volumetric flask
/ vol-yoo-MET-rik FLASK /
Picture a pear-shaped glass bottle with a long thin neck and a single ring etched around it; you fill to that ring and you have, very exactly, the stated volume. A volumetric flask is built for one job: holding one precise volume of solution.
A volumetric flask is a flat-bottomed glass vessel calibrated to contain a single, accurate volume (such as 100.0 mL or 250.0 mL) when filled so the bottom of the liquid's curved surface sits on its one marked line. It is the standard tool for making up a solution to an exact final volume.
It matters because accurate solution preparation — and therefore accurate concentration — depends on dialling in volume precisely; the narrow neck makes the level very sensitive, so a tiny rise in liquid is a tiny, controllable change in volume. Its limit: it measures only that one volume, and only at the temperature it was calibrated for.
To prepare a 250.0 mL standard solution, you dissolve the weighed solid in a little water in the flask, then add water carefully until the meniscus rests exactly on the 250 mL ring, and mix.
One ring, one exact volume — the tool for making up solutions.
A volumetric flask is calibrated 'to contain' (TC) its volume, whereas a volumetric pipette is calibrated 'to deliver' (TD). Never heat a volumetric flask, as heat permanently changes its calibrated volume.