A stir bar is a magnetic bar used to stir a liquid mixture

Mixture

In chemistry, a mixture is a material system made up by two or more different substances which are mixed together but are not combined chemically...

 or solution

Solution

In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...

, usually in a laboratory. The stir bar's motion is driven by a separate rotating magnet or assembly of electromagnets (i.e., coils) located beneath the vessel containing the liquid. Since glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 does not affect a magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 appreciably (it is transparent to magnetism), and most chemical reactions take place in glass vessels (i.e. see beaker (glassware)

Beaker (glassware)

A beaker is a simple container for stirring, mixing and heating liquids commonly used in many laboratories. Beakers are generally cylindrical in shape, with a flat bottom. Most also have a small spout to aid pouring as shown in the picture...

 or laboratory flasks), magnetic stir bars work well in glass vessels.

Stir bars are typically coated in teflon, or less often in glass. Glass coatings are used for liquid alkali metals (except lye

Lye

Lye is a corrosive alkaline substance, commonly sodium hydroxide or historically potassium hydroxide . Previously, lye was among the many different alkalis leached from hardwood ashes...

, which will eat through glass), alkali metal solutions in ammonia. Both coatings are chemically inert

Inert

-Chemistry:In chemistry, the term inert is used to describe a substance that is not chemically reactive.The noble gases were previously known as inert gases because of their perceived lack of participation in any chemical reactions...

 and do not contaminate or react with the reaction mixture they are in.

They are bar shaped and often octagonal in cross-section (sometimes circular), although a variety of special shapes exist for more efficient stirring. Most stir bars have a ridge around the center (called a pivot ring) on which they rotate. The smallest are only a few millimeters long and the largest a few centimeters.

A stir bar retriever is a separate magnet on the end of a long stick (usually coated with teflon) which can be used to remove stir bars from a vessel.

A magnetic stirrer

Magnetic stirrer

A magnetic stirrer or magnetic mixer is a laboratory device that employs a rotating magnetic field to cause a stir bar immersed in a liquid to spin very quickly, thus stirring it. The rotating field may be created either by a rotating magnet or a set of stationary electromagnets, placed beneath...

 can be as simple as a magnet spun by flowing air or water, on which the reaction vessel containing the stir bar sits. Most magnetic stirrers today spin their magnets with an electric motor

Electric motor

An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

, and some also have a heating element.

Stir bars work best with relatively small reactions (a few liters of volume at most) that are not very viscous. For larger volumes or more viscous liquids, some sort of mechanical stirring is typically needed.