistry, boiling point elevation is used to determine the molecular weight of unknown compounds by measuring the temperature change when a known mass is dissolved.