The boiling point of a liquid is a specific physical property used to characterize and identify chemical substances. It is the temperature at which the liquid’s vapor pressure equals the external pressure, causing the liquid to transition into a gas phase. Since every pure compound has a unique boiling point under standard conditions, measuring this temperature is a valuable tool for verifying the identity or assessing the purity of a product. The measurement method chosen depends largely on the amount of sample available, utilizing different techniques for large volumes versus micro-scale quantities.
Measuring the Boiling Point of Larger Samples
For samples of 5 to 10 milliliters or more, the boiling point is typically determined using a distillation setup. This method involves heating the liquid in a flask until it vaporizes, and then condensing the vapor back into a liquid. The necessary apparatus includes a round-bottom flask, a heating source (such as a heating mantle or hot plate), a distillation head, a condenser, and a thermometer.
Proper placement of the thermometer is important for an accurate reading. The bulb must be positioned precisely at the junction where the vapor is directed toward the condenser, ensuring it is fully bathed in the condensing, equilibrium vapor. The liquid sample should also contain a few boiling chips to promote smooth, controlled boiling and prevent superheating.
Heating must be controlled and gradual to allow the liquid and vapor phases to reach thermal equilibrium. The temperature recorded when the distillation is running smoothly and the thermometer reading stabilizes is the observed boiling point. For safety, especially with flammable liquids, a closed heating system like a heating mantle must be used instead of an open flame. The entire procedure is performed in a fume hood to manage the escape of volatile solvent vapors.
Determining Boiling Point Using Micro-Scale Techniques
When only a small quantity of liquid, typically less than 0.5 milliliters, is available, chemists use the inverted capillary tube method, sometimes referred to as the Siwoloboff method. This technique takes advantage of the definition of boiling point by equating the compound’s vapor pressure with the atmospheric pressure. The apparatus consists of a small test tube containing the liquid sample and an inverted capillary tube, both attached to a thermometer.
This assembly is placed into a heating bath, such as a Thiele tube filled with mineral oil, and heated gently. As the temperature rises and the liquid begins to boil, a steady stream of bubbles emerges from the open end of the inverted capillary tube. This stream indicates that the substance’s vapor pressure has exceeded the external atmospheric pressure.
The heat source is then removed, and the apparatus is allowed to cool while the thermometer is monitored. As the temperature drops, the vapor pressure inside the capillary decreases, causing the stream of bubbles to stop. The precise temperature at which the liquid is drawn up into the capillary tube is recorded as the boiling point, as this is when the internal vapor pressure equals the external atmospheric pressure.
Adjusting Measurements for Atmospheric Pressure
Boiling point measurements depend on the prevailing atmospheric pressure at the time of the experiment. The standard or “normal” boiling point is defined as the temperature at which a substance boils at 760 millimeters of mercury (mmHg). Since atmospheric pressure fluctuates with weather and altitude, laboratory measurements often deviate from this standard value.
For example, lower atmospheric pressure at higher elevations causes liquids to boil at a lower temperature than at sea level. To compare an observed boiling point to a reference value, a correction must be applied to normalize the measurement to 760 mmHg. This correction often involves consulting a pressure-temperature nomograph or using the Clausius-Clapeyron equation if the exact barometric pressure is known.
Applying this adjustment converts the measured temperature into the theoretical normal boiling point. Reporting the barometric pressure alongside the observed boiling point is important for scientific records, allowing others to verify and correct the measurement. This step ensures the determined value is a true characteristic of the substance, independent of external environmental conditions.