Why is it crucial to stopper the volumetric flask when preparing a standard solution?

Stopping the volumetric flask is essential primarily to avoid evaporation of the solution. When preparing a standard solution, it is important to have an accurate concentration, and evaporation can change the volume of the solution, thus affecting its concentration. If the solvent evaporates, the solution will become more concentrated than intended because the amount of solute remains the same while the volume decreases.

While it is true that preventing contamination from the air and maintaining the temperature are also important factors in the preparation of chemical solutions, they are secondary to the primary concern of volume stability. Furthermore, increasing the pressure inside the flask is not a relevant consideration in the context of preparing standard solutions, as this does not relate to maintaining the solution’s integrity. Thus, the most critical reason for stoppering the flask is to prevent evaporation, ensuring that the prepared solution remains at its intended concentration.