You are definitely on the right track. Your sugar and salt crystals are causing something physical to happen to the superheated liquid. Coffee heated in a microwave is somewhat different to coffee heated on a stove. The heating is a different kind of heating. The microwave heats by radio frequency excitation of the H-O bond in water molecules. The stove heats by convection, conduction and radiation. These differences make it relatively easy to create superheated water in a microwave, and your observations relate to the behaviour of superheated fluids and boiling. The following is taken from Bill Beaty's website on microwave experiments:
Water can only "boil" at places where the water touches a gas. If there are no bubbles, then "boiling" only occurs at the top surface of the water and not down within it. So, when you heat water on the stove, the extreme temperature at the bottom of the pot forces bubbles to form. The roiling bubbles act to cool the water and keep its temperature at or below 100C/212F degrees.
Things are different in a microwave oven. The water gets hot but the container usually does not. There are no "boiling-bubbles" triggered by a hot stove burner. Without those bubbles to cool it, the temperature of the water sometimes rises far higher than 100C. We call this "superheated water."
Superheated water is just waiting for some sort of trigger which will let bubbles form and allow boiling to commence. If the water becomes hot enough, a few bubbles will appear at the top, but these quickly rise and burst, and the water isn't cooled much at all. Even if your mug of water is bubbling slightly, don't trust it, since it's temperature has risen so high above 100C that bubbles are appearing spontaneously. If some unwitting victim should pour powder into the superheated water, this will carry thousands of tiny air bubbles into the water. Each of these micro bubbles expands into a 1cm steam bubble, and the result is a huge "explosion" of hot froth. It's just like dumping ice cream into rootbeer, but the froth can be so violent that the hot water sprays into the air.
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