Chapter 5: Mastering Laboratory Skills 71AATBoU BoUT
Instructions in the laboratory sessions later in this book
frequently say something like “transfer about 5 mL” of a
solution or “weigh about 5.1 g” of a solid. So what does
about 5 mL or about 5.1 g mean? The key is the number
of sig figs. In the first case, anything more than 4 mL
and less than 6 mL is acceptable (although you should
take reasonable care to get close to 5 mL). In the second
case, anything in the range of 5.0 g to 5.2 g is acceptable,
although again you should take reasonable care to stay as
close as possible to 5.1 g.
Often, the exact volume or mass you start with doesn’t
matter, but it’s important that it be known accurately.
For example, “weigh about 5 g of sodium hydroxide
and record the mass to 0.01 g” means you should start
with more or less 5 g of the chemical, but you need
to determine and record the actual mass of sodium
hydroxide to within 0.01 g.HANdLING CHEmICALS pRopERLy
- lways store chemicals away from your work area, ideally A
in cupboards or covered storage containers. - Never store chemicals in unlabeled bottles. In addition to
being unsafe, the risk of contamination is high, because one
chemical could easily be confused with another. - Have only one chemical bottle open at a time.
- Measure and transfer chemicals in an area that is remote
from any experimental procedures that are in progress.
Otherwise, even a small accident might contaminate the
entire bottle. - Always read the label, twice, before you open the bottle.
Make certain that you’re actually using the chemical you think
you’re using. - Before you open the bottle, wipe the exterior with a clean paper
towel to remove any accumulated dust or other contamination. - If a chemical bottle becomes grossly contaminated—for
example, by leakage from another chemical bottle—either rinse
the bottle thoroughly to remove all traces of the contamination
or discard the chemical and obtain a new supply.- Never return unused chemicals to their original containers.
Doing so risks contaminating the entire bottle. Take only as
much as you need from the original bottle, and safely discard
any excess. - Never place the bottle cap on the counter, where it may become
contaminated or switched with a cap from a different chemical
(which is another good reason to have only one chemical bottle
open at a time). Instead, hold the cap and chemical bottle in
the same hand. Depending on the size of the bottle and cap,
you may find it easier to hold the cap between your thumb and
forefinger, as shown in Figures 5-1 and 5-2, or between your
little finger and ring finger, as shown in Figure 5-3. If the cap and
bottle are too large to handle with one hand, either hold the cap
in your other hand or, if absolutely necessary, place the cap flat
with the inside surface up on a clean surface away from your
immediate work area. - When possible, avoid touching a solid chemical in the storage
bottle with a scoop, spatula, or similar tool. Many solid
chemicals are free-flowing crystals or powders. Transfer such
chemicals directly to the weighing paper by rotating and gently
Aside from safety, the most important goal of proper chemical handling is to avoid contamination.
To minimize the risk of contamination, use the following guidelines: