A very long time ago I performed an experiment in my kitchen, part of which was to remove all water from a solution. Unfortunately I don't remember what was in it, but the point is that as the solution got more concentrated, it gradually stopped boiling, and would instead remain completely still for seconds, until a huge bubble would form instantly at the bottom of the test tube, making a pop sound and shooting some of the solution out of the tube. The effect got stronger and stronger until it became too strong for me to feel safe to continue.
I was unable to get an explanation of this effect out of my school teachers at the time. What I expected was that as the solution got more concentrated, the dissolved compound would start to precipitate, until eventually no water is left. Why did the popping occur instead?
Answer
What you witnessed was probably something similar to superheating/bumping. Essentially, when a solution is heated too rapidly, large pockets of vapor can form beneath the surface of the liquid that have an internal temperature higher than the expected boiling temperature of the liquid. In those situations, even small physical disturbances and/or further addition of heat can provoke the bubbles to burst explosively as the vapor is released. Usually, boiling chips (made of inert, inorganic materials like aluminum oxide, silicates, carbonates, etc.) are added to the reaction vessel in order to prevent that occurrence. The boiling chips provide an uneven and porous surface around which smaller and more numerous vapor nuclei can form, rather than the large bubbles that can form in smooth containers which have relatively few nucleation sites. Furthermore, the chips themselves provide additional surface area for the radiation of heat, allowing more uniform heat distribution. The likelihood of bumping can also be reduced by using wider reaction vessels, which allow superheated bubbles to escape without disturbing all of the surrounding liquid above them.
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