88 DIY Science: Illustrated Guide to Home Chemistry Experiments
- When the sample appears to be completely dry, allow it to
cool, weigh it, and record the mass. - Return the sample to the oven or replace it under the heat
lamp, and heat it strongly for at least another 15 minutes. - Reweigh the sample, record the mass, and compare that
mass with the mass you previously recorded. If the new
mass is lower, not all of the water was extracted from the
sample when you made the previous weighing. - Repeat steps 5 and 6 until the mass no longer changes.
At that point, you can be sure that the sample is
completely dry.
FIGURE 5-19: Using a lamp to dry a sample
woRkING wITH GLASS TUBING
Glass tubing is used in conjunction with flexible rubber or plastic
tubing and holed stoppers to route liquids and gases between
containers. Glass tubing is available in various outside and
inside diameters, wall thicknesses, types of glass, and lengths.
The most important characteristic is the outside diameter,
which should be chosen to fit your stoppers and flexible tubing.
Tubing with an outside diameter (OD) of 5 mm is most common,
but verify the proper OD before you order.
Most glass tubing is made of ordinary flint glass, which softens
at the relatively low temperature provided by an alcohol lamp.
By heating the tubing until it softens, you can bend it, stretch it,
and otherwise manipulate it into the shapes needed to construct
distillation setups, gas-generating bottles, and similar apparatus.
For high-temperature use, glass tubing is available made from
Pyrex or similar heat-resistant borosilicate glasses. Such high-
temperature tubing is seldom needed, if ever, in a home lab, but
it costs little more than flint-glass tubing and is often stocked
by laboratory supply vendors. If you find that heating a piece of
glass tubing with an alcohol lamp doesn’t soften it, that glass
tubing is almost certainly made of heat-resistant glass. You
can still manipulate and shape such tubing, but you’ll need the
hotter flame of a gas burner to do so.
CGUTTInG LASS TUBInG
Glass tubing is supplied in standard lengths, typically 6"
(~150 mm), 12" (~300 mm), and longer. You’ll often need shorter
lengths, which are easy enough to cut from the stock tubing.
To do so, take the following steps:
- Use the edge of a steel file to score the glass once,
perpendicular to its length, as shown in Figure 5-20. Do
not saw away at the glass or attempt to cut deeply into
it. Just a slight notch suffices.- Place both thumbs near the notch, as shown in Figure
5-21, with the notch on the other side of the tubing from
your thumbs, and apply gentle pressure until the tubing
snaps in two. Wear eye protection, and use heavy gloves
or wrap the tubing in a towel to prevent cuts. - The two fresh ends of the tubing are quite sharp, and
need to be fire-polished before use. To fire polish the
tubing, hold the sharp end in the flame of an alcohol
lamp or gas burner, as shown in Figure 5-22, and rotate
the tubing slowly until the sharp end is smoothed. Don’t
overdo the fire-polishing or you’ll melt the end of the
tubing enough to block it. An alcohol lamp isn’t hot
enough to fire-polish Pyrex tubing, which requires a
gas burner.
BIEnDnG AnD DRAWInG GLASS TUBInG
Bending and drawing glass tubing is easy enough, but requires
a bit of practice. (Drawing is the process of stretching tubing to
reduce its diameter—for example, to make a nozzle for a wash
bottle.) To bend glass tubing, perform the following steps:- Hold the tubing horizontally in the hottest (blue) part
of the flame of an alcohol lamp or gas burner, as shown
in Figure 5-23. Heat the tubing at the point you want
to make the bend and for a centimeter or so on either
side. (If you use a gas burner, a flame spreader is very
helpful in distributing the heat evenly.) Rotate the tubing
continuously to make sure that it’s heated around its
full circumference.