Why do you move the test tube?

Answer 1

This is to ensure even and controlled heating of the reactants. Things that can go wrong include bumping, where one area becomes hotter than the others, and boil over, when the reactants are heated evenly but too strongly.

Answer 2

Consider that the liquid may suddenly boil at the point of contact with the area that the flame is heating, the boiling (massive change in volume of the liquid concerned to become a gas) will eject the liquid/gas at a high temperature in a dangerous manner. The movement of the test tube therefore can keep the liquid mobile (assist convection) and ensure that a larger surface area is being heated to reduce the Watts/sq cm heat transfer to the liquid.

Also consider that test tubes are made of glass (normally similar to Pyrex {trade-name}which is much superior to normal glass). Glass is a poor conductor of heat, as opposed to metals such as copper which are good conductors of heat. All materials will expand/contract to some degree with the application/removal of heat. So heating a poor conductor will in the case of a glass test tube tend to expand the surface where the flame impinges. Glass being a poor conductor means that heat in glass travels slowly and the temperature of the glass may be much cooler where the flame does not reach. The differential expansion of glass (glass being also a very brittle material) will or rather can cause cracking/ destruction of the test tube. Moving the test tube in the flame tends to heat more area of the glass and more uniformly. Test tubes normally have very thin walls. It is always better to apply heat with a small flame just to be sure. Consider a bi-metal strip of which one side expands greater than the other although it is being heated at the same rate and is actually the same temperature, these types of metals are not brittle, so the bi-metal strip just bends. If however you tried to make a bi-metal strip with cast iron and copper, it would fail because of the differential expansion of the different metals and the overriding fact that the cast iron side is very brittle and will break (fail) as the copper will force the cast iron to one side (bend) and the brittleness of the cast iron cannot take such distress.

Answer 3

To heat all parts of the test tube evenly.

See the Related Questions to the left for more information about test tubes.

Answer 4

Scientists use test tubes in order to test a larger sample. This allows them to try new things and new chemicals against the sample.

Answer 5

When heating a testube in a Bunsen flame the testtube should be gently moved to avoid any part of the liquid becoming gas and exploding the rest of the liquid on to you neighbour.

Answer 6

Because it is too hot to handle with your fingers if it has been in a hot or boiling water bath.