A reaction vial has a conical shape inside of a glass cylinder. The glass cylinder encasing the conical reaction vial allows for the tube to be secured to holding stands in an experiment apparatus, or set up. The round-bottom flask is sphere-shaped with a one, two or three round necks protruding out its top. The neck is slightly conical (female) and can accept similarly-sized tapered pieces (male).
Reaction vials are intended for smaller chemical reactions that can be contained in one of the four vial sizes, all measured in milliliters: 5, 3, 1 and 0.3. Round-bottom flasks are ideal for larger chemical reactions and liquid collection, with the largest flask being 5 liters and the smallest flask being 5 milliliters. The size of the vial or flask is painted on the side of the glass, usually in white.
The top of the reaction vial has threads that allow for a cap to close the reaction vial to the air. This allows you to shake and swirl the vial without the contents spilling out. The tops of round-bottom flasks are closed to air by stopping the neck with a cork top. Because cork is porous, the cap is not airtight.
Reaction vials are ideal for small chemical reactions. The conical shape allows you to swirl the contents easily and observe the results. The exterior cylinder shape is easily stored in a test-tube rack while the reaction proceeds. Round-bottom flasks can also be used for large chemical reactions, especially those that need a heat element. They are more often used in experiments where the boiling point needs to be kept constant, such as distillation experiments.