Boiling Point Calculator (Pressure–Temperature Relation)
Estimate boiling point changes with pressure, or solve required pressure from a target temperature, using Clausius–Clapeyron, visuals, and steps.
Background
A liquid boils when its vapor pressure equals the external pressure. Lower pressure lowers the boiling point; higher pressure raises it. This calculator supports a simple normal-boiling-point mode, a target-temperature pressure solve, an advanced two-reference-point mode, and an altitude estimate mode for student-friendly chemistry and lab problems.
How to use this calculator
- Choose Simple, Target temperature, Advanced, or Altitude mode.
- Enter temperatures in °C or K and pressures in atm, kPa, mmHg, bar, or psi.
- Click Calculate to get either the boiling point or the required pressure, plus the visual graph and steps.
- Use quick examples to compare lower-pressure, higher-pressure, and altitude cases.
How this calculator works
The calculator converts all temperatures to Kelvin and pressures to atm, then applies the Clausius–Clapeyron relationship. In Simple mode it uses a ΔHvap preset for water, ethanol, or benzene; custom mode estimates ΔHvap using Trouton’s rule. In Advanced mode, it derives ΔHvap from two pressure–temperature reference points before solving the target temperature.
The output also explains whether the pressure is lower or higher than 1 atm, because that direction predicts whether the boiling point should go down or up.
Formula & Equation Used
Clausius–Clapeyron: ln(P₂/P₁) = −(ΔHvap/R)(1/T₂ − 1/T₁)
Simple solve: 1/T₂ = 1/T₁ − (R/ΔHvap)ln(P₂/P₁)
Target temperature solve: P₂ = P₁ × exp[−(ΔHvap/R)(1/T₂ − 1/T₁)]
Altitude pressure estimate: pressure is estimated from altitude, then used as the target pressure.
Example Problems & Step-by-Step Solutions
Example 1: Water at lower pressure
At 0.80 atm, water boils below 100 °C because the external pressure is lower than normal atmospheric pressure.
Example 2: Two reference points
Use two known (P, T) points to estimate ΔHvap, then solve for the boiling point at a new target pressure.
Example 3: Target temperature
If you want water to boil at 90 °C, use Target temperature mode to solve the pressure needed instead of entering pressure first.
Example 4: Higher pressure
At pressure above 1 atm, boiling requires a higher vapor pressure, so the boiling point increases.
Frequently Asked Questions
Can I solve for pressure from a target temperature?
Yes. Use Target temperature mode when you know the desired boiling temperature and need the pressure required to reach it.
Why does boiling point change with pressure?
A liquid boils when vapor pressure equals external pressure. Lower external pressure means this happens at a lower temperature.
Why must temperature use Kelvin in the formula?
Clausius–Clapeyron uses absolute temperature, so Celsius values are converted to Kelvin before calculation.
Is this exact for every liquid?
It is an estimate, especially over large pressure ranges. Advanced mode is usually better when you have two reliable reference points.
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