Illustrated Guide to Home Chemistry Experiments

Chapter 3: Equipping a Home Chemistry Laboratory 37

Table 3-4 lists standard rubber stopper sizes used by most (but
not all) U.S. vendors, the hole sizes for predrilled stoppers, the
sizes of openings those stoppers fit, and the types of vessels they
are typically used with. Some stoppers use different dimensions
for the same stopper numbers, so it’s important to verify the
actual size of the stoppers before you order them. Either that, or
order a large assortment that includes solid, 1-hole, and 2-hole
stoppers in each size to make sure you always have a stopper
that fits whatever vessel you need to use it with. Verify that the
hole sizes of the stoppers you buy matches the outside diameter
of your supply of glass tubing.

The one- and two-hole versions of #1 rubber stoppers usually
have 4 mm holes, although we have seen two-hole #1 stoppers
with 3 mm holes. The one- and two-hole versions of #2 and
larger rubber stoppers all use the standard 5 mm hole size,
except some #6.5 stoppers that use the 4 mm size. The oddball
#6.5 size can be difficult to find, but is the best fit for a 3-liter
soda bottle.

Corks are less durable and less convenient to use than rubber
stoppers, but are also less expensive and more resistant

Stopper #Hole size Fits opening Typically fits

00 3 mm 10 mm to 13 mm Very small test tubes.

0 3 mm 13 mm to 15 mm Small test tubes; many 25 mL flasks.

1 3 mm or 4 mm15 mm to 17 mm Some medium test tubes; many 50 mL flasks.

2 5 mm 16 mm to 18.5 mmSome medium test tubes.

3 5 mm 18 mm to 21 mm 2-liter soda bottles; some large test tubes.

4 5 mm 20 mm to 23 mm Some large test tubes.

5 5 mm 23 mm to 25 mmMost 125 mL flasks; some very large test tubes.

6 5 mm 26 mm to 27 mmMany 250 mL flasks; some 500 mL flasks.

6.5 4 mm or 5 mm27 mm to 31.5 mm3-liter soda bottles; some 500 mL flasks.

7 5 mm 30 mm to 34 mm3-liter soda bottles; most 500 mL flasks.

8 5 mm 33 mm to 37 mmSome 500 mL flasks.

TABLE 3-4: U.S. standard rubber stopper sizes

to high temperatures. For historical reasons, corks use an

entirely different numbering system from stoppers. In fact,

cork measurements were originally specified in fractions of

an inch rather than SI (metric), and only later were those inch

denominations converted to SI.

Table 3-5 lists standard cork sizes used by most U.S. vendors.

The actual size of a cork may vary slightly, depending on whether

it is specified in U.S. customary units or SI. Corks are much more

compressible than are rubber stoppers. Proper use of corks

requires using a cork roller, which compresses the cork before it’s

inserted into the vessel. As the cork relaxes, it expands to seal the

vessel. This results in a much better seal than is possible simply

by pressing a cork into the opening using finger pressure.

Corks are usually sold without holes. If you want a 1-hole or 2-hole

cork, use a cork borer to make it yourself. If you buy the cheapest

corks, you’ll probably discover why they were so cheap the first

time you try to bore a hole in one. Cheap corks often have internal

cavities and cracks, and are prone to crumble when you attempt

to bore them. Better corks, usually sold as laboratory-grade, are

solid throughout and less prone to crumbling.