The boiling point of a compound is the specific temperature where the liquid’s vapor pressure equals the external pressure surrounding it. This physical property is a fundamental characteristic of a pure substance, making its determination a standard procedure in chemistry. Identifying this temperature is a reliable method for compound identification and helps assess the purity of a substance. The boiling point is an important parameter in large-scale industrial operations, such as distillation in petroleum refining or pharmaceutical synthesis, where precise temperature control is necessary.
Laboratory Techniques for Measurement
The most common laboratory method for determining a compound’s boiling point is simple distillation. The liquid is heated in a flask, and the resulting vapor travels upward to a thermometer bulb placed near the condenser. The temperature recorded remains constant while the pure compound is distilling, representing the point where the vapor is in equilibrium with the liquid at the prevailing atmospheric pressure. To obtain an accurate reading, the thermometer bulb must be fully immersed in the vapor stream.
For limited sample sizes, the microscale method, often called the Siwoloboff method, is used. This technique requires less than a milliliter of liquid placed in a test tube with a small inverted capillary tube. The assembly is heated gently in a temperature-controlled bath, such as a Thiele tube, until a continuous stream of bubbles emerges from the capillary. The boiling point is recorded when heating stops and the liquid is suddenly sucked back into the capillary, indicating the external pressure has overcome the internal vapor pressure.
Modern laboratories often utilize automated digital apparatuses, which offer quick measurements with minimal effort. These instruments use electronic heating blocks and built-in sensors or cameras to automatically detect the phase transition in small samples. A key advantage is the inclusion of calibrated pressure sensors, which measure ambient pressure and automatically apply corrections. This allows the device to report the boiling point at a standard reference pressure, streamlining the process and reducing potential human error.
Predicting Boiling Point Based on Molecular Structure
When a compound’s boiling point cannot be easily measured, or to verify experimental results, predicting this value relies on understanding the relationship between molecular structure and the forces holding the molecules together. The energy required to overcome these attractive forces, known as intermolecular forces (IMFs), directly dictates the boiling temperature. Stronger intermolecular forces necessitate more energy input, resulting in a higher boiling point.
London dispersion forces, present in all molecules, increase in strength with the total number of electrons and the surface area of the molecule. Nonpolar compounds rely solely on these forces, meaning larger, heavier molecules with more electrons will have higher boiling points due to stronger dispersion forces.
Molecules with permanent dipoles, where electron density is unevenly distributed, exhibit stronger dipole-dipole interactions in addition to dispersion forces. These stronger attractions lead to a higher boiling point compared to nonpolar molecules of a similar size. The strongest of the three forces is hydrogen bonding, which occurs when hydrogen is directly bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine. Hydrogen bonding increases the boiling point because it creates a strong attraction between molecules that requires significant energy to break.
Molecular shape also plays a role, especially when comparing compounds with the same molecular formula, called isomers. Linear molecules can pack more closely together, increasing the total surface area for London dispersion forces to act, leading to a higher boiling point. Conversely, molecules that are branched or more spherical in shape have a smaller effective surface area for interaction, which weakens the intermolecular forces and results in a lower boiling point.
Adjusting Measurements for Non-Standard Conditions
A reported boiling point is only accurate if the external pressure is specified, as boiling is defined by the liquid’s vapor pressure equaling the surrounding pressure. The standard, or “normal,” boiling point is referenced at a pressure of 760 millimeters of mercury (mmHg), equivalent to one atmosphere. Because atmospheric pressure fluctuates with weather and altitude, a measured boiling point must be corrected if the laboratory pressure deviates from this standard value.
A lower atmospheric pressure, such as at higher altitudes, results in a lower measured boiling point because less energy is needed for the liquid’s vapor pressure to match the external pressure. For pressures near one atmosphere, a drop of 10 mmHg in pressure can cause the boiling point to decrease by 0.3 to 0.5 degrees Celsius. For more precise corrections, especially at pressures far from standard, one can use established mathematical relationships like the Clausius-Clapeyron equation or refer to pressure-temperature charts called nomographs.
The purity of the sample is another factor that influences the measured boiling temperature. A pure compound exhibits a sharp, constant boiling point, whereas the presence of non-volatile impurities causes boiling point elevation. The impurities lower the vapor pressure, meaning more heat is required to reach the boiling condition. Consequently, the liquid boils at a temperature higher than the pure substance and over a range of temperatures, which indicates contamination.