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CR Scientific

CAUTION: If you choose to attempt any of the experiments or procedures described on this site, you do so at your own risk. Working with fire and/or chemicals can be dangerous and should be attempted only by competent, trained persons.

Information presented on this web site is not intended to substitute in any way for proper training.  Please review our Terms of Use.

Proper heating of Test Tubes

The test tube is one of the simplest pieces of lab glassware imaginable.  It is also one of the most common.  
Most test tubes today are made of borosilicate glass.  Even though borosilicate test tubes are resistant to breakage from thermal shock, there are right ways and wrong ways to heat them.  For example, test tubes can be made to shatter if heated too intensely, or if they're suddenly brought from a high temperature to a low temperature (such as quenching them in water).
One of the most common heating mistakes is to have too much liquid in the test tube.  Another is to heat the liquid too rapidly or for too long a time. Both factors can cause the boiling to be extremely sudden, forcing the liquid out of the tube.  This is called bumping.  The sudden boiling creates a steam bubble that acts like a gas piston.  Do not expect to have any warning signs that bumping is about to happen;  there aren't any.  The only way to avoid bumping is proper technique.

Safety Notes:

Here are some safety rules and tips for working with test tubes.  They are all very important.  If you want to learn to work with test tubes, start with plain water.  Learn to heat the water without "bumping" before you progress to chemical experiments.  Please be advised, however, that dissolved or undissolved additives can change how the liquid behaves with regard to "bumping".

1.)  Wear safety goggles!  There is no excuse for failing to do this.  It doesn't matter whether you're working with test tubes, measuring out reagents, doing some other experiment, or just cleaning glassware... wear your safety goggles.

2.)  Make sure the mouth of the test tube is pointed away from yourself and from anyone else.  

3.)  Heat gradually and slowly to avoid "bumping".  There are a few, different ways to do this.  
A.) Use a boiling stone and move the test tube back and forth across the flame.  
B.)  Heat the test tube only near the top of the liquid, not the bottom;  use a boiling stone if available..  
C.)  Heat the test tube on a water bath;  use a boiling stone if available.  

4.)  Limit the amount of liquid.  If you plan to heat the a test tube, it should not be more than half-full.  If possible, it should be only 1/4 to 1/3 full.  If the amount of liquid you wish to heat is more than this, consider using a bigger test tube.   A short column of liquid in a wide tube is much less prone to bumping than is a tall column of liquid in a narrow tube.
If using a water bath, you may have to clamp the test tube in place.  A tube that's only 1/4 full of liquid, for example, may overturn in the bath due to buoyant force.
3.)  Never heat a plugged or stoppered tube.  Also, never stopper or plug a test tube that contains any mixture that will release gases (e.g., acetic acid + sodium bicarbonate).  Pressure buildup can cause bursting of the tube or ejection of the stopper, leading to injury.  Borosilicate test tubes can withstand proper heating, but they were never intended to withstand high pressure.

4.)  Test tubes are one of the few types of glassware intended to be heated via direct flame.  This is partly due to their shape and their small size.  However, it is still possible to shatter them with incautious heating.  Do not put the test tube directly into the flame.  Hold it above the tip of the flame, preferably by a couple of inches.
If you are heating the test tube cautiously enough to avoid bumping, you are probably also heating it cautiously enough to avoid shattering due to rapid heating.  Generally, you should pass the tube back and forth over the flame instead of holding it in once place.  

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