Which apparatus is typically used to heat test tubes containing flammable liquids?

The apparatus typically used to heat test tubes containing flammable liquids is a hot water bath. A hot water bath provides a safe and controlled method for heating substances without exposing them directly to an open flame, thereby minimizing the risk of ignition of flammable vapors that can arise from the liquids. This technique allows for even heating and helps to maintain a stable temperature.

In contrast, a hot plate applies direct heat and poses a higher risk when working with flammable materials. An incubator is designed to maintain specific temperatures for biological cultures but is not suited for heating liquids in test tubes. A graduated cylinder is a measuring tool and does not provide a heating function at all. Therefore, the use of a hot water bath is the safest and most appropriate method for heating flammable liquids in a laboratory setting.