Chemistry Reference
In-Depth Information
Procedure: (a) The temperature of an ice-water mixture is measured. The
mixture is heated one minute in the beaker, after a good stirring, the
temperature is read off again.
(b) A test tube is filled to a quarter with naphthalene and is heated with
the Bunsen burner until all substance is melted; the melt is stirred with the
thermometer and monitored. While stirring all the time the temperature is
recorded every 30 s until the substance is condensing to a solid and
completely solid again.
Observation: As long as the ice melts, the temperature remains constant at
0
C. The naphthalene smells strongly of moth balls (since moth balls
contain this substance). As long as there is a mixture of liquid and solid
naphthalene, the temperature remains constant at 80
C, when the substance
is completely solid, the temperature sinks down to room temperature.
Disposal: The test tubes with the solid naphthalene are provided with a
stopper and kept in the laboratory until the experiment is performed again.
E2.2 Boiling Temperature of Water [
16
]
Problem: Students often know only the scientifically reduced statement:
“water boils at 100
C.” To show the relation of boiling temperatures and
air pressure, the flask for boiling water can be connected to a pump to
produce low air pressures and to measure lower boiling temperatures.
It is also possible to replace the air in the boiling flask by water vapor and to
condense it by cooling down with a wet cloth (see picture). By condensing of
water vapor the pressure in the flask is decreasing, and the measured boiling
temperature sinks to nearly 70
C. This results in the motivational incongru-
ence for the students that the boiling temperature of water is not, as usual,
reached by heating, but by “cooling.”
Material: Round flask (500 mL) with side tube and valve, plug with ther-
mometer (thermal sensor), water jet pump, tripod; water, boiling chips.
