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Boiling point

Temperature at which a liquid’s vapor pressure equals the surrounding pressure and it converts rapidly to vapor; depends strongly on pressure and dissolved substances.

Author: Leandro Alegsa Creation date: November 13, 2022 Last updated: June 14, 2026

simple.wikipedia.org · Public domain

Definition and overview. The boiling point of a substance is the temperature at which its vapor pressure equals the pressure of the surrounding gas and the liquid begins to form vapor rapidly throughout its bulk. This concept can be expressed in different wordings: temperature (temperature) where boiling occurs, or the condition when vapor pressure matches ambient pressure. For common reference, pure water (pure water) boils at 100° Celsius (212° Fahrenheit) under one atmosphere (one atmosphere) of pressure, roughly the pressure at sea level (sea level).

Physical basis and measurement

Boiling is distinct from simple evaporation: evaporation happens at the surface at temperatures below the boiling point, while boiling involves formation of vapor bubbles inside the liquid. A precise statement uses vapor pressure: the boiling point is the temperature at which the liquid’s vapor pressure equals the external pressure. Because of this, boiling points are pressure-dependent; changing the ambient pressure (pressure) shifts the boiling temperature. In practice, the boiling point is measured by heating a liquid and recording the temperature at which vigorous bubbling begins under a known pressure, and techniques such as distillation rely on this principle.

Dependence on pressure and elevation

As external pressure rises, the boiling point increases; as pressure falls, the boiling point decreases. At high altitudes, where atmospheric pressure is lower, water boils at substantially lower temperatures (for example, near the top of Mount Everest water boils at a temperature well below 100 °C). Conversely, pressure cookers raise the cooking temperature by increasing the pressure above atmospheric. To predict how boiling point changes with pressure, chemists use relations derived from the Clausius–Clapeyron equation, which links vapor pressure and temperature.

Effect of dissolved substances

Adding substances to a solvent typically alters its boiling point. Non-volatile solutes such as sugar (sugar) or salt (salt) usually elevate the boiling point — a colligative effect that depends on the concentration and the number of particles produced when the solute dissolves. Volatile additives such as alcohol (alcohol) can lower or otherwise change the apparent boiling behavior because they contribute their own vapor pressure. Complex mixtures may form azeotropes, which boil at compositions and temperatures different from either pure component.

Applications and practical considerations

Understanding boiling points is central to many technologies: distillation separates liquids by differences in boiling temperature; chemical reactors and heat exchange systems are designed around boiling behavior; and everyday cooking is affected by altitude and pressure. In laboratories, vacuum distillation lowers pressure to boil compounds at safer, lower temperatures; in industry, control of boiling prevents overheating, unwanted phase change, or equipment damage.

Notable phenomena and distinctions

Superheating: clean, smooth containers can allow liquids to be heated above their boiling point without bubble formation; a disturbance may then trigger violent boiling.
Boiling versus evaporation: evaporation is gradual and surface-limited; boiling is rapid and involves bulk vaporization.
Standard definitions: "normal boiling point" usually means the temperature at one atmosphere, but some references use 100 kPa as a standard; check the context in data tables.

For further reading on the principles and numerical data, see general references on phase transitions and practical guides to pressure-dependent boiling and colligative properties (boiling point, evaporation and vaporization, Celsius scale). Additional resources cover effects of pressure (pressure), altitude (altitude), and examples of solute influences such as sugar and salt, as well as the role of alcohol in mixtures. Practical demonstrations and data tables may be found via educational sites and laboratory manuals (temperature measurements, water, Fahrenheit, atmospheric pressure, sea level, basic concepts).

Phase diagram of an "ordinary" substance and water

Boiling process

→ Main article: Evaporation, heat of evaporation and boiling delay

Below and above the boiling point, heating the liquid or gas only leads to an increase in temperature. The energy supplied is converted into kinetic energy of the particles. During the phase transition of the liquid to the gas, however, the temperature remains constant, provided that the pressure also remains constant. All thermal energy supplied is invested in the change of state.

Once the boiling point is reached, the chemical-physical interactions between the particles are dissolved when further energy is added - the particles enter the gas phase. The temperature of the liquid stagnates, since the thermal energy supplied is used entirely for the dissolution of the intermolecular bonds. The energy required for this in the case of one mole of the substance is also referred to as the enthalpy of vaporization and its counterpart, which is not related to the quantity of substance, as the heat of vaporization. Only when all particles are in the gas phase does the temperature of the system rise again.

Water, hydrogen peroxide or alkalis (for example caustic soda) without dust particles or gas bubbles can also be heated above boiling temperature in clean vessels without boiling occurring. The smallest disturbances, such as vibrations, which cause mixing, can lead to an explosive separation of the liquid from the vapour phase, which is called boiling delay. To avoid this, so-called boiling stones made of clay or pumice are added to chemical liquids that tend to boil. These are not attacked by the chemical, but their porous structure facilitates the formation of small bubbles so that boiling delay does not occur.

See also: evaporation, gasification, evaporation, transpiration, Pictet-Trouton rule

Temperature change with time when heating a pure liquid substance

Boiling Point Curve

All temperature-pressure-value pairs at the gas-liquid phase boundary in a phase diagram together result in the boiling point curve, whereby a thermodynamic equilibrium prevails on it. The boiling point curve is often referred to as the boiling curve, boiling line, boiling pressure curve or boiling point curve. This curve is bounded by two points:

Triple point Pt: If the pressure-temperature value pair is lower than the triple temperature or the triple pressure, only a transition between solid and gaseous state, i.e. sublimation or resublimation, is possible.
Critical point Pc: If the pair of pressure-temperature values is higher than the critical temperature or the critical pressure, there is no longer any difference between the density of the liquid and that of the gaseous state, which is why they are no longer separated by a phase boundary line and the substance is therefore called a supercritical fluid in this state.

The equilibrium of the boiling point curve is a dynamic equilibrium. From a liquid, particles constantly pass into the gas phase - they evaporate. On the other hand, these particles also re-enter the liquid phase - they condense. The numerical ratio of the particles leaving the liquid phase and the particles re-entering it depends on both the temperature and the pressure: The higher the temperature, the more particles evaporate due to their higher velocity (see Maxwell-Boltzmann distribution). The more particles evaporate, the higher the vapor pressure, and the more particles condense again. Equilibrium is reached when as many particles pass into the gas phase as return to the liquid phase. Since the gas phase is saturated in this state, this is also referred to as saturation vapour pressure. The thermodynamic law from which the boiling point curve is quantitatively derived is known as the Clausius-Clapeyron equation. For water, this relationship between saturation vapour pressure and saturation temperature can also be determined using the approximate equations of the Magnus formula type.

Phase diagram of carbon dioxide (not to scale)