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Heating Sources



We often need to run chemical reactions at temperatures above room temperature or distill liquids. The dangers of Bunsen burners and open flames have led to many different ways to heat reactions to reflux. This guide details some of the more common methods used in a typical chemistry laboratory.

Disclaimer: Regardless of the method you use to heat something, a stuck contact, an electrical short circuit, runaway chemical reaction or other malfunction can cause a reaction to heat to dangerously high temperatures. Do not leave experiments unattended and always take proper precautions. Fires and explosions can often result from the heating of chemical reaction mixtures.

Heating Mantles

One of the most popular and safest methods to heat a reaction mixture in a synthetic organic, inorganic, or organometallic laboratory is a heating mantle, a resistively heated flexible fiberglass shell that conforms to the shape of the reaction flask. Most heating mantles are hemispherical in design and come in a variety of sizes designed to fit round bottom flasks from 50 mL up to 5000 mL or more.

Shown below are three examples of heating mantles. The one on the left has a shell that supports the flask, the middle one has a hemispherical outer aluminum shell and the one on the right has a flexible all-fiberglass design. Most chemists prefer the latter two types because these are easiest to use in combination with a magnetic stir plate. Combination heating/stirring mantles are also available, but are somewhat more expensive:

a heating mantle a heating mantle a heating mantle
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Some heating mantles come in two piece designs that zip or tie together to enclose almost the entire flask except for the neck(s), but these are usually only used for special apparatuses. Heating tapes are also available which can be used to keep distillation columns warm or to regenerate catalyst columns.

Heating mantles should NEVER be plugged directly into an outlet. A heating mantle can easily generate enough heat to vaporize most organic materials and will melt a flask if used on full power. ALWAYS plug the heating mantle into a grounded variable voltage transformer or percentage timer control (which runs at full power but cycles on and off to control the heat). Such devices are often known by the trademarked name, Variac (below, left) or MiniTrol (below, middle). Start with the lowest power setting -- 20-30% power is usually enough to cause most common organic liquids to reflux. A few examples of these are shown below:

a Variac the MiniTrol the PowrTrol
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Remember that heating mantles and Variacs are electrical appliances, so follow the appropriate precautions. To prolong the life of your heating mantle, clean up spills (solid or liquid) right away. Except for the cord, mantles are not generally repairable. If the fiberglass is brittle or cracked or if any wires are exposed then discard the heating mantle. Never immerse a heating mantle in water or other liquids (or place these in the mantle) and always use a properly grounded outlet. Do not allow the cords to dangle down the front of the lab bench or hood, a snag/trip danger.

Additional warning: Some older models of Variacs will keep whatever is plugged into them electrically live even though the Variac is switched off! Touching this device and ground at the same time could complete a circuit with your body and lead to electrocution.

Variacs also find use around the laboratory for other purposes, such as controlling the heating of a high vacuum line diffusion pump or the rate of a stirring motor.

Heating Baths

Bath Materials

Fluids have good thermal transfer coefficients and are an effective way of heating reactions. The simplest bath we can think of is a hot water bath, however even at 70 degrees C, water evaporates too quickly to make this a practical bath fluid for most purposes.

The key, then, is to find a bath fluid that does not evaporate readily and is not flammable at the temperatures being used. This depends on knowing the flash point of the material, i.e. the point at which the bath material can form a flammable mixture with air at the liquid interface. NEVER heat a bath fluid above its flash point (and as a good rule of thumb, not even within 20 degrees of its flash point).

Fluid heating baths are inherently dangerous, and a heating mantle or solid (dry) bath material should be used instead unless absolutely necessary (see below). A hot fluid can be easily spilled or spattered, especially if water is accidentally introduced into a hot fluid. In addition, there is an electrical heat source in close proximity and a hot bath is very difficult to deal with in an emergency situation. Bath fluids expand upon heating, so overflows are another potential pitfall.

