What is the boiling point?
The boiling point is the temperature at which a liquid transitions to a gas (vapor) at a given pressure. At this temperature, the vapor pressure of the liquid equals the surrounding atmospheric pressure. The normal boiling point is defined at 1 atm (760 mmHg) of pressure. See also our Bomb Calorimeter Calculator.
What's the boiling point of water?
Water boils at 100°C (212°F) at standard atmospheric pressure (1 atm or 760 mmHg). At higher altitudes where pressure is lower, water boils at a lower temperature — for example, at the top of Mount Everest (~253 mmHg), water boils at roughly 70°C.
How do I calculate the boiling point at a different pressure?
Use the Clausius–Clapeyron equation: ln(P2/P1) = (ΔHvap / R) × (1/T1 − 1/T2), where P1 and T1 are your known pressure and boiling point (in Kelvin), P2 is the target pressure, R is the gas constant (8.314 J/mol·K), and ΔHvap is the molar heat of vaporization. Solving for T2 gives the new boiling point.
Is boiling point a physical property?
Yes, boiling point is an intensive physical property of a substance. It depends on the intermolecular forces within the liquid and the external pressure. It does not depend on the amount of substance present, which makes it useful for identifying and characterizing compounds. You might also find our Ideal Gas Law Calculator useful.
Does salt lower the boiling point of water?
No — adding salt to water actually raises its boiling point slightly, a phenomenon called boiling point elevation. This is a colligative property: dissolved solutes increase the boiling point proportionally to their concentration. However, the effect is very small in typical cooking amounts and has negligible impact on cooking times.
What is the Clausius–Clapeyron equation?
The Clausius–Clapeyron equation describes how the vapor pressure of a substance changes with temperature. It relates two pressure-temperature pairs using the molar enthalpy of vaporization (ΔHvap) and the gas constant (R). It's the standard tool used to estimate boiling points at non-standard pressures in chemistry and chemical engineering.
Why does boiling point decrease at higher altitudes?
At higher altitudes, atmospheric pressure is lower. Since boiling occurs when a liquid's vapor pressure matches the surrounding pressure, a lower surrounding pressure means the liquid reaches that vapor pressure at a lower temperature. This is why water boils faster but at a cooler temperature in the mountains, requiring longer cooking times for foods.
What is heat of vaporization and why does it matter?
The molar heat of vaporization (ΔHvap) is the energy required to convert one mole of a liquid into vapor at constant pressure. It reflects the strength of intermolecular forces in the liquid. In the Clausius–Clapeyron equation, ΔHvap determines how sensitive a substance's boiling point is to pressure changes — a higher ΔHvap means the boiling point changes less dramatically with pressure. Check out our use the Enthalpy Calculator as well.