Which item spins rapidly in a rotating field to stir and dissolve?

• A. Safety Glasses
• B. Pipetting Device
• C. Laboratory Balance
• D. Magnetic Stir Bar

Answer: (D)

Magnetic stirring uses a rotating magnetic field to drive a stir bar inside a liquid. The stir plate creates a rotating magnetic field beneath the container, and the stir bar, which contains a magnet, responds to that field by spinning. This rotation creates turbulence and shear that rapidly distributes solute and solvent, helping substances dissolve more quickly and evenly. The bar is typically coated with PTFE to prevent sticking and to be chemically inert, and it must be fully submerged for effective mixing.

Safety glasses, pipetting devices, and laboratory balances serve protection, transfer, or measurement roles, not stirring via a magnetic field, so they don’t spin or dissolve solutions.