254 DIY Science: Illustrated Guide to Home Chemistry Experiments
CUTIOA nS
Use extreme care with dry ice. At atmospheric pressure, dry
ice sublimates (changes directly from a solid to a gas) at
–78.5°C, cold enough to cause severe frostbite almost
instantly if it contacts your skin. Handle dry ice only with tongs.
Make sure the alcohol lamp or gas burner is widely separated
from the beaker of alcohol—across the room is best. Wear
splash goggles, heavy-duty gloves, and protective clothing.z
value of the experiment, increase the danger level, and use up a lot
of vegetable oil for no good reason.
I wanted the temperature of my cold bath as low as possible.
In my first pass, I put the apparatus in a full-size freezer, which
got the temperature down to about –20°C. I was able to do a
bit better than that, though. As I was working on this chapter, I
happened to receive a FedEx shipment of food packed in dry ice.
Putting chunks of dry ice in a beaker full of alcohol allowed us to
reach a temperature considerably colder than our freezer. You can
purchase small amounts of dry ice locally. Check the Yellow Pages
for “dry ice.”
In this laboratory session, we’ll use an apparatus similar to the one
we used in the preceding lab. The difference is that in the previous
lab we held temperature constant and changed the pressure, and
in this lab we hold pressure constant and change the temperature.
Thus, we won’t need the container and the masses we used in the
preceding lab. Instead, we’ll record the volume contained by the
syringe at room temperature, and then immerse the syringe in
liquids at various temperatures and record the changes in volume.
SBSTITUTIU oNS ANd modIfICATIoNS- If you use dry ice, the thermometer needs to be
accurate down to at least –80°C. If you use a standard
freezer as your cold bath, the thermometer needs to
be accurate down to –20°C or so. Typical inexpensive
digital thermometers are accurate down to –40 or
–50°C, and typical inexpensive glass thermometers
are accurate down to –20°C. - If you do not have a ring stand, clamp, and so on, you
may substitute beaker tongs and a large steel bolt or
other weight. Tie the bolt securely to the body of the
syringe (not the plunger) and use the weight to keep
the syringe fully immersed in the hot and cold baths.
Use the beaker tongs to add and remove the syringe
from the baths.
POCEDURER
1.f you have not already done so, put on your splash I
goggles, gloves, and protective clothing.- Spread a tiny amount (~1 drop) of mineral oil or
petroleum jelly on the seal at the bottom of the plunger.
Use just enough to lubricate the seal so that it slides
freely within the body of the syringe. - With the cap removed, slide the plunger in and out of
the syringe several times to distribute the lubricant.
There should be little discernible resistance when
moving the plunger. - Adjust the plunger to about 75% of full scale. For
example, for my 10 mL syringe, I adjusted the plunger
to 7.5 mL. Once the plunger is in the correct position,
reinstall the cap on the syringe, and record the initial
volume on line A of Table 14-2. - Fill the beaker about 90% full of ethanol, isopropanol,
or acetone and add several chunks of dry ice. Insert
the thermometer and observe the temperature until it
stabilizes at its lowest point. (Continue adding chunks of
dry ice as necessary, as the dry ice is consumed.) Once
the temperature has stabilized, immerse the syringe,
making sure that the portion filled with gas is below the
surface of the alcohol. - Allow the syringe to remain in the dry ice bath for
two to three minutes to ensure that the gas has
equilibrated at the temperature of the bath, and then
withdraw the syringe and note the volume indicated.
(Bump the plunger once or twice to ensure that
it is actually at equilibrium position.) Record the
temperature of the bath and the volume of the syringe
on line B of Table 14-2.
FIGURE 14-3: Measuring gas volume in a bath of alcohol and dry ice