Illustrated Guide to Home Chemistry Experiments

74 DIY Science: Illustrated Guide to Home Chemistry Experiments

mEASURING LIqUIdS By voLUmE

Liquids are usually measured volumetrically, using glassware that is graduated to indicate the

volume that it contains. In general, volumetric accuracy varies inversely with the capacity of the

glassware. For example, a 100 mL graduated cylinder may be accurate to ± 1.0 mL, and a 10 mL

graduated cylinder may be accurate to ± 0.1 mL. Similarly, volumetric glassware with a small

bore is more accurate than glassware with a larger bore. For example, a beaker or flask may be

accurate to ± 5%, a graduated cylinder to ± 1%, and a pipette to ± 0.1%. (Remember that these

percentages refer to full capacity, so if you use a 10 mL graduated cylinder to measure 1.0 mL

of liquid, for example, your measurement is accurate to 0.1 mL, or 10%, rather than the 1% full-

scale accuracy.)

dAR. Rm y CHERvENAk CommENTS:

I was curious whether I could find another fluid,

other than mercury, that formed a convex meniscus.

Some molten metals and ink solutions form convex

meniscuses/menisci.

Unfortunately, the surface of a liquid in a glass container is
not flat, because the glass attracts or repels the liquid. This
attraction or repulsion causes the surface of the liquid to
assume a curved shape called a meniscus. Water and aqueous
solutions are attracted to glass and therefore exhibit a concave
meniscus, with the center of the column of fluid lower than the
edges. Figure 5-10 shows an example of a concave meniscus.
(Mercury and a few other fluids are repelled by glass, and
therefore form a convex meniscus, with the center of the column
of fluid higher than the edges, but convex menisci are seldom
seen in a home lab.)

For accurate volumetric measurements, you must take the
meniscus into account. Take the measurement with the
meniscus at eye level, recording the reading at the center
(bottom) of the meniscus. One advantage of plasticware
for volumetric measurements is that it neither attracts nor
repels water or aqueous solutions, so there is no meniscus to
complicate readings.

UISnG A vOLUMETRIC FLASK
Using a volumetric flask allows you to make up fixed
quantities of solutions with very high accuracy. For example,
a 100 mL volumetric flask allows you to make up exactly (within
stated error) 100 mL of a solution, and a 500 mL volumetric
flask allows you to make up exactly 500 mL of a solution.
To make up solutions with a volumetric flask, take the
following steps:

1. Fill the flask half to three-quarters full with distilled or
   deionized water.


2. Carefully weigh or measure the solid or liquid solute
   and transfer it to the volumetric flask using a funnel.
   Make sure to do a quantitative transfer, which is a fancy
   way of saying you should ensure that all of the chemical
   is transferred to the flask. For example, tap the weighing
   paper against the funnel to make sure none of the
   chemical adheres to the weighing paper, and rinse the

inside of the funnel two or three times with several mL

of distilled water from a wash bottle.

3. Using distilled water from a wash bottle, fill the flask
   until the liquid level reaches the neck of the flask, within
   a few centimeters of the index line on the neck.


4. Stopper the flask and swirl or invert it until the chemical
   dissolves completely.


5. When the solute appears to be fully dissolved, invert
   the flask several times to ensure that the solution is
   reasonably homogeneous. Allow a moment or two for
   the solution adhering to the upper part of the flask to
   drain into the body of the flask.


6. Rinse the stopper, directing the rinse water into the flask
   to make sure that any chemical adhering to the stopper
   is not lost.


7. Carefully fill the flask until the bottom of the meniscus
   just touches the index line. Use an eyedropper or Beral
   pipette to add the last mL or so of liquid.


8. Stopper the flask and again invert it repeatedly until
   the solution is thoroughly mixed. Allow the solution to
   drain from the neck, and verify that the bottom of the
   meniscus is just touching the index line.


9. Transfer the solution to a labeled storage bottle or
   other container.

Dissolving some chemicals such as concentrated acids and

strong bases is extremely exothermic (heat-producing),

sometimes producing enough heat to cause a volumetric flask

made of ordinary glass to crack. Although most volumetric