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A stir bar is a small magnetic device used primarily in laboratories to efficiently mix solutions. When placed in a liquid and positioned on a hot plate stirrer, it creates a vortex that keeps the solution evenly mixed throughout the process. This is crucial in various experiments where consistent concentration is necessary for accurate results. The stirring helps ensure that all components of the solution are uniformly distributed, allowing reactions to proceed evenly and efficiently.

Heat cannot be applied by the stir bar itself; it functions solely to mix, which makes it distinct from heating devices. Similarly, while temperature measurement is vital in many experimental setups, that task is not performed by a stir bar. Lastly, the term "primary stain" typically refers to a technique in microbiology to highlight cells or structures under a microscope, which is unrelated to the mechanical mixing role of a stir bar.