Final answer:
A stir bar is needed in a calorimeter to mix reactants more effectively and maintain a constant temperature throughout the calorimeter.
Step-by-step explanation:
In a calorimeter, a stir bar is needed primarily to mix reactants more effectively. It ensures that the reactants are evenly distributed throughout the solution, which promotes efficient heat transfer and accurate measurements. By continuously stirring, the stir bar helps to maintain a constant temperature throughout the calorimeter, which also contributes to the reliability of the results.