Heating test tubes and other containers

It is often necessary to heat a solution in a test tube, either to cause precipitation or to dissolve a precipitate. You can carry out this heating effectively and safely by partially immersing the test tube containing the mixture in a simmering boiling-water bath (remember to use a test tube holder!).

It is possible to reduce the volume of the solution in the test tube, i.e. to pre-concentrate the sample, by evaporation. Two different methods can be employed.

1. Transfer the solution to a small evaporating dish. Place the evaporating dish on a wire gauze located on a tripod stand, and apply heat using a micro-Bunsen burner. Note that the volumes of solutions in qualitative analysis are often small, and excessive heating might result in hardening of any residue, making it unusable.
2. Alternatively, evaporate the solution directly in a test tube by gentle heating over a micro-Bunsen burner. Remember to use a test tube holder. Position the test tube at an angle with the tip of the Bunsen burner flame positioned at the upper surface of the liquid. Place a glass rod inside the test tube and rotate constantly. This acts to disperse bubbles of steam that are given off. Extreme caution is required with this method of evaporation, as the steam bubbles can cause the solution to 'bump' out of the test tube. 'Bumping' can result in hot (and maybe toxic) substances being ejected over a suprisingly large distance. To prevent this it is normal to add antibumping granules.