Despite the potential hazards, fluid heating baths are sometimes used by chemists instead of heating mantles because of their convenience and ability to prevent thermal fluctuations. For example, it would be extremely difficult to heat an NMR tube with a heating mantle, and the temperature would be very difficult to monitor. For the best thermal stability, magnetically stir the bath during operation.

The bath container is typically a beaker or large crystallizing dish. This is convenient because you can usually see your reaction flask through the bath. For very high temperature materials, a metal container is sometime used, although this can present a shock hazard if the apparatus is not properly grounded.

Here are some bath materials commonly used in chemistry laboratories.

Bath MaterialFlash Point (°C)Useful Range (°C)Comments
WaterNA0 - 70
  • Non-flammable, non-toxic, cheap.
  • No waste disposal for used bath.
  • Easy to clean up if spilled.
  • Evaporates quickly at relatively low T.
Mineral Oil11325-100
  • Inexpensive, widely available.
  • Turns brown and polymerizes with prolonged use.
  • Spatter danger from leaking water.
  • Slippery, hard to handle, esp when hot.
  • Low flashpoint.
Dibutyl phthalate17125 - 150
  • Less tendency to discolor/degrade.
  • Relatively inexpensive.
  • High bp; tempting to take above flash pt.
  • Slippery, hard to handle, esp when hot.
Parrafin Waxvaries55 - 180
  • Doesn't degrade as easily as mineral oil.
  • Clean up by scraping and reusing wax.
  • Spatter danger from leaking water.
  • Slippery, hard to handle, esp when hot.
Silicone Oil150 - 35025 - 230
  • Greater working range than mineral oil.
  • Less tendency to discolor/degrade.
  • Moderately expensive.
  • Spatter danger from leaking water.
  • Slippery, hard to handle, esp when hot.
Wood's Metal
50% Bi, 25% Pb, 12.5% Sn, 12.5% Cd
  • High working temperature.
  • Does not degrade easily.
  • Stainless steel container required.
  • Toxic (use in fume hood!).
  • Moderately expensive.
  • Hard to handle, esp when hot.
  • Reaction flask can be hard to clean.
Eutectic ( ) salt mixtures
51.3% KNO3; 48.7% NaNO3
40% NaNO2; 7%NaNO3; 53%KNO3
NA 230 - 500
142 - 500
  • Good choice for moderately high T.
  • Does not degrade easily.
  • Can be corrosive or react with certain vapors.
  • Hard to handle, esp when hot.
  • Cleanup usually easy.
  • Caution: Some mixtures in the literature are reportedly explosive; never mix an oxidizer with a reducer!
  • No practical temperature limit, non-flammable.
  • Does not degrade. Inexpensive.
  • Can use a heating mantle as the container.
  • Non-toxic, easy to clean up and reuse.
  • Spatter danger much lower.
  • Chemically inert to organic materials.
  • Slowest to heat and cool.

If you need to use a fluid bath for a kinetics run, NMR tube experiment etc., silicone oil is a good choice for temperatures below 200 degrees and molten salts are a good choice for temperatures 200-500 degrees C.

You'll notice that we list sand as the last entry in the fluid table. Solid (dry) heating baths using sand, inert shot, and similar materials are superior as they pose no spatter danger, are non-flammable, are non-toxic, and generally don't degrade. Solid baths are a great choice when you have an odd-shaped apparatus and don't want to use a fluid. Commercially available solid baths are discussed in the next section.

Bath Heat Sources

Having a bath in which to immerse your reaction flask is not have to have a heat source. There are many different ways to heat your reaction; here are the the most common:

cover illustration
Synthesis and Technique
in Inorganic Chemistry:
A Laboratory Manual

by Girolami, Rauchfuss & Angelici

cover illustration
The Laboratory Companion:
A Practical Guide to
Materials, Equipment,
and Technique

by Gary S. Coyne


This page was last updated Wednesday, March 15, 2017.
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