Heating a Test Tube

Safety

• Check the test tube for cracks before heating.
• 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.

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.

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.

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.

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.

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.

Link to view the full-length video.