I'd like to ask when you would do the following: use the direct Bunsen flame to heat a substance; place the substance in a test tube, which is placed in a beaker of water, then boil the water with the flame while the test tube is in it?

Specifically, I'm doing an experiment where I synthesise aspirin by reacting salicylic acid with acetic anhydride using an 85% phosphoric acid catalyst. Is it potentially harmful to use a direct flame in this instance?

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    $\begingroup$ For safety's sake, use the water bath. $\endgroup$ – user55119 Jun 27 '20 at 15:48
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    $\begingroup$ SAFETY - The water bath gets the reactants hot enough to cause the reaction without the risk of causing the contents of the test tube to boil thereby turning the test tube into a cannon. $\endgroup$ – MaxW Jun 27 '20 at 16:07
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    $\begingroup$ Also, most organic chemicals are flammable, so if you spill any out of the test tube, that might be a fire / explosion hazard, whereas if it were to land in the water bath, it's not as significant a danger. $\endgroup$ – Zhe Jun 27 '20 at 17:04
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    $\begingroup$ Bunsen burners are not safe to use–even as the source of heat for a water bath–if you are working with chemicals that are flammable. There are plenty of ways to heat vessels that don't–at any point–involve naked flames. If you are, for example, working with diethyl ether, there should be no naked flames anywhere in the same laboratory, never mind being used to directly heat the vessel. Acetic anhydride might be less volatile than that but it is still wise to avoid any naked flames and to stick with electrical heaters. $\endgroup$ – matt_black Jun 27 '20 at 20:02
  • $\begingroup$ Besides safety concerns. Sorry but the answer should be quite obvious. This, in my opinion has nothing to do with chemistry. The Temperature of a body immersed in any bath cannot exceed that of the bath itself (when the latter is compatible). The T of the bath will be rather omogenous and and always below that of the heating source. Of course if you want to boil a water based solution, a water bath won't be suitable. Still, unless we are speaking of calcination, decomposition, or small test tube scale, you will alway use something to spread the flame/heat. Minus 1 to the Q. . $\endgroup$ – Alchimista Dec 2 '20 at 12:51

This reminds me of an incident that occured in my school lab: The lab assistant had set up an electrically heated water bath in a corner of the lab (Which was the only place where an electrical socket was available). Since this was a large lab, meant to accomodate 40+ students, it was a long walk to the bath. You had to place your test tube in the bath after marking it, wait for a few minutes and then pick it up again.

A few guys who were located in a corner of the lab didn't feel like walking the distance and waiting, so they turned on the bunsen burner and used it to heat the organic chemicals (Acetone and Formaldehyde) that they had to identify the functional groups for. While heating, the chemical started to boil so vigorously that a small amount 'cannoned' out of the mouth of the test tube and got stuck onto the ceiling. Needless to say, the students spent their afternoon in the headmaster's office.

TL;DR: While working with volatile chemicals, always use a water bath. It ensures slow, safe and even heating and as a bonus, the temperature does not rise above 100° C in the worst case. Most electrical water baths are temperature-controlled as well, meaning a thermostat senses the temperature and shuts off the heater coil if the temperature rises above a certain limit.

  • $\begingroup$ Would it make a difference if I placed the reaction mixture in a test tube, which I place in a beaker of water, and heated the water with the bunsen burner? Here's a picture of what I mean: [google.com/… The reason I'm considering using a bunsen burner even if a water bath is safer is because my experiment is to test different heating methods in making aspirin. $\endgroup$ – Ali Aug 4 '20 at 7:38
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    $\begingroup$ that would be safer than directly using a bunsen burner, but still risky. You should read @matt_black 's comment on your question. $\endgroup$ – Aniruddha Deb Aug 4 '20 at 17:37

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