Question

Why the volumetric flask must be stoppered and shaken?​

Answer 1

Explanation:

It is very difficult to dissolve the solid once the flask is filled to the mark, so make sure it is all dissolved before you fill to the mark. 

Answer 2

It must be stoppered and shook to prevent breakage.

• Because the considerable volume of a selected sample and a suitable solvent can typically change during mixing, the sample should not be diluted to the marked line without stopping.
• The ideal solution is thoroughly mixed after the final dilution by inverting the flask and gently shaking it.
• The mixing is not enough when it is carefully shaken without inverting. If the flask is tipped over, the stopper prevents the neck from promptly breaking.