Passage V

Some of the liquid in a closed container evaporates, forming a vapor that condenses, reforming the liquid. The pressure of the vapor at equilibrium (when the rates of evaporation and condensation are equal) is the liquid’s vapor pressure. A liquid in an open container boils when its vapor pressure equals the external pressure. The following experiments were performed to study vapor pressures.

Experiment 1

The apparatus shown in Figure 1 was assembled except for the tubing. The flask was placed in a 20°C H₂O bath. After 5 minutes the manometer was connected, and 2 mL of hexane was added to the flask from the dropper. Some of the hexane evaporated. The vapor pressure was determined by measuring the height of the mercury (Hg) after the Hg level had stabilized. Additional trials were performed at different temperatures and with other liquids in the flask. The results are shown in Table 1.

Experiment 2

A test tube containing a thermometer and hexane was heated in an oil bath until the hexane boiled gently. The temperature was recorded. The external pressure was 760 mm Hg. This procedure was repeated in a chamber at pressures of 400 mm Hg and 100 mm Hg. The boiling points of other liquids were also determined. The results are shown in Table 2.