When heating liquids in a test tube why the tube must be moved back and forth across the flame of the burner?

Answer 1

This is done so that the test tube is heated uniformly and not in just one place. If its heated in only one place, it could crack, or even explode. Usually, after passing the test tube back and forth for a few minutes, it will be hot enough to safely hold it directly over the open flame.

Notes: Even if the test tube is hot enough to be held directly over the flame, always be careful and keep your face away from the test tube and use proper safety equipment, like gloves, a lab coat, and goggles. Also, if what you're heating produces dangerous fumes, make sure to have proper ventilation, use a fume hood, and/or use an appropriate gas mask.

Answer 2

You should only heat liquids in the wider kind of test tube known as a boiling tube. Using an ordinary test tube often results in rapid local boiling and consequent shooting out of the liquid, which is known as 'bumping'. That said, many people prefer to move the tube around in the flame at first to prevent uneven heating of the contents, and to produce smoother boiling. There is a very small chance that concentrated heating may put stress on a weak spot in the glass and cause it to break.

Answer 3

To ensure that the liquid is heated evenly.

Answer 4

So that you do not form a hot spot that will boil and shoot out the contents.

Answer 5

The best way is to secure the test tube and its contents with a clamp to a ring stand, and then to heat it by moving the flame back and forth across the test tube.

Answer 6

Avoid hot spots.