All calculatorsBoiling Point Calculator (Simple / Advanced)
Predict how a liquid’s boiling point changes with pressure. Use the Simple mode with a normal boiling point and target pressure, or switch to Advanced to use two reference points (Clausius–Clapeyron).
Background
The boiling point is the temperature at which a liquid’s vapor pressure equals the external pressure. At a different pressure, the boiling point shifts. In Simple mode, we treat the normal boiling point as the temperature where the vapor pressure is 1 atm and adjust to your target pressure with a standard formula. In Advanced mode, you provide two (P, T) reference points and we apply the two-point Clausius–Clapeyron relation to find the temperature at your target pressure.
Formula & Equation Used
Advanced (two-point Clausius–Clapeyron): \(\ln\!\big(\tfrac{P_2}{P_1}\big) = -\tfrac{\Delta H_{\text{vap}}}{R}\left(\tfrac{1}{T_2}-\tfrac{1}{T_1}\right)\). Eliminating \(\Delta H_{\text{vap}}\) with two reference points lets us solve for the temperature at a new pressure \(P_{\text{target}}\).
Simple: starts from the normal boiling point at 1 atm and adjusts to the target pressure using a standard rearrangement consistent with Clausius–Clapeyron behavior near the boiling region.
Example Problems & Step-by-Step Solutions
Example 1 (Simple)
Water: normal boiling point = 100 °C at 1.00 atm. Estimate the boiling point at 0.80 atm.
- Enter: Tn = 100 °C, Ptarget = 0.80 atm, Mode = Simple.
- Use Clausius–Clapeyron with P1=1 atm and ΔHvap from the substance (or Trouton’s rule).
- Formula used (display):
1/T₂ = 1/Tₙ − (R/ΔHvap)·ln(P₂/P₁). - Conclusion: the computed T₂ will be below 100 °C (lower pressure → lower boiling point).
Example 2 (Advanced)
Ethanol: given (P1=1.00 atm, T1=78.37 °C) and (P2=0.80 atm, T2=72.0 °C). Find the boiling point at Ptarget=0.60 atm.
- Enter: P1=1.00 atm, T1=78.37 °C; P2=0.80 atm, T2=72.0 °C; Ptarget=0.60 atm; Mode = Advanced.
- Compute ΔHvap from the two reference points using:
ln(P₂/P₁) = −(ΔHvap/R)·(1/T₂ − 1/T₁). - Then solve for T at the target pressure:
1/Tt = 1/T₁ − (R/ΔHvap)·ln(Pt/P₁). - Result: the calculator outputs Tt in °C (and K).
Frequently Asked Questions
Q: Which units should I use for pressure and temperature?
Use any provided units—this tool converts internally. Temperatures for the math are handled in Kelvin, but you can enter °C and get °C out.
Q: Is the result exact?
It’s a good thermodynamic estimate. Real substances can deviate, especially far from the reference point or near critical conditions.
Q: Should I include significant figures?
Yes—match your given data and your course’s convention.
Q: What’s the difference between Simple and Advanced mode?
Simple mode uses a single reference boiling point and pressure for quick estimates. Advanced mode applies the Clausius–Clapeyron equation with two known points for greater accuracy.
Q: Can I input values in Fahrenheit?
Yes—enter °F and the calculator converts it internally to Kelvin. Results are displayed back in the same scale.
Q: Why might my calculated boiling point look different from lab data?
The model assumes ideal behavior. Real liquids can show deviations due to impurities, non-ideal vapor pressures, or elevation changes (altitude).
