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Definition of Boiling Point in Chemistry

The boiling point is affected by atmospheric pressure

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  • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
  • B.A., Physics and Mathematics, Hastings College
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The boiling point is the temperature at which the vapor pressure of a liquid equals the external pressure surrounding the liquid. Therefore, the boiling point of a liquid depends on atmospheric pressure. The boiling point becomes lower as the external pressure is reduced. As an example, at sea level the boiling point of water is 100 C (212 F), but at 6,600 feet the boiling point is 93.4 C (200.1 F).

Boiling vs. Evaporation

Boiling differs from evaporation. Evaporation is a surface phenomenon that occurs at any temperature in which molecules at the liquid edge escape as vapor because there is not enough liquid pressure on all sides to hold them. In contrast, boiling affects all molecules in the liquid, not just those on the surface. Because molecules within the liquid change to vapor, bubbles form.

Types of Boiling Points

​Boiling point is also known as saturation temperature. Sometimes boiling point is defined by the pressure at which the measurement was taken. In 1982, the International Union of Pure and Applied Chemistry (IUPAC0 defined the standard boiling point as the temperature of boiling under 1 bar of pressure. The normal boiling point or atmospheric boiling point is the temperature at which the vapor pressure of the liquid equals the pressure at sea level (1 atmosphere).

The definition of boiling point in chemistry takes into consideration the surrounding atmospheric pressure.

Distillation

Distillation is the process of heating a liquid until it boils, then condensing and collecting the resultant hot vapors. Mankind has applied the principles of distillation for thousands of years. Distillation was probably first used by ancient Arab chemists to isolate perfumes. Vessels with a trough on the rim to collect distillate, called diqarus, date back to 3500 BC.

In the modern organic chemistry laboratory, distillation is a powerful tool, both for the identification and the purification of organic compounds. The boiling point of a compound is one of the physical properties used to identify it. Distillation is used to purify a compound by separating it from a non-volatile or less-volatile material. When different compounds in a mixture have different boiling points, they separate into individual components when the mixture is carefully distilled.

Distillation for Boiling Point Determination

The organic teaching labs employ distillation routinely, both for the identification and the purification of organic compounds. The boiling point of a compound, determined by distillation, is well-defined and thus is one of the physical properties of a compound by which it can be identified. Distillation is used to purify a compound by separating it from a non-volatile or less-volatile material. Because different compounds often have different boiling points, the components often separate from a mixture when the mixture is distilled.

The boiling point is the temperature at which the vapor pressure of the liquid phase of a compound equals the external pressure acting on the surface of the liquid. The external pressure is usually the atmospheric pressure. For instance, consider a liquid heated in an open flask. The vapor pressure of the liquid will increase as the temperature of the liquid increases, and when the vapor pressure equals the atmospheric pressure, the liquid will boil. Different compounds boil at different temperatures because each has a different, characteristic vapor pressure: compounds with higher vapor pressures will boil at lower temperatures.

Boiling points are usually measured by recording the boiling point (or boiling range) on a thermometer while performing a distillation. This method is used whenever there is enough of the compound to perform a distillation. The distillation method of boiling point determination measures the temperature of the vapors above the liquid. Since these vapors are in equilibrium with the boiling liquid, they are the same temperature as the boiling liquid. The vapor temperature rather than the pot temperature is measured because if you put a thermometer actually in the boiling liquid mixture, the temperature reading would likely be higher than that of the vapors. This is because the liquid can be superheated or contaminated with other substances, and therefore its temperature is not an accurate measurement of the boiling temperature.

If you are using the boiling point to identify a solid compound which you have isolated in the lab, you will need to compare its boiling point with that of the true compound. Boiling points are listed in various sources of scientific data, as referenced on the Chemical Information page on this website.

If you look up the boiling point of a compound in more than one source, you may find that the values reported differ slightly. The literature boiling point depends on the method and ability of the technician taking the boiling point, and also on the purity of the compound. While theoretically all boiling points should be constant from source to source, in reality the reported boiling points sometimes vary. Therefore, always reference the source of the physical data which you write in your lab report.

Distillation for Compound Purification

Simple distillations are used frequently in the organic chemistry teaching labs. They are useful in the following circumstances:

  • the liquid is relatively pure to begin with (e.g., no more than 10% liquid contaminants)
  • the liquid has a non-volatile component, for example, a solid contaminant
  • the liquid is contaminated by a liquid with a boiling point that differs by at least 70°C

“Simple” distillation may be a misleading term to the beginning organic chemistry student, since it takes a lot of practice in simple distillation to become proficient in this technique. It is especially important to do a perfect simple distillation when determining a boiling point for identification purposes. You can see detailed photos of a simple distillation set-up here. Be sure to have correct placement of the thermometer, fill the flask to the correct level, and use a boiling chip.

Fractional Distillations

Mixtures of liquids whose boiling points are similar (separated by less than 70°C) cannot be separated by a single simple distillation. In these situations, a fractional distillation is used. You can see photos of a fractional distillation set-up here.

Vacuum Distillations

Vacuum distillation is distillation at a reduced pressure. Since the boiling point of a compound is lower at a lower external pressure, the compound will not have to be heated to as high a temperature in order for it to boil. Vacuum distillation is used to distill compounds that have a high boiling point or any compound which might undergo decomposition on heating at atmospheric pressure. The vacuum is provided either by a water aspirator or by a mechanical pump. You can see photos of a fractional distillation set-up here. Always check for star cracks in the flasks before beginning a vacuum distillation.

Distillation Technique Quiz

See how well you understand distillation by taking the online Distillation Technique Quiz!

Distillation Distillation is the process of heating a liquid until it boils, then condensing and collecting the resultant hot vapors. Mankind has applied the principles of distillation for