[Twigg]

I boiled some sucrose solutions in test tubes for a high school lab last night. I had 5mL of roughly 1.5 molal sucrose solution in a test tube with a marble chip, but the thing still flash boiled and almost shot out of the tube. I count myself lucky.
First question: Even before the solution started boiling at all, it started making a rumbling noise as if it were boiling already. This was in test tubes with 5mL of 6 molal, 3 molal, and 1.5 molal sucrose. Is this noise a sign of super-heating? Are there any signs?
Second question: Were the test tubes a bad idea? Could the small exposed area have caused the solution to superheat?

[Stepan]

I should not use test tubes for boiling. The diameter is too narrow. If it starts to boil, all content will be out. In old manuals it was always spelled. If you need to heat a test tube, you point it away from yourself and people around you. The best way is to use a water bath.

For boiling you use a flask or a beaker. Never fill more than 1/4 of the volume (for the same reasons as for a test-tube). The noise you heard was a sound of collapsing steam bubble as they reach cold layer. The bottom part of the tube was overheated, and upper part was still cold.