When using a boiling tube to boil a liquid, I am frequently advised to move the boiling tube in and out of the Bunsen flame and when the liquid starts to boil, remove the tube from the flame for a few seconds before resuming. Why is this so?
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5$\begingroup$ That piece of glassware is called test-tube. "Why it is so" seems pretty obvious. $\endgroup$– AlchimistaCommented May 19, 2021 at 7:28
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2$\begingroup$ You should also keep the open end pointed away from you (and everyone else!) and, of course, wear safety glasses. $\endgroup$– theoristCommented May 19, 2021 at 10:01
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1 Answer
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Usually, if people have experience of boiling water at home and think a while about it, they are able to answer it themselves. These "boiling tubes" are used primarily for ambient temperature testing, so they are called test tubes, or test-tubes.
Taking a test-tube in and out is for decreasing the heat flow to prevent violent overheating and the burst of boiling liquid shooting out of the tube. Similarly, removing the boiling test-tube from the flame serves the same purpose, avoiding violent outbursts. Bear in mind there is high ratio of provided heat versus the test-tube heat capacity, so warming up is fast and subsequent boiling can be violent.
Additionally, as a welcome side effect, moving of a test-tube keeps mechanical disturbances and traps air bubbles, creating boiling centers. This in large extent prevents overheating above the boiling point and then sudden boiling outbursts. This is especially important when heating liquid with strong tendency to be overheated, like strongly alkalic solutions. A burst of boiling hydroxide solution is nothing I would recommend to experience.
Another precaution is warming up rather the upper part of the liquid in a tilted test-tube. If an outburst happens in spite of being careful, less liquid is shot outside.
