Question

During the microscope boiling point determination of an unknown liquid, heating was discontinued at 154°C and the liquid immediately began to enter the inverted bell. Heating was begun again at once, and the liquid was forced out of the bell. Heating was discontinued at 165°C, at which time a rapid stream of bubbles emerged from the bell. On cooling, the rate of the bubbling gradually diminished until the liquid reached a temperature of 161°Cand entered and filled the bell. Explain the sequence of events. What was the boiling point of the liquid?

Answer 1

Answer:

First event: Heating was discontinued at 154C and the liquid began to enter the inverted bell which originally contains air and vapor of the unknown liquid which is due to its vapor pressure. In this case, the heating was discontinued before the boiling point could be reached.

Second event: Heating was begun and liquid was forced out of the bell. The inverted bell will still contain air and the vapor of the unknown liquid.

Third event: Heating was discontinued at 165 C and a stream of bubbles emerges from the bell. In this case, the apparatus was heated to a temperature greater than the boiling point so vapor pressure inside the bell will be greater than atmospheric pressure and this extra pressure is released as bubbles.

Final event: On cooling, the bubbling diminishes at 161 C and the liquid enters and fills the bell. Thus the boiling point of the liquid is 161 C. This is the temp at which the bubbling stopped. Boiling point is the temperature at which the vapor pressure is equal to atmospheric pressure. Upon cooling, the liquid then enters and fills the bell because the liquid vapor pressure is now below atmospheric pressure as the temp reaches below boiling point on cooling.