Always ensure that the mouth of the test tube is pointed away
from you and any other persons around you in the lab, including
those on the opposite side of the bench..
Potential Difficulties
Bumping occurs when one localized area of the test tube
wall becomes hotter than the rest of the tube and the liquid within
it. The liquid in the tube adjacent to this "hot spot"
boils (vaporizes) suddenly creating a large bubble which forces
its way up the tube, pushing liquid ahead of it and often out
of the tube.
Boil-over occurs when the tube is heated evenly but
too strongly. Rapid boiling creates a large amount of bubbles
in the tube which "foam" up out of the tube.
Crackling occurs when a dissolve salt has been heated
to dryness and has initially formed a hydrate. Continued heating
decomposes the hydrate. As the hydrate water molecules are forced
out of the solid, the particles jump around in the tube, and sometimes
may jump out of the tube. Some crackling often is unavoidable
as a hydrate decomposes. The goal is to avoid crackling that is
so violent that it throws material out of the tube.
Video. Heat the test tube evenly.
Correct technique
Proper heating technique is shown in the video at right.
It is important to heat the tube gently and uniformly.
Use a medium blue flame and move the test tube around in the flame
to heat the entire surface of the tube as evenly as possible. This
is important to minimize bumping and cracking.
Video. Move the test tube in and out of the flame to
allow time for heat transfer.
Move the test tube out of the flame every few seconds to
allow heat to pass from the glass to the solution. This is particularly
important in preventing boil-over.
Video. Apply more heat closer to the surface of the
liquid.
Try to heat the area near the liquid surface slightly more
than the bottom of the tube. This concentrates boiling near the
surface of the liquid and reduces the potential for bumping and
boil-over.
Video. Heat the upper part of the test tube to prevent
recondensation.
When heating a tube and its contents to dryness, make sure
to heat the upper portion of the tube as well to prevent the vapors
from recondensing.
Video. Gentle shaking increases the rate of evaporation.
As the volume of liquid in the tube becomes quite small,
shaking the tube gently will spread it out over the wall of the
tube, increasing the evaporation rate and reducing the potential
for bumping and boil-over.
Click on the link below to download and view
a single (5.1 MB) video showing a solution in
a test tube being heated to dryness.
